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llvm-mirror/lib/Transforms/Utils/SimplifyCFG.cpp
Chandler Carruth ae65e281f3 Update the file headers across all of the LLVM projects in the monorepo 
to reflect the new license.

We understand that people may be surprised that we're moving the header
entirely to discuss the new license. We checked this carefully with the
Foundation's lawyer and we believe this is the correct approach.

Essentially, all code in the project is now made available by the LLVM
project under our new license, so you will see that the license headers
include that license only. Some of our contributors have contributed
code under our old license, and accordingly, we have retained a copy of
our old license notice in the top-level files in each project and
repository.

llvm-svn: 351636
2019-01-19 08:50:56 +00:00

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//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/EHPersonalities.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/NoFolder.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <climits>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <map>
#include <set>
#include <tuple>
#include <utility>
#include <vector>
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "simplifycfg"
// Chosen as 2 so as to be cheap, but still to have enough power to fold
// a select, so the "clamp" idiom (of a min followed by a max) will be caught.
// To catch this, we need to fold a compare and a select, hence '2' being the
// minimum reasonable default.
static cl::opt<unsigned> PHINodeFoldingThreshold(
"phi-node-folding-threshold", cl::Hidden, cl::init(2),
cl::desc(
"Control the amount of phi node folding to perform (default = 2)"));
static cl::opt<bool> DupRet(
"simplifycfg-dup-ret", cl::Hidden, cl::init(false),
cl::desc("Duplicate return instructions into unconditional branches"));
static cl::opt<bool>
SinkCommon("simplifycfg-sink-common", cl::Hidden, cl::init(true),
cl::desc("Sink common instructions down to the end block"));
static cl::opt<bool> HoistCondStores(
"simplifycfg-hoist-cond-stores", cl::Hidden, cl::init(true),
cl::desc("Hoist conditional stores if an unconditional store precedes"));
static cl::opt<bool> MergeCondStores(
"simplifycfg-merge-cond-stores", cl::Hidden, cl::init(true),
cl::desc("Hoist conditional stores even if an unconditional store does not "
"precede - hoist multiple conditional stores into a single "
"predicated store"));
static cl::opt<bool> MergeCondStoresAggressively(
"simplifycfg-merge-cond-stores-aggressively", cl::Hidden, cl::init(false),
cl::desc("When merging conditional stores, do so even if the resultant "
"basic blocks are unlikely to be if-converted as a result"));
static cl::opt<bool> SpeculateOneExpensiveInst(
"speculate-one-expensive-inst", cl::Hidden, cl::init(true),
cl::desc("Allow exactly one expensive instruction to be speculatively "
"executed"));
static cl::opt<unsigned> MaxSpeculationDepth(
"max-speculation-depth", cl::Hidden, cl::init(10),
cl::desc("Limit maximum recursion depth when calculating costs of "
"speculatively executed instructions"));
STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
STATISTIC(NumLinearMaps,
"Number of switch instructions turned into linear mapping");
STATISTIC(NumLookupTables,
"Number of switch instructions turned into lookup tables");
STATISTIC(
NumLookupTablesHoles,
"Number of switch instructions turned into lookup tables (holes checked)");
STATISTIC(NumTableCmpReuses, "Number of reused switch table lookup compares");
STATISTIC(NumSinkCommons,
"Number of common instructions sunk down to the end block");
STATISTIC(NumSpeculations, "Number of speculative executed instructions");
namespace {
// The first field contains the value that the switch produces when a certain
// case group is selected, and the second field is a vector containing the
// cases composing the case group.
using SwitchCaseResultVectorTy =
SmallVector<std::pair<Constant *, SmallVector<ConstantInt *, 4>>, 2>;
// The first field contains the phi node that generates a result of the switch
// and the second field contains the value generated for a certain case in the
// switch for that PHI.
using SwitchCaseResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
/// ValueEqualityComparisonCase - Represents a case of a switch.
struct ValueEqualityComparisonCase {
ConstantInt *Value;
BasicBlock *Dest;
ValueEqualityComparisonCase(ConstantInt *Value, BasicBlock *Dest)
: Value(Value), Dest(Dest) {}
bool operator<(ValueEqualityComparisonCase RHS) const {
// Comparing pointers is ok as we only rely on the order for uniquing.
return Value < RHS.Value;
}
bool operator==(BasicBlock *RHSDest) const { return Dest == RHSDest; }
};
class SimplifyCFGOpt {
const TargetTransformInfo &TTI;
const DataLayout &DL;
SmallPtrSetImpl<BasicBlock *> *LoopHeaders;
const SimplifyCFGOptions &Options;
bool Resimplify;
Value *isValueEqualityComparison(Instruction *TI);
BasicBlock *GetValueEqualityComparisonCases(
Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases);
bool SimplifyEqualityComparisonWithOnlyPredecessor(Instruction *TI,
BasicBlock *Pred,
IRBuilder<> &Builder);
bool FoldValueComparisonIntoPredecessors(Instruction *TI,
IRBuilder<> &Builder);
bool SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder);
bool SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder);
bool SimplifySingleResume(ResumeInst *RI);
bool SimplifyCommonResume(ResumeInst *RI);
bool SimplifyCleanupReturn(CleanupReturnInst *RI);
bool SimplifyUnreachable(UnreachableInst *UI);
bool SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder);
bool SimplifyIndirectBr(IndirectBrInst *IBI);
bool SimplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder);
bool SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder);
bool tryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
IRBuilder<> &Builder);
public:
SimplifyCFGOpt(const TargetTransformInfo &TTI, const DataLayout &DL,
SmallPtrSetImpl<BasicBlock *> *LoopHeaders,
const SimplifyCFGOptions &Opts)
: TTI(TTI), DL(DL), LoopHeaders(LoopHeaders), Options(Opts) {}
bool run(BasicBlock *BB);
bool simplifyOnce(BasicBlock *BB);
// Helper to set Resimplify and return change indication.
bool requestResimplify() {
Resimplify = true;
return true;
}
};
} // end anonymous namespace
/// Return true if it is safe to merge these two
/// terminator instructions together.
static bool
SafeToMergeTerminators(Instruction *SI1, Instruction *SI2,
SmallSetVector<BasicBlock *, 4> *FailBlocks = nullptr) {
if (SI1 == SI2)
return false; // Can't merge with self!
// It is not safe to merge these two switch instructions if they have a common
// successor, and if that successor has a PHI node, and if *that* PHI node has
// conflicting incoming values from the two switch blocks.
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
bool Fail = false;
for (BasicBlock *Succ : successors(SI2BB))
if (SI1Succs.count(Succ))
for (BasicBlock::iterator BBI = Succ->begin(); isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
if (PN->getIncomingValueForBlock(SI1BB) !=
PN->getIncomingValueForBlock(SI2BB)) {
if (FailBlocks)
FailBlocks->insert(Succ);
Fail = true;
}
}
return !Fail;
}
/// Return true if it is safe and profitable to merge these two terminator
/// instructions together, where SI1 is an unconditional branch. PhiNodes will
/// store all PHI nodes in common successors.
static bool
isProfitableToFoldUnconditional(BranchInst *SI1, BranchInst *SI2,
Instruction *Cond,
SmallVectorImpl<PHINode *> &PhiNodes) {
if (SI1 == SI2)
return false; // Can't merge with self!
assert(SI1->isUnconditional() && SI2->isConditional());
// We fold the unconditional branch if we can easily update all PHI nodes in
// common successors:
// 1> We have a constant incoming value for the conditional branch;
// 2> We have "Cond" as the incoming value for the unconditional branch;
// 3> SI2->getCondition() and Cond have same operands.
CmpInst *Ci2 = dyn_cast<CmpInst>(SI2->getCondition());
if (!Ci2)
return false;
if (!(Cond->getOperand(0) == Ci2->getOperand(0) &&
Cond->getOperand(1) == Ci2->getOperand(1)) &&
!(Cond->getOperand(0) == Ci2->getOperand(1) &&
Cond->getOperand(1) == Ci2->getOperand(0)))
return false;
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
SmallPtrSet<BasicBlock *, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
for (BasicBlock *Succ : successors(SI2BB))
if (SI1Succs.count(Succ))
for (BasicBlock::iterator BBI = Succ->begin(); isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
if (PN->getIncomingValueForBlock(SI1BB) != Cond ||
!isa<ConstantInt>(PN->getIncomingValueForBlock(SI2BB)))
return false;
PhiNodes.push_back(PN);
}
return true;
}
/// Update PHI nodes in Succ to indicate that there will now be entries in it
/// from the 'NewPred' block. The values that will be flowing into the PHI nodes
/// will be the same as those coming in from ExistPred, an existing predecessor
/// of Succ.
static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
BasicBlock *ExistPred) {
for (PHINode &PN : Succ->phis())
PN.addIncoming(PN.getIncomingValueForBlock(ExistPred), NewPred);
}
/// Compute an abstract "cost" of speculating the given instruction,
/// which is assumed to be safe to speculate. TCC_Free means cheap,
/// TCC_Basic means less cheap, and TCC_Expensive means prohibitively
/// expensive.
static unsigned ComputeSpeculationCost(const User *I,
const TargetTransformInfo &TTI) {
assert(isSafeToSpeculativelyExecute(I) &&
"Instruction is not safe to speculatively execute!");
return TTI.getUserCost(I);
}
/// If we have a merge point of an "if condition" as accepted above,
/// return true if the specified value dominates the block. We
/// don't handle the true generality of domination here, just a special case
/// which works well enough for us.
///
/// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
/// see if V (which must be an instruction) and its recursive operands
/// that do not dominate BB have a combined cost lower than CostRemaining and
/// are non-trapping. If both are true, the instruction is inserted into the
/// set and true is returned.
///
/// The cost for most non-trapping instructions is defined as 1 except for
/// Select whose cost is 2.
///
/// After this function returns, CostRemaining is decreased by the cost of
/// V plus its non-dominating operands. If that cost is greater than
/// CostRemaining, false is returned and CostRemaining is undefined.
static bool DominatesMergePoint(Value *V, BasicBlock *BB,
SmallPtrSetImpl<Instruction *> &AggressiveInsts,
unsigned &CostRemaining,
const TargetTransformInfo &TTI,
unsigned Depth = 0) {
// It is possible to hit a zero-cost cycle (phi/gep instructions for example),
// so limit the recursion depth.
// TODO: While this recursion limit does prevent pathological behavior, it
// would be better to track visited instructions to avoid cycles.
if (Depth == MaxSpeculationDepth)
return false;
Instruction *I = dyn_cast<Instruction>(V);
if (!I) {
// Non-instructions all dominate instructions, but not all constantexprs
// can be executed unconditionally.
if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
if (C->canTrap())
return false;
return true;
}
BasicBlock *PBB = I->getParent();
// We don't want to allow weird loops that might have the "if condition" in
// the bottom of this block.
if (PBB == BB)
return false;
// If this instruction is defined in a block that contains an unconditional
// branch to BB, then it must be in the 'conditional' part of the "if
// statement". If not, it definitely dominates the region.
BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator());
if (!BI || BI->isConditional() || BI->getSuccessor(0) != BB)
return true;
// If we have seen this instruction before, don't count it again.
if (AggressiveInsts.count(I))
return true;
// Okay, it looks like the instruction IS in the "condition". Check to
// see if it's a cheap instruction to unconditionally compute, and if it
// only uses stuff defined outside of the condition. If so, hoist it out.
if (!isSafeToSpeculativelyExecute(I))
return false;
unsigned Cost = ComputeSpeculationCost(I, TTI);
// Allow exactly one instruction to be speculated regardless of its cost
// (as long as it is safe to do so).
// This is intended to flatten the CFG even if the instruction is a division
// or other expensive operation. The speculation of an expensive instruction
// is expected to be undone in CodeGenPrepare if the speculation has not
// enabled further IR optimizations.
if (Cost > CostRemaining &&
(!SpeculateOneExpensiveInst || !AggressiveInsts.empty() || Depth > 0))
return false;
// Avoid unsigned wrap.
CostRemaining = (Cost > CostRemaining) ? 0 : CostRemaining - Cost;
// Okay, we can only really hoist these out if their operands do
// not take us over the cost threshold.
for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i)
if (!DominatesMergePoint(*i, BB, AggressiveInsts, CostRemaining, TTI,
Depth + 1))
return false;
// Okay, it's safe to do this! Remember this instruction.
AggressiveInsts.insert(I);
return true;
}
/// Extract ConstantInt from value, looking through IntToPtr
/// and PointerNullValue. Return NULL if value is not a constant int.
static ConstantInt *GetConstantInt(Value *V, const DataLayout &DL) {
// Normal constant int.
ConstantInt *CI = dyn_cast<ConstantInt>(V);
if (CI || !isa<Constant>(V) || !V->getType()->isPointerTy())
return CI;
// This is some kind of pointer constant. Turn it into a pointer-sized
// ConstantInt if possible.
IntegerType *PtrTy = cast<IntegerType>(DL.getIntPtrType(V->getType()));
// Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
if (isa<ConstantPointerNull>(V))
return ConstantInt::get(PtrTy, 0);
// IntToPtr const int.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
if (CE->getOpcode() == Instruction::IntToPtr)
if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
// The constant is very likely to have the right type already.
if (CI->getType() == PtrTy)
return CI;
else
return cast<ConstantInt>(
ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false));
}
return nullptr;
}
namespace {
/// Given a chain of or (||) or and (&&) comparison of a value against a
/// constant, this will try to recover the information required for a switch
/// structure.
/// It will depth-first traverse the chain of comparison, seeking for patterns
/// like %a == 12 or %a < 4 and combine them to produce a set of integer
/// representing the different cases for the switch.
/// Note that if the chain is composed of '||' it will build the set of elements
/// that matches the comparisons (i.e. any of this value validate the chain)
/// while for a chain of '&&' it will build the set elements that make the test
/// fail.
struct ConstantComparesGatherer {
const DataLayout &DL;
/// Value found for the switch comparison
Value *CompValue = nullptr;
/// Extra clause to be checked before the switch
Value *Extra = nullptr;
/// Set of integers to match in switch
SmallVector<ConstantInt *, 8> Vals;
/// Number of comparisons matched in the and/or chain
unsigned UsedICmps = 0;
/// Construct and compute the result for the comparison instruction Cond
ConstantComparesGatherer(Instruction *Cond, const DataLayout &DL) : DL(DL) {
gather(Cond);
}
ConstantComparesGatherer(const ConstantComparesGatherer &) = delete;
ConstantComparesGatherer &
operator=(const ConstantComparesGatherer &) = delete;
private:
/// Try to set the current value used for the comparison, it succeeds only if
/// it wasn't set before or if the new value is the same as the old one
bool setValueOnce(Value *NewVal) {
if (CompValue && CompValue != NewVal)
return false;
CompValue = NewVal;
return (CompValue != nullptr);
}
/// Try to match Instruction "I" as a comparison against a constant and
/// populates the array Vals with the set of values that match (or do not
/// match depending on isEQ).
/// Return false on failure. On success, the Value the comparison matched
/// against is placed in CompValue.
/// If CompValue is already set, the function is expected to fail if a match
/// is found but the value compared to is different.
bool matchInstruction(Instruction *I, bool isEQ) {
// If this is an icmp against a constant, handle this as one of the cases.
ICmpInst *ICI;
ConstantInt *C;
if (!((ICI = dyn_cast<ICmpInst>(I)) &&
(C = GetConstantInt(I->getOperand(1), DL)))) {
return false;
}
Value *RHSVal;
const APInt *RHSC;
// Pattern match a special case
// (x & ~2^z) == y --> x == y || x == y|2^z
// This undoes a transformation done by instcombine to fuse 2 compares.
if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE)) {
// It's a little bit hard to see why the following transformations are
// correct. Here is a CVC3 program to verify them for 64-bit values:
/*
ONE : BITVECTOR(64) = BVZEROEXTEND(0bin1, 63);
x : BITVECTOR(64);
y : BITVECTOR(64);
z : BITVECTOR(64);
mask : BITVECTOR(64) = BVSHL(ONE, z);
QUERY( (y & ~mask = y) =>
((x & ~mask = y) <=> (x = y OR x = (y | mask)))
);
QUERY( (y | mask = y) =>
((x | mask = y) <=> (x = y OR x = (y & ~mask)))
);
*/
// Please note that each pattern must be a dual implication (<--> or
// iff). One directional implication can create spurious matches. If the
// implication is only one-way, an unsatisfiable condition on the left
// side can imply a satisfiable condition on the right side. Dual
// implication ensures that satisfiable conditions are transformed to
// other satisfiable conditions and unsatisfiable conditions are
// transformed to other unsatisfiable conditions.
// Here is a concrete example of a unsatisfiable condition on the left
// implying a satisfiable condition on the right:
//
// mask = (1 << z)
// (x & ~mask) == y --> (x == y || x == (y | mask))
//
// Substituting y = 3, z = 0 yields:
// (x & -2) == 3 --> (x == 3 || x == 2)
// Pattern match a special case:
/*
QUERY( (y & ~mask = y) =>
((x & ~mask = y) <=> (x = y OR x = (y | mask)))
);
*/
if (match(ICI->getOperand(0),
m_And(m_Value(RHSVal), m_APInt(RHSC)))) {
APInt Mask = ~*RHSC;
if (Mask.isPowerOf2() && (C->getValue() & ~Mask) == C->getValue()) {
// If we already have a value for the switch, it has to match!
if (!setValueOnce(RHSVal))
return false;
Vals.push_back(C);
Vals.push_back(
ConstantInt::get(C->getContext(),
C->getValue() | Mask));
UsedICmps++;
return true;
}
}
// Pattern match a special case:
/*
QUERY( (y | mask = y) =>
((x | mask = y) <=> (x = y OR x = (y & ~mask)))
);
*/
if (match(ICI->getOperand(0),
m_Or(m_Value(RHSVal), m_APInt(RHSC)))) {
APInt Mask = *RHSC;
if (Mask.isPowerOf2() && (C->getValue() | Mask) == C->getValue()) {
// If we already have a value for the switch, it has to match!
if (!setValueOnce(RHSVal))
return false;
Vals.push_back(C);
Vals.push_back(ConstantInt::get(C->getContext(),
C->getValue() & ~Mask));
UsedICmps++;
return true;
}
}
// If we already have a value for the switch, it has to match!
if (!setValueOnce(ICI->getOperand(0)))
return false;
UsedICmps++;
Vals.push_back(C);
return ICI->getOperand(0);
}
// If we have "x ult 3", for example, then we can add 0,1,2 to the set.
ConstantRange Span = ConstantRange::makeAllowedICmpRegion(
ICI->getPredicate(), C->getValue());
// Shift the range if the compare is fed by an add. This is the range
// compare idiom as emitted by instcombine.
Value *CandidateVal = I->getOperand(0);
if (match(I->getOperand(0), m_Add(m_Value(RHSVal), m_APInt(RHSC)))) {
Span = Span.subtract(*RHSC);
CandidateVal = RHSVal;
}
// If this is an and/!= check, then we are looking to build the set of
// value that *don't* pass the and chain. I.e. to turn "x ugt 2" into
// x != 0 && x != 1.
if (!isEQ)
Span = Span.inverse();
// If there are a ton of values, we don't want to make a ginormous switch.
if (Span.isSizeLargerThan(8) || Span.isEmptySet()) {
return false;
}
// If we already have a value for the switch, it has to match!
if (!setValueOnce(CandidateVal))
return false;
// Add all values from the range to the set
for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
Vals.push_back(ConstantInt::get(I->getContext(), Tmp));
UsedICmps++;
return true;
}
/// Given a potentially 'or'd or 'and'd together collection of icmp
/// eq/ne/lt/gt instructions that compare a value against a constant, extract
/// the value being compared, and stick the list constants into the Vals
/// vector.
/// One "Extra" case is allowed to differ from the other.
void gather(Value *V) {
Instruction *I = dyn_cast<Instruction>(V);
bool isEQ = (I->getOpcode() == Instruction::Or);
// Keep a stack (SmallVector for efficiency) for depth-first traversal
SmallVector<Value *, 8> DFT;
SmallPtrSet<Value *, 8> Visited;
// Initialize
Visited.insert(V);
DFT.push_back(V);
while (!DFT.empty()) {
V = DFT.pop_back_val();
if (Instruction *I = dyn_cast<Instruction>(V)) {
// If it is a || (or && depending on isEQ), process the operands.
if (I->getOpcode() == (isEQ ? Instruction::Or : Instruction::And)) {
if (Visited.insert(I->getOperand(1)).second)
DFT.push_back(I->getOperand(1));
if (Visited.insert(I->getOperand(0)).second)
DFT.push_back(I->getOperand(0));
continue;
}
// Try to match the current instruction
if (matchInstruction(I, isEQ))
// Match succeed, continue the loop
continue;
}
// One element of the sequence of || (or &&) could not be match as a
// comparison against the same value as the others.
// We allow only one "Extra" case to be checked before the switch
if (!Extra) {
Extra = V;
continue;
}
// Failed to parse a proper sequence, abort now
CompValue = nullptr;
break;
}
}
};
} // end anonymous namespace
static void EraseTerminatorAndDCECond(Instruction *TI) {
Instruction *Cond = nullptr;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cond = dyn_cast<Instruction>(SI->getCondition());
} else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isConditional())
Cond = dyn_cast<Instruction>(BI->getCondition());
} else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(TI)) {
Cond = dyn_cast<Instruction>(IBI->getAddress());
}
TI->eraseFromParent();
if (Cond)
RecursivelyDeleteTriviallyDeadInstructions(Cond);
}
/// Return true if the specified terminator checks
/// to see if a value is equal to constant integer value.
Value *SimplifyCFGOpt::isValueEqualityComparison(Instruction *TI) {
Value *CV = nullptr;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
// Do not permit merging of large switch instructions into their
// predecessors unless there is only one predecessor.
if (!SI->getParent()->hasNPredecessorsOrMore(128 / SI->getNumSuccessors()))
CV = SI->getCondition();
} else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
if (BI->isConditional() && BI->getCondition()->hasOneUse())
if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) {
if (ICI->isEquality() && GetConstantInt(ICI->getOperand(1), DL))
CV = ICI->getOperand(0);
}
// Unwrap any lossless ptrtoint cast.
if (CV) {
if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV)) {
Value *Ptr = PTII->getPointerOperand();
if (PTII->getType() == DL.getIntPtrType(Ptr->getType()))
CV = Ptr;
}
}
return CV;
}
/// Given a value comparison instruction,
/// decode all of the 'cases' that it represents and return the 'default' block.
BasicBlock *SimplifyCFGOpt::GetValueEqualityComparisonCases(
Instruction *TI, std::vector<ValueEqualityComparisonCase> &Cases) {
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cases.reserve(SI->getNumCases());
for (auto Case : SI->cases())
Cases.push_back(ValueEqualityComparisonCase(Case.getCaseValue(),
Case.getCaseSuccessor()));
return SI->getDefaultDest();
}
BranchInst *BI = cast<BranchInst>(TI);
ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
BasicBlock *Succ = BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_NE);
Cases.push_back(ValueEqualityComparisonCase(
GetConstantInt(ICI->getOperand(1), DL), Succ));
return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
}
/// Given a vector of bb/value pairs, remove any entries
/// in the list that match the specified block.
static void
EliminateBlockCases(BasicBlock *BB,
std::vector<ValueEqualityComparisonCase> &Cases) {
Cases.erase(std::remove(Cases.begin(), Cases.end(), BB), Cases.end());
}
/// Return true if there are any keys in C1 that exist in C2 as well.
static bool ValuesOverlap(std::vector<ValueEqualityComparisonCase> &C1,
std::vector<ValueEqualityComparisonCase> &C2) {
std::vector<ValueEqualityComparisonCase> *V1 = &C1, *V2 = &C2;
// Make V1 be smaller than V2.
if (V1->size() > V2->size())
std::swap(V1, V2);
if (V1->empty())
return false;
if (V1->size() == 1) {
// Just scan V2.
ConstantInt *TheVal = (*V1)[0].Value;
for (unsigned i = 0, e = V2->size(); i != e; ++i)
if (TheVal == (*V2)[i].Value)
return true;
}
// Otherwise, just sort both lists and compare element by element.
array_pod_sort(V1->begin(), V1->end());
array_pod_sort(V2->begin(), V2->end());
unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
while (i1 != e1 && i2 != e2) {
if ((*V1)[i1].Value == (*V2)[i2].Value)
return true;
if ((*V1)[i1].Value < (*V2)[i2].Value)
++i1;
else
++i2;
}
return false;
}
// Set branch weights on SwitchInst. This sets the metadata if there is at
// least one non-zero weight.
static void setBranchWeights(SwitchInst *SI, ArrayRef<uint32_t> Weights) {
// Check that there is at least one non-zero weight. Otherwise, pass
// nullptr to setMetadata which will erase the existing metadata.
MDNode *N = nullptr;
if (llvm::any_of(Weights, [](uint32_t W) { return W != 0; }))
N = MDBuilder(SI->getParent()->getContext()).createBranchWeights(Weights);
SI->setMetadata(LLVMContext::MD_prof, N);
}
// Similar to the above, but for branch and select instructions that take
// exactly 2 weights.
static void setBranchWeights(Instruction *I, uint32_t TrueWeight,
uint32_t FalseWeight) {
assert(isa<BranchInst>(I) || isa<SelectInst>(I));
// Check that there is at least one non-zero weight. Otherwise, pass
// nullptr to setMetadata which will erase the existing metadata.
MDNode *N = nullptr;
if (TrueWeight || FalseWeight)
N = MDBuilder(I->getParent()->getContext())
.createBranchWeights(TrueWeight, FalseWeight);
I->setMetadata(LLVMContext::MD_prof, N);
}
/// If TI is known to be a terminator instruction and its block is known to
/// only have a single predecessor block, check to see if that predecessor is
/// also a value comparison with the same value, and if that comparison
/// determines the outcome of this comparison. If so, simplify TI. This does a
/// very limited form of jump threading.
bool SimplifyCFGOpt::SimplifyEqualityComparisonWithOnlyPredecessor(
Instruction *TI, BasicBlock *Pred, IRBuilder<> &Builder) {
Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
if (!PredVal)
return false; // Not a value comparison in predecessor.
Value *ThisVal = isValueEqualityComparison(TI);
assert(ThisVal && "This isn't a value comparison!!");
if (ThisVal != PredVal)
return false; // Different predicates.
// TODO: Preserve branch weight metadata, similarly to how
// FoldValueComparisonIntoPredecessors preserves it.
// Find out information about when control will move from Pred to TI's block.
std::vector<ValueEqualityComparisonCase> PredCases;
BasicBlock *PredDef =
GetValueEqualityComparisonCases(Pred->getTerminator(), PredCases);
EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
// Find information about how control leaves this block.
std::vector<ValueEqualityComparisonCase> ThisCases;
BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
// If TI's block is the default block from Pred's comparison, potentially
// simplify TI based on this knowledge.
if (PredDef == TI->getParent()) {
// If we are here, we know that the value is none of those cases listed in
// PredCases. If there are any cases in ThisCases that are in PredCases, we
// can simplify TI.
if (!ValuesOverlap(PredCases, ThisCases))
return false;
if (isa<BranchInst>(TI)) {
// Okay, one of the successors of this condbr is dead. Convert it to a
// uncond br.
assert(ThisCases.size() == 1 && "Branch can only have one case!");
// Insert the new branch.
Instruction *NI = Builder.CreateBr(ThisDef);
(void)NI;
// Remove PHI node entries for the dead edge.
ThisCases[0].Dest->removePredecessor(TI->getParent());
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI
<< "\n");
EraseTerminatorAndDCECond(TI);
return true;
}
SwitchInst *SI = cast<SwitchInst>(TI);
// Okay, TI has cases that are statically dead, prune them away.
SmallPtrSet<Constant *, 16> DeadCases;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
DeadCases.insert(PredCases[i].Value);
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI);
// Collect branch weights into a vector.
SmallVector<uint32_t, 8> Weights;
MDNode *MD = SI->getMetadata(LLVMContext::MD_prof);
bool HasWeight = MD && (MD->getNumOperands() == 2 + SI->getNumCases());
if (HasWeight)
for (unsigned MD_i = 1, MD_e = MD->getNumOperands(); MD_i < MD_e;
++MD_i) {
ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(MD_i));
Weights.push_back(CI->getValue().getZExtValue());
}
for (SwitchInst::CaseIt i = SI->case_end(), e = SI->case_begin(); i != e;) {
--i;
if (DeadCases.count(i->getCaseValue())) {
if (HasWeight) {
std::swap(Weights[i->getCaseIndex() + 1], Weights.back());
Weights.pop_back();
}
i->getCaseSuccessor()->removePredecessor(TI->getParent());
SI->removeCase(i);
}
}
if (HasWeight && Weights.size() >= 2)
setBranchWeights(SI, Weights);
LLVM_DEBUG(dbgs() << "Leaving: " << *TI << "\n");
return true;
}
// Otherwise, TI's block must correspond to some matched value. Find out
// which value (or set of values) this is.
ConstantInt *TIV = nullptr;
BasicBlock *TIBB = TI->getParent();
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest == TIBB) {
if (TIV)
return false; // Cannot handle multiple values coming to this block.
TIV = PredCases[i].Value;
}
assert(TIV && "No edge from pred to succ?");
// Okay, we found the one constant that our value can be if we get into TI's
// BB. Find out which successor will unconditionally be branched to.
BasicBlock *TheRealDest = nullptr;
for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
if (ThisCases[i].Value == TIV) {
TheRealDest = ThisCases[i].Dest;
break;
}
// If not handled by any explicit cases, it is handled by the default case.
if (!TheRealDest)
TheRealDest = ThisDef;
// Remove PHI node entries for dead edges.
BasicBlock *CheckEdge = TheRealDest;
for (BasicBlock *Succ : successors(TIBB))
if (Succ != CheckEdge)
Succ->removePredecessor(TIBB);
else
CheckEdge = nullptr;
// Insert the new branch.
Instruction *NI = Builder.CreateBr(TheRealDest);
(void)NI;
LLVM_DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI
<< "\n");
EraseTerminatorAndDCECond(TI);
return true;
}
namespace {
/// This class implements a stable ordering of constant
/// integers that does not depend on their address. This is important for
/// applications that sort ConstantInt's to ensure uniqueness.
struct ConstantIntOrdering {
bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
return LHS->getValue().ult(RHS->getValue());
}
};
} // end anonymous namespace
static int ConstantIntSortPredicate(ConstantInt *const *P1,
ConstantInt *const *P2) {
const ConstantInt *LHS = *P1;
const ConstantInt *RHS = *P2;
if (LHS == RHS)
return 0;
return LHS->getValue().ult(RHS->getValue()) ? 1 : -1;
}
static inline bool HasBranchWeights(const Instruction *I) {
MDNode *ProfMD = I->getMetadata(LLVMContext::MD_prof);
if (ProfMD && ProfMD->getOperand(0))
if (MDString *MDS = dyn_cast<MDString>(ProfMD->getOperand(0)))
return MDS->getString().equals("branch_weights");
return false;
}
/// Get Weights of a given terminator, the default weight is at the front
/// of the vector. If TI is a conditional eq, we need to swap the branch-weight
/// metadata.
static void GetBranchWeights(Instruction *TI,
SmallVectorImpl<uint64_t> &Weights) {
MDNode *MD = TI->getMetadata(LLVMContext::MD_prof);
assert(MD);
for (unsigned i = 1, e = MD->getNumOperands(); i < e; ++i) {
ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(i));
Weights.push_back(CI->getValue().getZExtValue());
}
// If TI is a conditional eq, the default case is the false case,
// and the corresponding branch-weight data is at index 2. We swap the
// default weight to be the first entry.
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
assert(Weights.size() == 2);
ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
std::swap(Weights.front(), Weights.back());
}
}
/// Keep halving the weights until all can fit in uint32_t.
static void FitWeights(MutableArrayRef<uint64_t> Weights) {
uint64_t Max = *std::max_element(Weights.begin(), Weights.end());
if (Max > UINT_MAX) {
unsigned Offset = 32 - countLeadingZeros(Max);
for (uint64_t &I : Weights)
I >>= Offset;
}
}
/// The specified terminator is a value equality comparison instruction
/// (either a switch or a branch on "X == c").
/// See if any of the predecessors of the terminator block are value comparisons
/// on the same value. If so, and if safe to do so, fold them together.
bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(Instruction *TI,
IRBuilder<> &Builder) {
BasicBlock *BB = TI->getParent();
Value *CV = isValueEqualityComparison(TI); // CondVal
assert(CV && "Not a comparison?");
bool Changed = false;
SmallVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.pop_back_val();
// See if the predecessor is a comparison with the same value.
Instruction *PTI = Pred->getTerminator();
Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
if (PCV == CV && TI != PTI) {
SmallSetVector<BasicBlock*, 4> FailBlocks;
if (!SafeToMergeTerminators(TI, PTI, &FailBlocks)) {
for (auto *Succ : FailBlocks) {
if (!SplitBlockPredecessors(Succ, TI->getParent(), ".fold.split"))
return false;
}
}
// Figure out which 'cases' to copy from SI to PSI.
std::vector<ValueEqualityComparisonCase> BBCases;
BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
std::vector<ValueEqualityComparisonCase> PredCases;
BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
// Based on whether the default edge from PTI goes to BB or not, fill in
// PredCases and PredDefault with the new switch cases we would like to
// build.
SmallVector<BasicBlock *, 8> NewSuccessors;
// Update the branch weight metadata along the way
SmallVector<uint64_t, 8> Weights;
bool PredHasWeights = HasBranchWeights(PTI);
bool SuccHasWeights = HasBranchWeights(TI);
if (PredHasWeights) {
GetBranchWeights(PTI, Weights);
// branch-weight metadata is inconsistent here.
if (Weights.size() != 1 + PredCases.size())
PredHasWeights = SuccHasWeights = false;
} else if (SuccHasWeights)
// If there are no predecessor weights but there are successor weights,
// populate Weights with 1, which will later be scaled to the sum of
// successor's weights
Weights.assign(1 + PredCases.size(), 1);
SmallVector<uint64_t, 8> SuccWeights;
if (SuccHasWeights) {
GetBranchWeights(TI, SuccWeights);
// branch-weight metadata is inconsistent here.
if (SuccWeights.size() != 1 + BBCases.size())
PredHasWeights = SuccHasWeights = false;
} else if (PredHasWeights)
SuccWeights.assign(1 + BBCases.size(), 1);
if (PredDefault == BB) {
// If this is the default destination from PTI, only the edges in TI
// that don't occur in PTI, or that branch to BB will be activated.
std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest != BB)
PTIHandled.insert(PredCases[i].Value);
else {
// The default destination is BB, we don't need explicit targets.
std::swap(PredCases[i], PredCases.back());
if (PredHasWeights || SuccHasWeights) {
// Increase weight for the default case.
Weights[0] += Weights[i + 1];
std::swap(Weights[i + 1], Weights.back());
Weights.pop_back();
}
PredCases.pop_back();
--i;
--e;
}
// Reconstruct the new switch statement we will be building.
if (PredDefault != BBDefault) {
PredDefault->removePredecessor(Pred);
PredDefault = BBDefault;
NewSuccessors.push_back(BBDefault);
}
unsigned CasesFromPred = Weights.size();
uint64_t ValidTotalSuccWeight = 0;
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (!PTIHandled.count(BBCases[i].Value) &&
BBCases[i].Dest != BBDefault) {
PredCases.push_back(BBCases[i]);
NewSuccessors.push_back(BBCases[i].Dest);
if (SuccHasWeights || PredHasWeights) {
// The default weight is at index 0, so weight for the ith case
// should be at index i+1. Scale the cases from successor by
// PredDefaultWeight (Weights[0]).
Weights.push_back(Weights[0] * SuccWeights[i + 1]);
ValidTotalSuccWeight += SuccWeights[i + 1];
}
}
if (SuccHasWeights || PredHasWeights) {
ValidTotalSuccWeight += SuccWeights[0];
// Scale the cases from predecessor by ValidTotalSuccWeight.
for (unsigned i = 1; i < CasesFromPred; ++i)
Weights[i] *= ValidTotalSuccWeight;
// Scale the default weight by SuccDefaultWeight (SuccWeights[0]).
Weights[0] *= SuccWeights[0];
}
} else {
// If this is not the default destination from PSI, only the edges
// in SI that occur in PSI with a destination of BB will be
// activated.
std::set<ConstantInt *, ConstantIntOrdering> PTIHandled;
std::map<ConstantInt *, uint64_t> WeightsForHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].Dest == BB) {
PTIHandled.insert(PredCases[i].Value);
if (PredHasWeights || SuccHasWeights) {
WeightsForHandled[PredCases[i].Value] = Weights[i + 1];
std::swap(Weights[i + 1], Weights.back());
Weights.pop_back();
}
std::swap(PredCases[i], PredCases.back());
PredCases.pop_back();
--i;
--e;
}
// Okay, now we know which constants were sent to BB from the
// predecessor. Figure out where they will all go now.
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (PTIHandled.count(BBCases[i].Value)) {
// If this is one we are capable of getting...
if (PredHasWeights || SuccHasWeights)
Weights.push_back(WeightsForHandled[BBCases[i].Value]);
PredCases.push_back(BBCases[i]);
NewSuccessors.push_back(BBCases[i].Dest);
PTIHandled.erase(
BBCases[i].Value); // This constant is taken care of
}
// If there are any constants vectored to BB that TI doesn't handle,
// they must go to the default destination of TI.
for (ConstantInt *I : PTIHandled) {
if (PredHasWeights || SuccHasWeights)
Weights.push_back(WeightsForHandled[I]);
PredCases.push_back(ValueEqualityComparisonCase(I, BBDefault));
NewSuccessors.push_back(BBDefault);
}
}
// Okay, at this point, we know which new successor Pred will get. Make
// sure we update the number of entries in the PHI nodes for these
// successors.
for (BasicBlock *NewSuccessor : NewSuccessors)
AddPredecessorToBlock(NewSuccessor, Pred, BB);
Builder.SetInsertPoint(PTI);
// Convert pointer to int before we switch.
if (CV->getType()->isPointerTy()) {
CV = Builder.CreatePtrToInt(CV, DL.getIntPtrType(CV->getType()),
"magicptr");
}
// Now that the successors are updated, create the new Switch instruction.
SwitchInst *NewSI =
Builder.CreateSwitch(CV, PredDefault, PredCases.size());
NewSI->setDebugLoc(PTI->getDebugLoc());
for (ValueEqualityComparisonCase &V : PredCases)
NewSI->addCase(V.Value, V.Dest);
if (PredHasWeights || SuccHasWeights) {
// Halve the weights if any of them cannot fit in an uint32_t
FitWeights(Weights);
SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
setBranchWeights(NewSI, MDWeights);
}
EraseTerminatorAndDCECond(PTI);
// Okay, last check. If BB is still a successor of PSI, then we must
// have an infinite loop case. If so, add an infinitely looping block
// to handle the case to preserve the behavior of the code.
BasicBlock *InfLoopBlock = nullptr;
for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
if (NewSI->getSuccessor(i) == BB) {
if (!InfLoopBlock) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
InfLoopBlock = BasicBlock::Create(BB->getContext(), "infloop",
BB->getParent());
BranchInst::Create(InfLoopBlock, InfLoopBlock);
}
NewSI->setSuccessor(i, InfLoopBlock);
}
Changed = true;
}
}
return Changed;
}
// If we would need to insert a select that uses the value of this invoke
// (comments in HoistThenElseCodeToIf explain why we would need to do this), we
// can't hoist the invoke, as there is nowhere to put the select in this case.
static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
Instruction *I1, Instruction *I2) {
for (BasicBlock *Succ : successors(BB1)) {
for (const PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V != BB2V && (BB1V == I1 || BB2V == I2)) {
return false;
}
}
}
return true;
}
static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I);
/// Given a conditional branch that goes to BB1 and BB2, hoist any common code
/// in the two blocks up into the branch block. The caller of this function
/// guarantees that BI's block dominates BB1 and BB2.
static bool HoistThenElseCodeToIf(BranchInst *BI,
const TargetTransformInfo &TTI) {
// This does very trivial matching, with limited scanning, to find identical
// instructions in the two blocks. In particular, we don't want to get into
// O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
// such, we currently just scan for obviously identical instructions in an
// identical order.
BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
BasicBlock::iterator BB1_Itr = BB1->begin();
BasicBlock::iterator BB2_Itr = BB2->begin();
Instruction *I1 = &*BB1_Itr++, *I2 = &*BB2_Itr++;
// Skip debug info if it is not identical.
DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
while (isa<DbgInfoIntrinsic>(I1))
I1 = &*BB1_Itr++;
while (isa<DbgInfoIntrinsic>(I2))
I2 = &*BB2_Itr++;
}
if (isa<PHINode>(I1) || !I1->isIdenticalToWhenDefined(I2) ||
(isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)))
return false;
BasicBlock *BIParent = BI->getParent();
bool Changed = false;
do {
// If we are hoisting the terminator instruction, don't move one (making a
// broken BB), instead clone it, and remove BI.
if (I1->isTerminator())
goto HoistTerminator;
// If we're going to hoist a call, make sure that the two instructions we're
// commoning/hoisting are both marked with musttail, or neither of them is
// marked as such. Otherwise, we might end up in a situation where we hoist
// from a block where the terminator is a `ret` to a block where the terminator
// is a `br`, and `musttail` calls expect to be followed by a return.
auto *C1 = dyn_cast<CallInst>(I1);
auto *C2 = dyn_cast<CallInst>(I2);
if (C1 && C2)
if (C1->isMustTailCall() != C2->isMustTailCall())
return Changed;
if (!TTI.isProfitableToHoist(I1) || !TTI.isProfitableToHoist(I2))
return Changed;
if (isa<DbgInfoIntrinsic>(I1) || isa<DbgInfoIntrinsic>(I2)) {
assert (isa<DbgInfoIntrinsic>(I1) && isa<DbgInfoIntrinsic>(I2));
// The debug location is an integral part of a debug info intrinsic
// and can't be separated from it or replaced. Instead of attempting
// to merge locations, simply hoist both copies of the intrinsic.
BIParent->getInstList().splice(BI->getIterator(),
BB1->getInstList(), I1);
BIParent->getInstList().splice(BI->getIterator(),
BB2->getInstList(), I2);
Changed = true;
} else {
// For a normal instruction, we just move one to right before the branch,
// then replace all uses of the other with the first. Finally, we remove
// the now redundant second instruction.
BIParent->getInstList().splice(BI->getIterator(),
BB1->getInstList(), I1);
if (!I2->use_empty())
I2->replaceAllUsesWith(I1);
I1->andIRFlags(I2);
unsigned KnownIDs[] = {LLVMContext::MD_tbaa,
LLVMContext::MD_range,
LLVMContext::MD_fpmath,
LLVMContext::MD_invariant_load,
LLVMContext::MD_nonnull,
LLVMContext::MD_invariant_group,
LLVMContext::MD_align,
LLVMContext::MD_dereferenceable,
LLVMContext::MD_dereferenceable_or_null,
LLVMContext::MD_mem_parallel_loop_access,
LLVMContext::MD_access_group};
combineMetadata(I1, I2, KnownIDs, true);
// I1 and I2 are being combined into a single instruction. Its debug
// location is the merged locations of the original instructions.
I1->applyMergedLocation(I1->getDebugLoc(), I2->getDebugLoc());
I2->eraseFromParent();
Changed = true;
}
I1 = &*BB1_Itr++;
I2 = &*BB2_Itr++;
// Skip debug info if it is not identical.
DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
while (isa<DbgInfoIntrinsic>(I1))
I1 = &*BB1_Itr++;
while (isa<DbgInfoIntrinsic>(I2))
I2 = &*BB2_Itr++;
}
} while (I1->isIdenticalToWhenDefined(I2));
return true;
HoistTerminator:
// It may not be possible to hoist an invoke.
if (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))
return Changed;
for (BasicBlock *Succ : successors(BB1)) {
for (PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V == BB2V)
continue;
// Check for passingValueIsAlwaysUndefined here because we would rather
// eliminate undefined control flow then converting it to a select.
if (passingValueIsAlwaysUndefined(BB1V, &PN) ||
passingValueIsAlwaysUndefined(BB2V, &PN))
return Changed;
if (isa<ConstantExpr>(BB1V) && !isSafeToSpeculativelyExecute(BB1V))
return Changed;
if (isa<ConstantExpr>(BB2V) && !isSafeToSpeculativelyExecute(BB2V))
return Changed;
}
}
// Okay, it is safe to hoist the terminator.
Instruction *NT = I1->clone();
BIParent->getInstList().insert(BI->getIterator(), NT);
if (!NT->getType()->isVoidTy()) {
I1->replaceAllUsesWith(NT);
I2->replaceAllUsesWith(NT);
NT->takeName(I1);
}
// Ensure terminator gets a debug location, even an unknown one, in case
// it involves inlinable calls.
NT->applyMergedLocation(I1->getDebugLoc(), I2->getDebugLoc());
// PHIs created below will adopt NT's merged DebugLoc.
IRBuilder<NoFolder> Builder(NT);
// Hoisting one of the terminators from our successor is a great thing.
// Unfortunately, the successors of the if/else blocks may have PHI nodes in
// them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
// nodes, so we insert select instruction to compute the final result.
std::map<std::pair<Value *, Value *>, SelectInst *> InsertedSelects;
for (BasicBlock *Succ : successors(BB1)) {
for (PHINode &PN : Succ->phis()) {
Value *BB1V = PN.getIncomingValueForBlock(BB1);
Value *BB2V = PN.getIncomingValueForBlock(BB2);
if (BB1V == BB2V)
continue;
// These values do not agree. Insert a select instruction before NT
// that determines the right value.
SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
if (!SI)
SI = cast<SelectInst>(
Builder.CreateSelect(BI->getCondition(), BB1V, BB2V,
BB1V->getName() + "." + BB2V->getName(), BI));
// Make the PHI node use the select for all incoming values for BB1/BB2
for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
if (PN.getIncomingBlock(i) == BB1 || PN.getIncomingBlock(i) == BB2)
PN.setIncomingValue(i, SI);
}
}
// Update any PHI nodes in our new successors.
for (BasicBlock *Succ : successors(BB1))
AddPredecessorToBlock(Succ, BIParent, BB1);
EraseTerminatorAndDCECond(BI);
return true;
}
// All instructions in Insts belong to different blocks that all unconditionally
// branch to a common successor. Analyze each instruction and return true if it
// would be possible to sink them into their successor, creating one common
// instruction instead. For every value that would be required to be provided by
// PHI node (because an operand varies in each input block), add to PHIOperands.
static bool canSinkInstructions(
ArrayRef<Instruction *> Insts,
DenseMap<Instruction *, SmallVector<Value *, 4>> &PHIOperands) {
// Prune out obviously bad instructions to move. Any non-store instruction
// must have exactly one use, and we check later that use is by a single,
// common PHI instruction in the successor.
for (auto *I : Insts) {
// These instructions may change or break semantics if moved.
if (isa<PHINode>(I) || I->isEHPad() || isa<AllocaInst>(I) ||
I->getType()->isTokenTy())
return false;
// Conservatively return false if I is an inline-asm instruction. Sinking
// and merging inline-asm instructions can potentially create arguments
// that cannot satisfy the inline-asm constraints.
if (const auto *C = dyn_cast<CallInst>(I))
if (C->isInlineAsm())
return false;
// Everything must have only one use too, apart from stores which
// have no uses.
if (!isa<StoreInst>(I) && !I->hasOneUse())
return false;
}
const Instruction *I0 = Insts.front();
for (auto *I : Insts)
if (!I->isSameOperationAs(I0))
return false;
// All instructions in Insts are known to be the same opcode. If they aren't
// stores, check the only user of each is a PHI or in the same block as the
// instruction, because if a user is in the same block as an instruction
// we're contemplating sinking, it must already be determined to be sinkable.
if (!isa<StoreInst>(I0)) {
auto *PNUse = dyn_cast<PHINode>(*I0->user_begin());
auto *Succ = I0->getParent()->getTerminator()->getSuccessor(0);
if (!all_of(Insts, [&PNUse,&Succ](const Instruction *I) -> bool {
auto *U = cast<Instruction>(*I->user_begin());
return (PNUse &&
PNUse->getParent() == Succ &&
PNUse->getIncomingValueForBlock(I->getParent()) == I) ||
U->getParent() == I->getParent();
}))
return false;
}
// Because SROA can't handle speculating stores of selects, try not
// to sink loads or stores of allocas when we'd have to create a PHI for
// the address operand. Also, because it is likely that loads or stores
// of allocas will disappear when Mem2Reg/SROA is run, don't sink them.
// This can cause code churn which can have unintended consequences down
// the line - see https://llvm.org/bugs/show_bug.cgi?id=30244.
// FIXME: This is a workaround for a deficiency in SROA - see
// https://llvm.org/bugs/show_bug.cgi?id=30188
if (isa<StoreInst>(I0) && any_of(Insts, [](const Instruction *I) {
return isa<AllocaInst>(I->getOperand(1));
}))
return false;
if (isa<LoadInst>(I0) && any_of(Insts, [](const Instruction *I) {
return isa<AllocaInst>(I->getOperand(0));
}))
return false;
for (unsigned OI = 0, OE = I0->getNumOperands(); OI != OE; ++OI) {
if (I0->getOperand(OI)->getType()->isTokenTy())
// Don't touch any operand of token type.
return false;
auto SameAsI0 = [&I0, OI](const Instruction *I) {
assert(I->getNumOperands() == I0->getNumOperands());
return I->getOperand(OI) == I0->getOperand(OI);
};
if (!all_of(Insts, SameAsI0)) {
if (!canReplaceOperandWithVariable(I0, OI))
// We can't create a PHI from this GEP.
return false;
// Don't create indirect calls! The called value is the final operand.
if ((isa<CallInst>(I0) || isa<InvokeInst>(I0)) && OI == OE - 1) {
// FIXME: if the call was *already* indirect, we should do this.
return false;
}
for (auto *I : Insts)
PHIOperands[I].push_back(I->getOperand(OI));
}
}
return true;
}
// Assuming canSinkLastInstruction(Blocks) has returned true, sink the last
// instruction of every block in Blocks to their common successor, commoning
// into one instruction.
static bool sinkLastInstruction(ArrayRef<BasicBlock*> Blocks) {
auto *BBEnd = Blocks[0]->getTerminator()->getSuccessor(0);
// canSinkLastInstruction returning true guarantees that every block has at
// least one non-terminator instruction.
SmallVector<Instruction*,4> Insts;
for (auto *BB : Blocks) {
Instruction *I = BB->getTerminator();
do {
I = I->getPrevNode();
} while (isa<DbgInfoIntrinsic>(I) && I != &BB->front());
if (!isa<DbgInfoIntrinsic>(I))
Insts.push_back(I);
}
// The only checking we need to do now is that all users of all instructions
// are the same PHI node. canSinkLastInstruction should have checked this but
// it is slightly over-aggressive - it gets confused by commutative instructions
// so double-check it here.
Instruction *I0 = Insts.front();
if (!isa<StoreInst>(I0)) {
auto *PNUse = dyn_cast<PHINode>(*I0->user_begin());
if (!all_of(Insts, [&PNUse](const Instruction *I) -> bool {
auto *U = cast<Instruction>(*I->user_begin());
return U == PNUse;
}))
return false;
}
// We don't need to do any more checking here; canSinkLastInstruction should
// have done it all for us.
SmallVector<Value*, 4> NewOperands;
for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O) {
// This check is different to that in canSinkLastInstruction. There, we
// cared about the global view once simplifycfg (and instcombine) have
// completed - it takes into account PHIs that become trivially
// simplifiable. However here we need a more local view; if an operand
// differs we create a PHI and rely on instcombine to clean up the very
// small mess we may make.
bool NeedPHI = any_of(Insts, [&I0, O](const Instruction *I) {
return I->getOperand(O) != I0->getOperand(O);
});
if (!NeedPHI) {
NewOperands.push_back(I0->getOperand(O));
continue;
}
// Create a new PHI in the successor block and populate it.
auto *Op = I0->getOperand(O);
assert(!Op->getType()->isTokenTy() && "Can't PHI tokens!");
auto *PN = PHINode::Create(Op->getType(), Insts.size(),
Op->getName() + ".sink", &BBEnd->front());
for (auto *I : Insts)
PN->addIncoming(I->getOperand(O), I->getParent());
NewOperands.push_back(PN);
}
// Arbitrarily use I0 as the new "common" instruction; remap its operands
// and move it to the start of the successor block.
for (unsigned O = 0, E = I0->getNumOperands(); O != E; ++O)
I0->getOperandUse(O).set(NewOperands[O]);
I0->moveBefore(&*BBEnd->getFirstInsertionPt());
// Update metadata and IR flags, and merge debug locations.
for (auto *I : Insts)
if (I != I0) {
// The debug location for the "common" instruction is the merged locations
// of all the commoned instructions. We start with the original location
// of the "common" instruction and iteratively merge each location in the
// loop below.
// This is an N-way merge, which will be inefficient if I0 is a CallInst.
// However, as N-way merge for CallInst is rare, so we use simplified API
// instead of using complex API for N-way merge.
I0->applyMergedLocation(I0->getDebugLoc(), I->getDebugLoc());
combineMetadataForCSE(I0, I, true);
I0->andIRFlags(I);
}
if (!isa<StoreInst>(I0)) {
// canSinkLastInstruction checked that all instructions were used by
// one and only one PHI node. Find that now, RAUW it to our common
// instruction and nuke it.
assert(I0->hasOneUse());
auto *PN = cast<PHINode>(*I0->user_begin());
PN->replaceAllUsesWith(I0);
PN->eraseFromParent();
}
// Finally nuke all instructions apart from the common instruction.
for (auto *I : Insts)
if (I != I0)
I->eraseFromParent();
return true;
}
namespace {
// LockstepReverseIterator - Iterates through instructions
// in a set of blocks in reverse order from the first non-terminator.
// For example (assume all blocks have size n):
// LockstepReverseIterator I([B1, B2, B3]);
// *I-- = [B1[n], B2[n], B3[n]];
// *I-- = [B1[n-1], B2[n-1], B3[n-1]];
// *I-- = [B1[n-2], B2[n-2], B3[n-2]];
// ...
class LockstepReverseIterator {
ArrayRef<BasicBlock*> Blocks;
SmallVector<Instruction*,4> Insts;
bool Fail;
public:
LockstepReverseIterator(ArrayRef<BasicBlock*> Blocks) : Blocks(Blocks) {
reset();
}
void reset() {
Fail = false;
Insts.clear();
for (auto *BB : Blocks) {
Instruction *Inst = BB->getTerminator();
for (Inst = Inst->getPrevNode(); Inst && isa<DbgInfoIntrinsic>(Inst);)
Inst = Inst->getPrevNode();
if (!Inst) {
// Block wasn't big enough.
Fail = true;
return;
}
Insts.push_back(Inst);
}
}
bool isValid() const {
return !Fail;
}
void operator--() {
if (Fail)
return;
for (auto *&Inst : Insts) {
for (Inst = Inst->getPrevNode(); Inst && isa<DbgInfoIntrinsic>(Inst);)
Inst = Inst->getPrevNode();
// Already at beginning of block.
if (!Inst) {
Fail = true;
return;
}
}
}
ArrayRef<Instruction*> operator * () const {
return Insts;
}
};
} // end anonymous namespace
/// Check whether BB's predecessors end with unconditional branches. If it is
/// true, sink any common code from the predecessors to BB.
/// We also allow one predecessor to end with conditional branch (but no more
/// than one).
static bool SinkCommonCodeFromPredecessors(BasicBlock *BB) {
// We support two situations:
// (1) all incoming arcs are unconditional
// (2) one incoming arc is conditional
//
// (2) is very common in switch defaults and
// else-if patterns;
//
// if (a) f(1);
// else if (b) f(2);
//
// produces:
//
// [if]
// / \
// [f(1)] [if]
// | | \
// | | |
// | [f(2)]|
// \ | /
// [ end ]
//
// [end] has two unconditional predecessor arcs and one conditional. The
// conditional refers to the implicit empty 'else' arc. This conditional
// arc can also be caused by an empty default block in a switch.
//
// In this case, we attempt to sink code from all *unconditional* arcs.
// If we can sink instructions from these arcs (determined during the scan
// phase below) we insert a common successor for all unconditional arcs and
// connect that to [end], to enable sinking:
//
// [if]
// / \
// [x(1)] [if]
// | | \
// | | \
// | [x(2)] |
// \ / |
// [sink.split] |
// \ /
// [ end ]
//
SmallVector<BasicBlock*,4> UnconditionalPreds;
Instruction *Cond = nullptr;
for (auto *B : predecessors(BB)) {
auto *T = B->getTerminator();
if (isa<BranchInst>(T) && cast<BranchInst>(T)->isUnconditional())
UnconditionalPreds.push_back(B);
else if ((isa<BranchInst>(T) || isa<SwitchInst>(T)) && !Cond)
Cond = T;
else
return false;
}
if (UnconditionalPreds.size() < 2)
return false;
bool Changed = false;
// We take a two-step approach to tail sinking. First we scan from the end of
// each block upwards in lockstep. If the n'th instruction from the end of each
// block can be sunk, those instructions are added to ValuesToSink and we
// carry on. If we can sink an instruction but need to PHI-merge some operands
// (because they're not identical in each instruction) we add these to
// PHIOperands.
unsigned ScanIdx = 0;
SmallPtrSet<Value*,4> InstructionsToSink;
DenseMap<Instruction*, SmallVector<Value*,4>> PHIOperands;
LockstepReverseIterator LRI(UnconditionalPreds);
while (LRI.isValid() &&
canSinkInstructions(*LRI, PHIOperands)) {
LLVM_DEBUG(dbgs() << "SINK: instruction can be sunk: " << *(*LRI)[0]
<< "\n");
InstructionsToSink.insert((*LRI).begin(), (*LRI).end());
++ScanIdx;
--LRI;
}
auto ProfitableToSinkInstruction = [&](LockstepReverseIterator &LRI) {
unsigned NumPHIdValues = 0;
for (auto *I : *LRI)
for (auto *V : PHIOperands[I])
if (InstructionsToSink.count(V) == 0)
++NumPHIdValues;
LLVM_DEBUG(dbgs() << "SINK: #phid values: " << NumPHIdValues << "\n");
unsigned NumPHIInsts = NumPHIdValues / UnconditionalPreds.size();
if ((NumPHIdValues % UnconditionalPreds.size()) != 0)
NumPHIInsts++;
return NumPHIInsts <= 1;
};
if (ScanIdx > 0 && Cond) {
// Check if we would actually sink anything first! This mutates the CFG and
// adds an extra block. The goal in doing this is to allow instructions that
// couldn't be sunk before to be sunk - obviously, speculatable instructions
// (such as trunc, add) can be sunk and predicated already. So we check that
// we're going to sink at least one non-speculatable instruction.
LRI.reset();
unsigned Idx = 0;
bool Profitable = false;
while (ProfitableToSinkInstruction(LRI) && Idx < ScanIdx) {
if (!isSafeToSpeculativelyExecute((*LRI)[0])) {
Profitable = true;
break;
}
--LRI;
++Idx;
}
if (!Profitable)
return false;
LLVM_DEBUG(dbgs() << "SINK: Splitting edge\n");
// We have a conditional edge and we're going to sink some instructions.
// Insert a new block postdominating all blocks we're going to sink from.
if (!SplitBlockPredecessors(BB, UnconditionalPreds, ".sink.split"))
// Edges couldn't be split.
return false;
Changed = true;
}
// Now that we've analyzed all potential sinking candidates, perform the
// actual sink. We iteratively sink the last non-terminator of the source
// blocks into their common successor unless doing so would require too
// many PHI instructions to be generated (currently only one PHI is allowed
// per sunk instruction).
//
// We can use InstructionsToSink to discount values needing PHI-merging that will
// actually be sunk in a later iteration. This allows us to be more
// aggressive in what we sink. This does allow a false positive where we
// sink presuming a later value will also be sunk, but stop half way through
// and never actually sink it which means we produce more PHIs than intended.
// This is unlikely in practice though.
for (unsigned SinkIdx = 0; SinkIdx != ScanIdx; ++SinkIdx) {
LLVM_DEBUG(dbgs() << "SINK: Sink: "
<< *UnconditionalPreds[0]->getTerminator()->getPrevNode()
<< "\n");
// Because we've sunk every instruction in turn, the current instruction to
// sink is always at index 0.
LRI.reset();
if (!ProfitableToSinkInstruction(LRI)) {
// Too many PHIs would be created.
LLVM_DEBUG(
dbgs() << "SINK: stopping here, too many PHIs would be created!\n");
break;
}
if (!sinkLastInstruction(UnconditionalPreds))
return Changed;
NumSinkCommons++;
Changed = true;
}
return Changed;
}
/// Determine if we can hoist sink a sole store instruction out of a
/// conditional block.
///
/// We are looking for code like the following:
/// BrBB:
/// store i32 %add, i32* %arrayidx2
/// ... // No other stores or function calls (we could be calling a memory
/// ... // function).
/// %cmp = icmp ult %x, %y
/// br i1 %cmp, label %EndBB, label %ThenBB
/// ThenBB:
/// store i32 %add5, i32* %arrayidx2
/// br label EndBB
/// EndBB:
/// ...
/// We are going to transform this into:
/// BrBB:
/// store i32 %add, i32* %arrayidx2
/// ... //
/// %cmp = icmp ult %x, %y
/// %add.add5 = select i1 %cmp, i32 %add, %add5
/// store i32 %add.add5, i32* %arrayidx2
/// ...
///
/// \return The pointer to the value of the previous store if the store can be
/// hoisted into the predecessor block. 0 otherwise.
static Value *isSafeToSpeculateStore(Instruction *I, BasicBlock *BrBB,
BasicBlock *StoreBB, BasicBlock *EndBB) {
StoreInst *StoreToHoist = dyn_cast<StoreInst>(I);
if (!StoreToHoist)
return nullptr;
// Volatile or atomic.
if (!StoreToHoist->isSimple())
return nullptr;
Value *StorePtr = StoreToHoist->getPointerOperand();
// Look for a store to the same pointer in BrBB.
unsigned MaxNumInstToLookAt = 9;
for (Instruction &CurI : reverse(BrBB->instructionsWithoutDebug())) {
if (!MaxNumInstToLookAt)
break;
--MaxNumInstToLookAt;
// Could be calling an instruction that affects memory like free().
if (CurI.mayHaveSideEffects() && !isa<StoreInst>(CurI))
return nullptr;
if (auto *SI = dyn_cast<StoreInst>(&CurI)) {
// Found the previous store make sure it stores to the same location.
if (SI->getPointerOperand() == StorePtr)
// Found the previous store, return its value operand.
return SI->getValueOperand();
return nullptr; // Unknown store.
}
}
return nullptr;
}
/// Speculate a conditional basic block flattening the CFG.
///
/// Note that this is a very risky transform currently. Speculating
/// instructions like this is most often not desirable. Instead, there is an MI
/// pass which can do it with full awareness of the resource constraints.
/// However, some cases are "obvious" and we should do directly. An example of
/// this is speculating a single, reasonably cheap instruction.
///
/// There is only one distinct advantage to flattening the CFG at the IR level:
/// it makes very common but simplistic optimizations such as are common in
/// instcombine and the DAG combiner more powerful by removing CFG edges and
/// modeling their effects with easier to reason about SSA value graphs.
///
///
/// An illustration of this transform is turning this IR:
/// \code
/// BB:
/// %cmp = icmp ult %x, %y
/// br i1 %cmp, label %EndBB, label %ThenBB
/// ThenBB:
/// %sub = sub %x, %y
/// br label BB2
/// EndBB:
/// %phi = phi [ %sub, %ThenBB ], [ 0, %EndBB ]
/// ...
/// \endcode
///
/// Into this IR:
/// \code
/// BB:
/// %cmp = icmp ult %x, %y
/// %sub = sub %x, %y
/// %cond = select i1 %cmp, 0, %sub
/// ...
/// \endcode
///
/// \returns true if the conditional block is removed.
static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
const TargetTransformInfo &TTI) {
// Be conservative for now. FP select instruction can often be expensive.
Value *BrCond = BI->getCondition();
if (isa<FCmpInst>(BrCond))
return false;
BasicBlock *BB = BI->getParent();
BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0);
// If ThenBB is actually on the false edge of the conditional branch, remember
// to swap the select operands later.
bool Invert = false;
if (ThenBB != BI->getSuccessor(0)) {
assert(ThenBB == BI->getSuccessor(1) && "No edge from 'if' block?");
Invert = true;
}
assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block");
// Keep a count of how many times instructions are used within ThenBB when
// they are candidates for sinking into ThenBB. Specifically:
// - They are defined in BB, and
// - They have no side effects, and
// - All of their uses are in ThenBB.
SmallDenseMap<Instruction *, unsigned, 4> SinkCandidateUseCounts;
SmallVector<Instruction *, 4> SpeculatedDbgIntrinsics;
unsigned SpeculationCost = 0;
Value *SpeculatedStoreValue = nullptr;
StoreInst *SpeculatedStore = nullptr;
for (BasicBlock::iterator BBI = ThenBB->begin(),
BBE = std::prev(ThenBB->end());
BBI != BBE; ++BBI) {
Instruction *I = &*BBI;
// Skip debug info.
if (isa<DbgInfoIntrinsic>(I)) {
SpeculatedDbgIntrinsics.push_back(I);
continue;
}
// Only speculatively execute a single instruction (not counting the
// terminator) for now.
++SpeculationCost;
if (SpeculationCost > 1)
return false;
// Don't hoist the instruction if it's unsafe or expensive.
if (!isSafeToSpeculativelyExecute(I) &&
!(HoistCondStores && (SpeculatedStoreValue = isSafeToSpeculateStore(
I, BB, ThenBB, EndBB))))
return false;
if (!SpeculatedStoreValue &&
ComputeSpeculationCost(I, TTI) >
PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic)
return false;
// Store the store speculation candidate.
if (SpeculatedStoreValue)
SpeculatedStore = cast<StoreInst>(I);
// Do not hoist the instruction if any of its operands are defined but not
// used in BB. The transformation will prevent the operand from
// being sunk into the use block.
for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i) {
Instruction *OpI = dyn_cast<Instruction>(*i);
if (!OpI || OpI->getParent() != BB || OpI->mayHaveSideEffects())
continue; // Not a candidate for sinking.
++SinkCandidateUseCounts[OpI];
}
}
// Consider any sink candidates which are only used in ThenBB as costs for
// speculation. Note, while we iterate over a DenseMap here, we are summing
// and so iteration order isn't significant.
for (SmallDenseMap<Instruction *, unsigned, 4>::iterator
I = SinkCandidateUseCounts.begin(),
E = SinkCandidateUseCounts.end();
I != E; ++I)
if (I->first->hasNUses(I->second)) {
++SpeculationCost;
if (SpeculationCost > 1)
return false;
}
// Check that the PHI nodes can be converted to selects.
bool HaveRewritablePHIs = false;
for (PHINode &PN : EndBB->phis()) {
Value *OrigV = PN.getIncomingValueForBlock(BB);
Value *ThenV = PN.getIncomingValueForBlock(ThenBB);
// FIXME: Try to remove some of the duplication with HoistThenElseCodeToIf.
// Skip PHIs which are trivial.
if (ThenV == OrigV)
continue;
// Don't convert to selects if we could remove undefined behavior instead.
if (passingValueIsAlwaysUndefined(OrigV, &PN) ||
passingValueIsAlwaysUndefined(ThenV, &PN))
return false;
HaveRewritablePHIs = true;
ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
if (!OrigCE && !ThenCE)
continue; // Known safe and cheap.
if ((ThenCE && !isSafeToSpeculativelyExecute(ThenCE)) ||
(OrigCE && !isSafeToSpeculativelyExecute(OrigCE)))
return false;
unsigned OrigCost = OrigCE ? ComputeSpeculationCost(OrigCE, TTI) : 0;
unsigned ThenCost = ThenCE ? ComputeSpeculationCost(ThenCE, TTI) : 0;
unsigned MaxCost =
2 * PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic;
if (OrigCost + ThenCost > MaxCost)
return false;
// Account for the cost of an unfolded ConstantExpr which could end up
// getting expanded into Instructions.
// FIXME: This doesn't account for how many operations are combined in the
// constant expression.
++SpeculationCost;
if (SpeculationCost > 1)
return false;
}
// If there are no PHIs to process, bail early. This helps ensure idempotence
// as well.
if (!HaveRewritablePHIs && !(HoistCondStores && SpeculatedStoreValue))
return false;
// If we get here, we can hoist the instruction and if-convert.
LLVM_DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *ThenBB << "\n";);
// Insert a select of the value of the speculated store.
if (SpeculatedStoreValue) {
IRBuilder<NoFolder> Builder(BI);
Value *TrueV = SpeculatedStore->getValueOperand();
Value *FalseV = SpeculatedStoreValue;
if (Invert)
std::swap(TrueV, FalseV);
Value *S = Builder.CreateSelect(
BrCond, TrueV, FalseV, "spec.store.select", BI);
SpeculatedStore->setOperand(0, S);
SpeculatedStore->applyMergedLocation(BI->getDebugLoc(),
SpeculatedStore->getDebugLoc());
}
// Metadata can be dependent on the condition we are hoisting above.
// Conservatively strip all metadata on the instruction.
for (auto &I : *ThenBB)
I.dropUnknownNonDebugMetadata();
// Hoist the instructions.
BB->getInstList().splice(BI->getIterator(), ThenBB->getInstList(),
ThenBB->begin(), std::prev(ThenBB->end()));
// Insert selects and rewrite the PHI operands.
IRBuilder<NoFolder> Builder(BI);
for (PHINode &PN : EndBB->phis()) {
unsigned OrigI = PN.getBasicBlockIndex(BB);
unsigned ThenI = PN.getBasicBlockIndex(ThenBB);
Value *OrigV = PN.getIncomingValue(OrigI);
Value *ThenV = PN.getIncomingValue(ThenI);
// Skip PHIs which are trivial.
if (OrigV == ThenV)
continue;
// Create a select whose true value is the speculatively executed value and
// false value is the preexisting value. Swap them if the branch
// destinations were inverted.
Value *TrueV = ThenV, *FalseV = OrigV;
if (Invert)
std::swap(TrueV, FalseV);
Value *V = Builder.CreateSelect(
BrCond, TrueV, FalseV, "spec.select", BI);
PN.setIncomingValue(OrigI, V);
PN.setIncomingValue(ThenI, V);
}
// Remove speculated dbg intrinsics.
// FIXME: Is it possible to do this in a more elegant way? Moving/merging the
// dbg value for the different flows and inserting it after the select.
for (Instruction *I : SpeculatedDbgIntrinsics)
I->eraseFromParent();
++NumSpeculations;
return true;
}
/// Return true if we can thread a branch across this block.
static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
unsigned Size = 0;
for (Instruction &I : BB->instructionsWithoutDebug()) {
if (Size > 10)
return false; // Don't clone large BB's.
++Size;
// We can only support instructions that do not define values that are
// live outside of the current basic block.
for (User *U : I.users()) {
Instruction *UI = cast<Instruction>(U);
if (UI->getParent() != BB || isa<PHINode>(UI))
return false;
}
// Looks ok, continue checking.
}
return true;
}
/// If we have a conditional branch on a PHI node value that is defined in the
/// same block as the branch and if any PHI entries are constants, thread edges
/// corresponding to that entry to be branches to their ultimate destination.
static bool FoldCondBranchOnPHI(BranchInst *BI, const DataLayout &DL,
AssumptionCache *AC) {
BasicBlock *BB = BI->getParent();
PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
// NOTE: we currently cannot transform this case if the PHI node is used
// outside of the block.
if (!PN || PN->getParent() != BB || !PN->hasOneUse())
return false;
// Degenerate case of a single entry PHI.
if (PN->getNumIncomingValues() == 1) {
FoldSingleEntryPHINodes(PN->getParent());
return true;
}
// Now we know that this block has multiple preds and two succs.
if (!BlockIsSimpleEnoughToThreadThrough(BB))
return false;
// Can't fold blocks that contain noduplicate or convergent calls.
if (any_of(*BB, [](const Instruction &I) {
const CallInst *CI = dyn_cast<CallInst>(&I);
return CI && (CI->cannotDuplicate() || CI->isConvergent());
}))
return false;
// Okay, this is a simple enough basic block. See if any phi values are
// constants.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i));
if (!CB || !CB->getType()->isIntegerTy(1))
continue;
// Okay, we now know that all edges from PredBB should be revectored to
// branch to RealDest.
BasicBlock *PredBB = PN->getIncomingBlock(i);
BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
if (RealDest == BB)
continue; // Skip self loops.
// Skip if the predecessor's terminator is an indirect branch.
if (isa<IndirectBrInst>(PredBB->getTerminator()))
continue;
// The dest block might have PHI nodes, other predecessors and other
// difficult cases. Instead of being smart about this, just insert a new
// block that jumps to the destination block, effectively splitting
// the edge we are about to create.
BasicBlock *EdgeBB =
BasicBlock::Create(BB->getContext(), RealDest->getName() + ".critedge",
RealDest->getParent(), RealDest);
BranchInst::Create(RealDest, EdgeBB);
// Update PHI nodes.
AddPredecessorToBlock(RealDest, EdgeBB, BB);
// BB may have instructions that are being threaded over. Clone these
// instructions into EdgeBB. We know that there will be no uses of the
// cloned instructions outside of EdgeBB.
BasicBlock::iterator InsertPt = EdgeBB->begin();
DenseMap<Value *, Value *> TranslateMap; // Track translated values.
for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
continue;
}
// Clone the instruction.
Instruction *N = BBI->clone();
if (BBI->hasName())
N->setName(BBI->getName() + ".c");
// Update operands due to translation.
for (User::op_iterator i = N->op_begin(), e = N->op_end(); i != e; ++i) {
DenseMap<Value *, Value *>::iterator PI = TranslateMap.find(*i);
if (PI != TranslateMap.end())
*i = PI->second;
}
// Check for trivial simplification.
if (Value *V = SimplifyInstruction(N, {DL, nullptr, nullptr, AC})) {
if (!BBI->use_empty())
TranslateMap[&*BBI] = V;
if (!N->mayHaveSideEffects()) {
N->deleteValue(); // Instruction folded away, don't need actual inst
N = nullptr;
}
} else {
if (!BBI->use_empty())
TranslateMap[&*BBI] = N;
}
// Insert the new instruction into its new home.
if (N)
EdgeBB->getInstList().insert(InsertPt, N);
// Register the new instruction with the assumption cache if necessary.
if (auto *II = dyn_cast_or_null<IntrinsicInst>(N))
if (II->getIntrinsicID() == Intrinsic::assume)
AC->registerAssumption(II);
}
// Loop over all of the edges from PredBB to BB, changing them to branch
// to EdgeBB instead.
Instruction *PredBBTI = PredBB->getTerminator();
for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
if (PredBBTI->getSuccessor(i) == BB) {
BB->removePredecessor(PredBB);
PredBBTI->setSuccessor(i, EdgeBB);
}
// Recurse, simplifying any other constants.
return FoldCondBranchOnPHI(BI, DL, AC) || true;
}
return false;
}
/// Given a BB that starts with the specified two-entry PHI node,
/// see if we can eliminate it.
static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
const DataLayout &DL) {
// Ok, this is a two entry PHI node. Check to see if this is a simple "if
// statement", which has a very simple dominance structure. Basically, we
// are trying to find the condition that is being branched on, which
// subsequently causes this merge to happen. We really want control
// dependence information for this check, but simplifycfg can't keep it up
// to date, and this catches most of the cases we care about anyway.
BasicBlock *BB = PN->getParent();
const Function *Fn = BB->getParent();
if (Fn && Fn->hasFnAttribute(Attribute::OptForFuzzing))
return false;
BasicBlock *IfTrue, *IfFalse;
Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
if (!IfCond ||
// Don't bother if the branch will be constant folded trivially.
isa<ConstantInt>(IfCond))
return false;
// Okay, we found that we can merge this two-entry phi node into a select.
// Doing so would require us to fold *all* two entry phi nodes in this block.
// At some point this becomes non-profitable (particularly if the target
// doesn't support cmov's). Only do this transformation if there are two or
// fewer PHI nodes in this block.
unsigned NumPhis = 0;
for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
if (NumPhis > 2)
return false;
// Loop over the PHI's seeing if we can promote them all to select
// instructions. While we are at it, keep track of the instructions
// that need to be moved to the dominating block.
SmallPtrSet<Instruction *, 4> AggressiveInsts;
unsigned MaxCostVal0 = PHINodeFoldingThreshold,
MaxCostVal1 = PHINodeFoldingThreshold;
MaxCostVal0 *= TargetTransformInfo::TCC_Basic;
MaxCostVal1 *= TargetTransformInfo::TCC_Basic;
for (BasicBlock::iterator II = BB->begin(); isa<PHINode>(II);) {
PHINode *PN = cast<PHINode>(II++);
if (Value *V = SimplifyInstruction(PN, {DL, PN})) {
PN->replaceAllUsesWith(V);
PN->eraseFromParent();
continue;
}
if (!DominatesMergePoint(PN->getIncomingValue(0), BB, AggressiveInsts,
MaxCostVal0, TTI) ||
!DominatesMergePoint(PN->getIncomingValue(1), BB, AggressiveInsts,
MaxCostVal1, TTI))
return false;
}
// If we folded the first phi, PN dangles at this point. Refresh it. If
// we ran out of PHIs then we simplified them all.
PN = dyn_cast<PHINode>(BB->begin());
if (!PN)
return true;
// Don't fold i1 branches on PHIs which contain binary operators. These can
// often be turned into switches and other things.
if (PN->getType()->isIntegerTy(1) &&
(isa<BinaryOperator>(PN->getIncomingValue(0)) ||
isa<BinaryOperator>(PN->getIncomingValue(1)) ||
isa<BinaryOperator>(IfCond)))
return false;
// If all PHI nodes are promotable, check to make sure that all instructions
// in the predecessor blocks can be promoted as well. If not, we won't be able
// to get rid of the control flow, so it's not worth promoting to select
// instructions.
BasicBlock *DomBlock = nullptr;
BasicBlock *IfBlock1 = PN->getIncomingBlock(0);
BasicBlock *IfBlock2 = PN->getIncomingBlock(1);
if (cast<BranchInst>(IfBlock1->getTerminator())->isConditional()) {
IfBlock1 = nullptr;
} else {
DomBlock = *pred_begin(IfBlock1);
for (BasicBlock::iterator I = IfBlock1->begin(); !I->isTerminator(); ++I)
if (!AggressiveInsts.count(&*I) && !isa<DbgInfoIntrinsic>(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control flow, so
// the xform is not worth it.
return false;
}
}
if (cast<BranchInst>(IfBlock2->getTerminator())->isConditional()) {
IfBlock2 = nullptr;
} else {
DomBlock = *pred_begin(IfBlock2);
for (BasicBlock::iterator I = IfBlock2->begin(); !I->isTerminator(); ++I)
if (!AggressiveInsts.count(&*I) && !isa<DbgInfoIntrinsic>(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control flow, so
// the xform is not worth it.
return false;
}
}
LLVM_DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond
<< " T: " << IfTrue->getName()
<< " F: " << IfFalse->getName() << "\n");
// If we can still promote the PHI nodes after this gauntlet of tests,
// do all of the PHI's now.
Instruction *InsertPt = DomBlock->getTerminator();
IRBuilder<NoFolder> Builder(InsertPt);
// Move all 'aggressive' instructions, which are defined in the
// conditional parts of the if's up to the dominating block.
if (IfBlock1)
hoistAllInstructionsInto(DomBlock, InsertPt, IfBlock1);
if (IfBlock2)
hoistAllInstructionsInto(DomBlock, InsertPt, IfBlock2);
while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
// Change the PHI node into a select instruction.
Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
Value *Sel = Builder.CreateSelect(IfCond, TrueVal, FalseVal, "", InsertPt);
PN->replaceAllUsesWith(Sel);
Sel->takeName(PN);
PN->eraseFromParent();
}
// At this point, IfBlock1 and IfBlock2 are both empty, so our if statement
// has been flattened. Change DomBlock to jump directly to our new block to
// avoid other simplifycfg's kicking in on the diamond.
Instruction *OldTI = DomBlock->getTerminator();
Builder.SetInsertPoint(OldTI);
Builder.CreateBr(BB);
OldTI->eraseFromParent();
return true;
}
/// If we found a conditional branch that goes to two returning blocks,
/// try to merge them together into one return,
/// introducing a select if the return values disagree.
static bool SimplifyCondBranchToTwoReturns(BranchInst *BI,
IRBuilder<> &Builder) {
assert(BI->isConditional() && "Must be a conditional branch");
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
ReturnInst *TrueRet = cast<ReturnInst>(TrueSucc->getTerminator());
ReturnInst *FalseRet = cast<ReturnInst>(FalseSucc->getTerminator());
// Check to ensure both blocks are empty (just a return) or optionally empty
// with PHI nodes. If there are other instructions, merging would cause extra
// computation on one path or the other.
if (!TrueSucc->getFirstNonPHIOrDbg()->isTerminator())
return false;
if (!FalseSucc->getFirstNonPHIOrDbg()->isTerminator())
return false;
Builder.SetInsertPoint(BI);
// Okay, we found a branch that is going to two return nodes. If
// there is no return value for this function, just change the
// branch into a return.
if (FalseRet->getNumOperands() == 0) {
TrueSucc->removePredecessor(BI->getParent());
FalseSucc->removePredecessor(BI->getParent());
Builder.CreateRetVoid();
EraseTerminatorAndDCECond(BI);
return true;
}
// Otherwise, figure out what the true and false return values are
// so we can insert a new select instruction.
Value *TrueValue = TrueRet->getReturnValue();
Value *FalseValue = FalseRet->getReturnValue();
// Unwrap any PHI nodes in the return blocks.
if (PHINode *TVPN = dyn_cast_or_null<PHINode>(TrueValue))
if (TVPN->getParent() == TrueSucc)
TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
if (PHINode *FVPN = dyn_cast_or_null<PHINode>(FalseValue))
if (FVPN->getParent() == FalseSucc)
FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
// In order for this transformation to be safe, we must be able to
// unconditionally execute both operands to the return. This is
// normally the case, but we could have a potentially-trapping
// constant expression that prevents this transformation from being
// safe.
if (ConstantExpr *TCV = dyn_cast_or_null<ConstantExpr>(TrueValue))
if (TCV->canTrap())
return false;
if (ConstantExpr *FCV = dyn_cast_or_null<ConstantExpr>(FalseValue))
if (FCV->canTrap())
return false;
// Okay, we collected all the mapped values and checked them for sanity, and
// defined to really do this transformation. First, update the CFG.
TrueSucc->removePredecessor(BI->getParent());
FalseSucc->removePredecessor(BI->getParent());
// Insert select instructions where needed.
Value *BrCond = BI->getCondition();
if (TrueValue) {
// Insert a select if the results differ.
if (TrueValue == FalseValue || isa<UndefValue>(FalseValue)) {
} else if (isa<UndefValue>(TrueValue)) {
TrueValue = FalseValue;
} else {
TrueValue =
Builder.CreateSelect(BrCond, TrueValue, FalseValue, "retval", BI);
}
}
Value *RI =
!TrueValue ? Builder.CreateRetVoid() : Builder.CreateRet(TrueValue);
(void)RI;
LLVM_DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
<< "\n " << *BI << "NewRet = " << *RI << "TRUEBLOCK: "
<< *TrueSucc << "FALSEBLOCK: " << *FalseSucc);
EraseTerminatorAndDCECond(BI);
return true;
}
/// Return true if the given instruction is available
/// in its predecessor block. If yes, the instruction will be removed.
static bool tryCSEWithPredecessor(Instruction *Inst, BasicBlock *PB) {
if (!isa<BinaryOperator>(Inst) && !isa<CmpInst>(Inst))
return false;
for (Instruction &I : *PB) {
Instruction *PBI = &I;
// Check whether Inst and PBI generate the same value.
if (Inst->isIdenticalTo(PBI)) {
Inst->replaceAllUsesWith(PBI);
Inst->eraseFromParent();
return true;
}
}
return false;
}
/// Return true if either PBI or BI has branch weight available, and store
/// the weights in {Pred|Succ}{True|False}Weight. If one of PBI and BI does
/// not have branch weight, use 1:1 as its weight.
static bool extractPredSuccWeights(BranchInst *PBI, BranchInst *BI,
uint64_t &PredTrueWeight,
uint64_t &PredFalseWeight,
uint64_t &SuccTrueWeight,
uint64_t &SuccFalseWeight) {
bool PredHasWeights =
PBI->extractProfMetadata(PredTrueWeight, PredFalseWeight);
bool SuccHasWeights =
BI->extractProfMetadata(SuccTrueWeight, SuccFalseWeight);
if (PredHasWeights || SuccHasWeights) {
if (!PredHasWeights)
PredTrueWeight = PredFalseWeight = 1;
if (!SuccHasWeights)
SuccTrueWeight = SuccFalseWeight = 1;
return true;
} else {
return false;
}
}
/// If this basic block is simple enough, and if a predecessor branches to us
/// and one of our successors, fold the block into the predecessor and use
/// logical operations to pick the right destination.
bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
BasicBlock *BB = BI->getParent();
const unsigned PredCount = pred_size(BB);
Instruction *Cond = nullptr;
if (BI->isConditional())
Cond = dyn_cast<Instruction>(BI->getCondition());
else {
// For unconditional branch, check for a simple CFG pattern, where
// BB has a single predecessor and BB's successor is also its predecessor's
// successor. If such pattern exists, check for CSE between BB and its
// predecessor.
if (BasicBlock *PB = BB->getSinglePredecessor())
if (BranchInst *PBI = dyn_cast<BranchInst>(PB->getTerminator()))
if (PBI->isConditional() &&
(BI->getSuccessor(0) == PBI->getSuccessor(0) ||
BI->getSuccessor(0) == PBI->getSuccessor(1))) {
for (auto I = BB->instructionsWithoutDebug().begin(),
E = BB->instructionsWithoutDebug().end();
I != E;) {
Instruction *Curr = &*I++;
if (isa<CmpInst>(Curr)) {
Cond = Curr;
break;
}
// Quit if we can't remove this instruction.
if (!tryCSEWithPredecessor(Curr, PB))
return false;
}
}
if (!Cond)
return false;
}
if (!Cond || (!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
Cond->getParent() != BB || !Cond->hasOneUse())
return false;
// Make sure the instruction after the condition is the cond branch.
BasicBlock::iterator CondIt = ++Cond->getIterator();
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(CondIt))
++CondIt;
if (&*CondIt != BI)
return false;
// Only allow this transformation if computing the condition doesn't involve
// too many instructions and these involved instructions can be executed
// unconditionally. We denote all involved instructions except the condition
// as "bonus instructions", and only allow this transformation when the
// number of the bonus instructions we'll need to create when cloning into
// each predecessor does not exceed a certain threshold.
unsigned NumBonusInsts = 0;
for (auto I = BB->begin(); Cond != &*I; ++I) {
// Ignore dbg intrinsics.
if (isa<DbgInfoIntrinsic>(I))
continue;
if (!I->hasOneUse() || !isSafeToSpeculativelyExecute(&*I))
return false;
// I has only one use and can be executed unconditionally.
Instruction *User = dyn_cast<Instruction>(I->user_back());
if (User == nullptr || User->getParent() != BB)
return false;
// I is used in the same BB. Since BI uses Cond and doesn't have more slots
// to use any other instruction, User must be an instruction between next(I)
// and Cond.
// Account for the cost of duplicating this instruction into each
// predecessor.
NumBonusInsts += PredCount;
// Early exits once we reach the limit.
if (NumBonusInsts > BonusInstThreshold)
return false;
}
// Cond is known to be a compare or binary operator. Check to make sure that
// neither operand is a potentially-trapping constant expression.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
if (CE->canTrap())
return false;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1)))
if (CE->canTrap())
return false;
// Finally, don't infinitely unroll conditional loops.
BasicBlock *TrueDest = BI->getSuccessor(0);
BasicBlock *FalseDest = (BI->isConditional()) ? BI->getSuccessor(1) : nullptr;
if (TrueDest == BB || FalseDest == BB)
return false;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *PredBlock = *PI;
BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator());
// Check that we have two conditional branches. If there is a PHI node in
// the common successor, verify that the same value flows in from both
// blocks.
SmallVector<PHINode *, 4> PHIs;
if (!PBI || PBI->isUnconditional() ||
(BI->isConditional() && !SafeToMergeTerminators(BI, PBI)) ||
(!BI->isConditional() &&
!isProfitableToFoldUnconditional(BI, PBI, Cond, PHIs)))
continue;
// Determine if the two branches share a common destination.
Instruction::BinaryOps Opc = Instruction::BinaryOpsEnd;
bool InvertPredCond = false;
if (BI->isConditional()) {
if (PBI->getSuccessor(0) == TrueDest) {
Opc = Instruction::Or;
} else if (PBI->getSuccessor(1) == FalseDest) {
Opc = Instruction::And;
} else if (PBI->getSuccessor(0) == FalseDest) {
Opc = Instruction::And;
InvertPredCond = true;
} else if (PBI->getSuccessor(1) == TrueDest) {
Opc = Instruction::Or;
InvertPredCond = true;
} else {
continue;
}
} else {
if (PBI->getSuccessor(0) != TrueDest && PBI->getSuccessor(1) != TrueDest)
continue;
}
LLVM_DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
IRBuilder<> Builder(PBI);
// If we need to invert the condition in the pred block to match, do so now.
if (InvertPredCond) {
Value *NewCond = PBI->getCondition();
if (NewCond->hasOneUse() && isa<CmpInst>(NewCond)) {
CmpInst *CI = cast<CmpInst>(NewCond);
CI->setPredicate(CI->getInversePredicate());
} else {
NewCond =
Builder.CreateNot(NewCond, PBI->getCondition()->getName() + ".not");
}
PBI->setCondition(NewCond);
PBI->swapSuccessors();
}
// If we have bonus instructions, clone them into the predecessor block.
// Note that there may be multiple predecessor blocks, so we cannot move
// bonus instructions to a predecessor block.
ValueToValueMapTy VMap; // maps original values to cloned values
// We already make sure Cond is the last instruction before BI. Therefore,
// all instructions before Cond other than DbgInfoIntrinsic are bonus
// instructions.
for (auto BonusInst = BB->begin(); Cond != &*BonusInst; ++BonusInst) {
if (isa<DbgInfoIntrinsic>(BonusInst))
continue;
Instruction *NewBonusInst = BonusInst->clone();
RemapInstruction(NewBonusInst, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
VMap[&*BonusInst] = NewBonusInst;
// If we moved a load, we cannot any longer claim any knowledge about
// its potential value. The previous information might have been valid
// only given the branch precondition.
// For an analogous reason, we must also drop all the metadata whose
// semantics we don't understand.
NewBonusInst->dropUnknownNonDebugMetadata();
PredBlock->getInstList().insert(PBI->getIterator(), NewBonusInst);
NewBonusInst->takeName(&*BonusInst);
BonusInst->setName(BonusInst->getName() + ".old");
}
// Clone Cond into the predecessor basic block, and or/and the
// two conditions together.
Instruction *CondInPred = Cond->clone();
RemapInstruction(CondInPred, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
PredBlock->getInstList().insert(PBI->getIterator(), CondInPred);
CondInPred->takeName(Cond);
Cond->setName(CondInPred->getName() + ".old");
if (BI->isConditional()) {
Instruction *NewCond = cast<Instruction>(
Builder.CreateBinOp(Opc, PBI->getCondition(), CondInPred, "or.cond"));
PBI->setCondition(NewCond);
uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
bool HasWeights =
extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
SuccTrueWeight, SuccFalseWeight);
SmallVector<uint64_t, 8> NewWeights;
if (PBI->getSuccessor(0) == BB) {
if (HasWeights) {
// PBI: br i1 %x, BB, FalseDest
// BI: br i1 %y, TrueDest, FalseDest
// TrueWeight is TrueWeight for PBI * TrueWeight for BI.
NewWeights.push_back(PredTrueWeight * SuccTrueWeight);
// FalseWeight is FalseWeight for PBI * TotalWeight for BI +
// TrueWeight for PBI * FalseWeight for BI.
// We assume that total weights of a BranchInst can fit into 32 bits.
// Therefore, we will not have overflow using 64-bit arithmetic.
NewWeights.push_back(PredFalseWeight *
(SuccFalseWeight + SuccTrueWeight) +
PredTrueWeight * SuccFalseWeight);
}
AddPredecessorToBlock(TrueDest, PredBlock, BB);
PBI->setSuccessor(0, TrueDest);
}
if (PBI->getSuccessor(1) == BB) {
if (HasWeights) {
// PBI: br i1 %x, TrueDest, BB
// BI: br i1 %y, TrueDest, FalseDest
// TrueWeight is TrueWeight for PBI * TotalWeight for BI +
// FalseWeight for PBI * TrueWeight for BI.
NewWeights.push_back(PredTrueWeight *
(SuccFalseWeight + SuccTrueWeight) +
PredFalseWeight * SuccTrueWeight);
// FalseWeight is FalseWeight for PBI * FalseWeight for BI.
NewWeights.push_back(PredFalseWeight * SuccFalseWeight);
}
AddPredecessorToBlock(FalseDest, PredBlock, BB);
PBI->setSuccessor(1, FalseDest);
}
if (NewWeights.size() == 2) {
// Halve the weights if any of them cannot fit in an uint32_t
FitWeights(NewWeights);
SmallVector<uint32_t, 8> MDWeights(NewWeights.begin(),
NewWeights.end());
setBranchWeights(PBI, MDWeights[0], MDWeights[1]);
} else
PBI->setMetadata(LLVMContext::MD_prof, nullptr);
} else {
// Update PHI nodes in the common successors.
for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
ConstantInt *PBI_C = cast<ConstantInt>(
PHIs[i]->getIncomingValueForBlock(PBI->getParent()));
assert(PBI_C->getType()->isIntegerTy(1));
Instruction *MergedCond = nullptr;
if (PBI->getSuccessor(0) == TrueDest) {
// Create (PBI_Cond and PBI_C) or (!PBI_Cond and BI_Value)
// PBI_C is true: PBI_Cond or (!PBI_Cond and BI_Value)
// is false: !PBI_Cond and BI_Value
Instruction *NotCond = cast<Instruction>(
Builder.CreateNot(PBI->getCondition(), "not.cond"));
MergedCond = cast<Instruction>(
Builder.CreateBinOp(Instruction::And, NotCond, CondInPred,
"and.cond"));
if (PBI_C->isOne())
MergedCond = cast<Instruction>(Builder.CreateBinOp(
Instruction::Or, PBI->getCondition(), MergedCond, "or.cond"));
} else {
// Create (PBI_Cond and BI_Value) or (!PBI_Cond and PBI_C)
// PBI_C is true: (PBI_Cond and BI_Value) or (!PBI_Cond)
// is false: PBI_Cond and BI_Value
MergedCond = cast<Instruction>(Builder.CreateBinOp(
Instruction::And, PBI->getCondition(), CondInPred, "and.cond"));
if (PBI_C->isOne()) {
Instruction *NotCond = cast<Instruction>(
Builder.CreateNot(PBI->getCondition(), "not.cond"));
MergedCond = cast<Instruction>(Builder.CreateBinOp(
Instruction::Or, NotCond, MergedCond, "or.cond"));
}
}
// Update PHI Node.
PHIs[i]->setIncomingValue(PHIs[i]->getBasicBlockIndex(PBI->getParent()),
MergedCond);
}
// Change PBI from Conditional to Unconditional.
BranchInst *New_PBI = BranchInst::Create(TrueDest, PBI);
EraseTerminatorAndDCECond(PBI);
PBI = New_PBI;
}
// If BI was a loop latch, it may have had associated loop metadata.
// We need to copy it to the new latch, that is, PBI.
if (MDNode *LoopMD = BI->getMetadata(LLVMContext::MD_loop))
PBI->setMetadata(LLVMContext::MD_loop, LoopMD);
// TODO: If BB is reachable from all paths through PredBlock, then we
// could replace PBI's branch probabilities with BI's.
// Copy any debug value intrinsics into the end of PredBlock.
for (Instruction &I : *BB)
if (isa<DbgInfoIntrinsic>(I))
I.clone()->insertBefore(PBI);
return true;
}
return false;
}
// If there is only one store in BB1 and BB2, return it, otherwise return
// nullptr.
static StoreInst *findUniqueStoreInBlocks(BasicBlock *BB1, BasicBlock *BB2) {
StoreInst *S = nullptr;
for (auto *BB : {BB1, BB2}) {
if (!BB)
continue;
for (auto &I : *BB)
if (auto *SI = dyn_cast<StoreInst>(&I)) {
if (S)
// Multiple stores seen.
return nullptr;
else
S = SI;
}
}
return S;
}
static Value *ensureValueAvailableInSuccessor(Value *V, BasicBlock *BB,
Value *AlternativeV = nullptr) {
// PHI is going to be a PHI node that allows the value V that is defined in
// BB to be referenced in BB's only successor.
//
// If AlternativeV is nullptr, the only value we care about in PHI is V. It
// doesn't matter to us what the other operand is (it'll never get used). We
// could just create a new PHI with an undef incoming value, but that could
// increase register pressure if EarlyCSE/InstCombine can't fold it with some
// other PHI. So here we directly look for some PHI in BB's successor with V
// as an incoming operand. If we find one, we use it, else we create a new
// one.
//
// If AlternativeV is not nullptr, we care about both incoming values in PHI.
// PHI must be exactly: phi <ty> [ %BB, %V ], [ %OtherBB, %AlternativeV]
// where OtherBB is the single other predecessor of BB's only successor.
PHINode *PHI = nullptr;
BasicBlock *Succ = BB->getSingleSuccessor();
for (auto I = Succ->begin(); isa<PHINode>(I); ++I)
if (cast<PHINode>(I)->getIncomingValueForBlock(BB) == V) {
PHI = cast<PHINode>(I);
if (!AlternativeV)
break;
assert(Succ->hasNPredecessors(2));
auto PredI = pred_begin(Succ);
BasicBlock *OtherPredBB = *PredI == BB ? *++PredI : *PredI;
if (PHI->getIncomingValueForBlock(OtherPredBB) == AlternativeV)
break;
PHI = nullptr;
}
if (PHI)
return PHI;
// If V is not an instruction defined in BB, just return it.
if (!AlternativeV &&
(!isa<Instruction>(V) || cast<Instruction>(V)->getParent() != BB))
return V;
PHI = PHINode::Create(V->getType(), 2, "simplifycfg.merge", &Succ->front());
PHI->addIncoming(V, BB);
for (BasicBlock *PredBB : predecessors(Succ))
if (PredBB != BB)
PHI->addIncoming(
AlternativeV ? AlternativeV : UndefValue::get(V->getType()), PredBB);
return PHI;
}
static bool mergeConditionalStoreToAddress(BasicBlock *PTB, BasicBlock *PFB,
BasicBlock *QTB, BasicBlock *QFB,
BasicBlock *PostBB, Value *Address,
bool InvertPCond, bool InvertQCond,
const DataLayout &DL) {
auto IsaBitcastOfPointerType = [](const Instruction &I) {
return Operator::getOpcode(&I) == Instruction::BitCast &&
I.getType()->isPointerTy();
};
// If we're not in aggressive mode, we only optimize if we have some
// confidence that by optimizing we'll allow P and/or Q to be if-converted.
auto IsWorthwhile = [&](BasicBlock *BB) {
if (!BB)
return true;
// Heuristic: if the block can be if-converted/phi-folded and the
// instructions inside are all cheap (arithmetic/GEPs), it's worthwhile to
// thread this store.
unsigned N = 0;
for (auto &I : BB->instructionsWithoutDebug()) {
// Cheap instructions viable for folding.
if (isa<BinaryOperator>(I) || isa<GetElementPtrInst>(I) ||
isa<StoreInst>(I))
++N;
// Free instructions.
else if (I.isTerminator() || IsaBitcastOfPointerType(I))
continue;
else
return false;
}
// The store we want to merge is counted in N, so add 1 to make sure
// we're counting the instructions that would be left.
return N <= (PHINodeFoldingThreshold + 1);
};
if (!MergeCondStoresAggressively &&
(!IsWorthwhile(PTB) || !IsWorthwhile(PFB) || !IsWorthwhile(QTB) ||
!IsWorthwhile(QFB)))
return false;
// For every pointer, there must be exactly two stores, one coming from
// PTB or PFB, and the other from QTB or QFB. We don't support more than one
// store (to any address) in PTB,PFB or QTB,QFB.
// FIXME: We could relax this restriction with a bit more work and performance
// testing.
StoreInst *PStore = findUniqueStoreInBlocks(PTB, PFB);
StoreInst *QStore = findUniqueStoreInBlocks(QTB, QFB);
if (!PStore || !QStore)
return false;
// Now check the stores are compatible.
if (!QStore->isUnordered() || !PStore->isUnordered())
return false;
// Check that sinking the store won't cause program behavior changes. Sinking
// the store out of the Q blocks won't change any behavior as we're sinking
// from a block to its unconditional successor. But we're moving a store from
// the P blocks down through the middle block (QBI) and past both QFB and QTB.
// So we need to check that there are no aliasing loads or stores in
// QBI, QTB and QFB. We also need to check there are no conflicting memory
// operations between PStore and the end of its parent block.
//
// The ideal way to do this is to query AliasAnalysis, but we don't
// preserve AA currently so that is dangerous. Be super safe and just
// check there are no other memory operations at all.
for (auto &I : *QFB->getSinglePredecessor())
if (I.mayReadOrWriteMemory())
return false;
for (auto &I : *QFB)
if (&I != QStore && I.mayReadOrWriteMemory())
return false;
if (QTB)
for (auto &I : *QTB)
if (&I != QStore && I.mayReadOrWriteMemory())
return false;
for (auto I = BasicBlock::iterator(PStore), E = PStore->getParent()->end();
I != E; ++I)
if (&*I != PStore && I->mayReadOrWriteMemory())
return false;
// If PostBB has more than two predecessors, we need to split it so we can
// sink the store.
if (std::next(pred_begin(PostBB), 2) != pred_end(PostBB)) {
// We know that QFB's only successor is PostBB. And QFB has a single
// predecessor. If QTB exists, then its only successor is also PostBB.
// If QTB does not exist, then QFB's only predecessor has a conditional
// branch to QFB and PostBB.
BasicBlock *TruePred = QTB ? QTB : QFB->getSinglePredecessor();
BasicBlock *NewBB = SplitBlockPredecessors(PostBB, { QFB, TruePred},
"condstore.split");
if (!NewBB)
return false;
PostBB = NewBB;
}
// OK, we're going to sink the stores to PostBB. The store has to be
// conditional though, so first create the predicate.
Value *PCond = cast<BranchInst>(PFB->getSinglePredecessor()->getTerminator())
->getCondition();
Value *QCond = cast<BranchInst>(QFB->getSinglePredecessor()->getTerminator())
->getCondition();
Value *PPHI = ensureValueAvailableInSuccessor(PStore->getValueOperand(),
PStore->getParent());
Value *QPHI = ensureValueAvailableInSuccessor(QStore->getValueOperand(),
QStore->getParent(), PPHI);
IRBuilder<> QB(&*PostBB->getFirstInsertionPt());
Value *PPred = PStore->getParent() == PTB ? PCond : QB.CreateNot(PCond);
Value *QPred = QStore->getParent() == QTB ? QCond : QB.CreateNot(QCond);
if (InvertPCond)
PPred = QB.CreateNot(PPred);
if (InvertQCond)
QPred = QB.CreateNot(QPred);
Value *CombinedPred = QB.CreateOr(PPred, QPred);
auto *T =
SplitBlockAndInsertIfThen(CombinedPred, &*QB.GetInsertPoint(), false);
QB.SetInsertPoint(T);
StoreInst *SI = cast<StoreInst>(QB.CreateStore(QPHI, Address));
AAMDNodes AAMD;
PStore->getAAMetadata(AAMD, /*Merge=*/false);
PStore->getAAMetadata(AAMD, /*Merge=*/true);
SI->setAAMetadata(AAMD);
unsigned PAlignment = PStore->getAlignment();
unsigned QAlignment = QStore->getAlignment();
unsigned TypeAlignment =
DL.getABITypeAlignment(SI->getValueOperand()->getType());
unsigned MinAlignment;
unsigned MaxAlignment;
std::tie(MinAlignment, MaxAlignment) = std::minmax(PAlignment, QAlignment);
// Choose the minimum alignment. If we could prove both stores execute, we
// could use biggest one. In this case, though, we only know that one of the
// stores executes. And we don't know it's safe to take the alignment from a
// store that doesn't execute.
if (MinAlignment != 0) {
// Choose the minimum of all non-zero alignments.
SI->setAlignment(MinAlignment);
} else if (MaxAlignment != 0) {
// Choose the minimal alignment between the non-zero alignment and the ABI
// default alignment for the type of the stored value.
SI->setAlignment(std::min(MaxAlignment, TypeAlignment));
} else {
// If both alignments are zero, use ABI default alignment for the type of
// the stored value.
SI->setAlignment(TypeAlignment);
}
QStore->eraseFromParent();
PStore->eraseFromParent();
return true;
}
static bool mergeConditionalStores(BranchInst *PBI, BranchInst *QBI,
const DataLayout &DL) {
// The intention here is to find diamonds or triangles (see below) where each
// conditional block contains a store to the same address. Both of these
// stores are conditional, so they can't be unconditionally sunk. But it may
// be profitable to speculatively sink the stores into one merged store at the
// end, and predicate the merged store on the union of the two conditions of
// PBI and QBI.
//
// This can reduce the number of stores executed if both of the conditions are
// true, and can allow the blocks to become small enough to be if-converted.
// This optimization will also chain, so that ladders of test-and-set
// sequences can be if-converted away.
//
// We only deal with simple diamonds or triangles:
//
// PBI or PBI or a combination of the two
// / \ | \
// PTB PFB | PFB
// \ / | /
// QBI QBI
// / \ | \
// QTB QFB | QFB
// \ / | /
// PostBB PostBB
//
// We model triangles as a type of diamond with a nullptr "true" block.
// Triangles are canonicalized so that the fallthrough edge is represented by
// a true condition, as in the diagram above.
BasicBlock *PTB = PBI->getSuccessor(0);
BasicBlock *PFB = PBI->getSuccessor(1);
BasicBlock *QTB = QBI->getSuccessor(0);
BasicBlock *QFB = QBI->getSuccessor(1);
BasicBlock *PostBB = QFB->getSingleSuccessor();
// Make sure we have a good guess for PostBB. If QTB's only successor is
// QFB, then QFB is a better PostBB.
if (QTB->getSingleSuccessor() == QFB)
PostBB = QFB;
// If we couldn't find a good PostBB, stop.
if (!PostBB)
return false;
bool InvertPCond = false, InvertQCond = false;
// Canonicalize fallthroughs to the true branches.
if (PFB == QBI->getParent()) {
std::swap(PFB, PTB);
InvertPCond = true;
}
if (QFB == PostBB) {
std::swap(QFB, QTB);
InvertQCond = true;
}
// From this point on we can assume PTB or QTB may be fallthroughs but PFB
// and QFB may not. Model fallthroughs as a nullptr block.
if (PTB == QBI->getParent())
PTB = nullptr;
if (QTB == PostBB)
QTB = nullptr;
// Legality bailouts. We must have at least the non-fallthrough blocks and
// the post-dominating block, and the non-fallthroughs must only have one
// predecessor.
auto HasOnePredAndOneSucc = [](BasicBlock *BB, BasicBlock *P, BasicBlock *S) {
return BB->getSinglePredecessor() == P && BB->getSingleSuccessor() == S;
};
if (!HasOnePredAndOneSucc(PFB, PBI->getParent(), QBI->getParent()) ||
!HasOnePredAndOneSucc(QFB, QBI->getParent(), PostBB))
return false;
if ((PTB && !HasOnePredAndOneSucc(PTB, PBI->getParent(), QBI->getParent())) ||
(QTB && !HasOnePredAndOneSucc(QTB, QBI->getParent(), PostBB)))
return false;
if (!QBI->getParent()->hasNUses(2))
return false;
// OK, this is a sequence of two diamonds or triangles.
// Check if there are stores in PTB or PFB that are repeated in QTB or QFB.
SmallPtrSet<Value *, 4> PStoreAddresses, QStoreAddresses;
for (auto *BB : {PTB, PFB}) {
if (!BB)
continue;
for (auto &I : *BB)
if (StoreInst *SI = dyn_cast<StoreInst>(&I))
PStoreAddresses.insert(SI->getPointerOperand());
}
for (auto *BB : {QTB, QFB}) {
if (!BB)
continue;
for (auto &I : *BB)
if (StoreInst *SI = dyn_cast<StoreInst>(&I))
QStoreAddresses.insert(SI->getPointerOperand());
}
set_intersect(PStoreAddresses, QStoreAddresses);
// set_intersect mutates PStoreAddresses in place. Rename it here to make it
// clear what it contains.
auto &CommonAddresses = PStoreAddresses;
bool Changed = false;
for (auto *Address : CommonAddresses)
Changed |= mergeConditionalStoreToAddress(
PTB, PFB, QTB, QFB, PostBB, Address, InvertPCond, InvertQCond, DL);
return Changed;
}
/// If we have a conditional branch as a predecessor of another block,
/// this function tries to simplify it. We know
/// that PBI and BI are both conditional branches, and BI is in one of the
/// successor blocks of PBI - PBI branches to BI.
static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
const DataLayout &DL) {
assert(PBI->isConditional() && BI->isConditional());
BasicBlock *BB = BI->getParent();
// If this block ends with a branch instruction, and if there is a
// predecessor that ends on a branch of the same condition, make
// this conditional branch redundant.
if (PBI->getCondition() == BI->getCondition() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
// Okay, the outcome of this conditional branch is statically
// knowable. If this block had a single pred, handle specially.
if (BB->getSinglePredecessor()) {
// Turn this into a branch on constant.
bool CondIsTrue = PBI->getSuccessor(0) == BB;
BI->setCondition(
ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue));
return true; // Nuke the branch on constant.
}
// Otherwise, if there are multiple predecessors, insert a PHI that merges
// in the constant and simplify the block result. Subsequent passes of
// simplifycfg will thread the block.
if (BlockIsSimpleEnoughToThreadThrough(BB)) {
pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
PHINode *NewPN = PHINode::Create(
Type::getInt1Ty(BB->getContext()), std::distance(PB, PE),
BI->getCondition()->getName() + ".pr", &BB->front());
// Okay, we're going to insert the PHI node. Since PBI is not the only
// predecessor, compute the PHI'd conditional value for all of the preds.
// Any predecessor where the condition is not computable we keep symbolic.
for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
if ((PBI = dyn_cast<BranchInst>(P->getTerminator())) && PBI != BI &&
PBI->isConditional() && PBI->getCondition() == BI->getCondition() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
bool CondIsTrue = PBI->getSuccessor(0) == BB;
NewPN->addIncoming(
ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue),
P);
} else {
NewPN->addIncoming(BI->getCondition(), P);
}
}
BI->setCondition(NewPN);
return true;
}
}
if (auto *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
if (CE->canTrap())
return false;
// If both branches are conditional and both contain stores to the same
// address, remove the stores from the conditionals and create a conditional
// merged store at the end.
if (MergeCondStores && mergeConditionalStores(PBI, BI, DL))
return true;
// If this is a conditional branch in an empty block, and if any
// predecessors are a conditional branch to one of our destinations,
// fold the conditions into logical ops and one cond br.
// Ignore dbg intrinsics.
if (&*BB->instructionsWithoutDebug().begin() != BI)
return false;
int PBIOp, BIOp;
if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
PBIOp = 0;
BIOp = 0;
} else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
PBIOp = 0;
BIOp = 1;
} else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
PBIOp = 1;
BIOp = 0;
} else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
PBIOp = 1;
BIOp = 1;
} else {
return false;
}
// Check to make sure that the other destination of this branch
// isn't BB itself. If so, this is an infinite loop that will
// keep getting unwound.
if (PBI->getSuccessor(PBIOp) == BB)
return false;
// Do not perform this transformation if it would require
// insertion of a large number of select instructions. For targets
// without predication/cmovs, this is a big pessimization.
// Also do not perform this transformation if any phi node in the common
// destination block can trap when reached by BB or PBB (PR17073). In that
// case, it would be unsafe to hoist the operation into a select instruction.
BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
unsigned NumPhis = 0;
for (BasicBlock::iterator II = CommonDest->begin(); isa<PHINode>(II);
++II, ++NumPhis) {
if (NumPhis > 2) // Disable this xform.
return false;
PHINode *PN = cast<PHINode>(II);
Value *BIV = PN->getIncomingValueForBlock(BB);
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BIV))
if (CE->canTrap())
return false;
unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
Value *PBIV = PN->getIncomingValue(PBBIdx);
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(PBIV))
if (CE->canTrap())
return false;
}
// Finally, if everything is ok, fold the branches to logical ops.
BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
LLVM_DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
<< "AND: " << *BI->getParent());
// If OtherDest *is* BB, then BB is a basic block with a single conditional
// branch in it, where one edge (OtherDest) goes back to itself but the other
// exits. We don't *know* that the program avoids the infinite loop
// (even though that seems likely). If we do this xform naively, we'll end up
// recursively unpeeling the loop. Since we know that (after the xform is
// done) that the block *is* infinite if reached, we just make it an obviously
// infinite loop with no cond branch.
if (OtherDest == BB) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
BasicBlock *InfLoopBlock =
BasicBlock::Create(BB->getContext(), "infloop", BB->getParent());
BranchInst::Create(InfLoopBlock, InfLoopBlock);
OtherDest = InfLoopBlock;
}
LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
// BI may have other predecessors. Because of this, we leave
// it alone, but modify PBI.
// Make sure we get to CommonDest on True&True directions.
Value *PBICond = PBI->getCondition();
IRBuilder<NoFolder> Builder(PBI);
if (PBIOp)
PBICond = Builder.CreateNot(PBICond, PBICond->getName() + ".not");
Value *BICond = BI->getCondition();
if (BIOp)
BICond = Builder.CreateNot(BICond, BICond->getName() + ".not");
// Merge the conditions.
Value *Cond = Builder.CreateOr(PBICond, BICond, "brmerge");
// Modify PBI to branch on the new condition to the new dests.
PBI->setCondition(Cond);
PBI->setSuccessor(0, CommonDest);
PBI->setSuccessor(1, OtherDest);
// Update branch weight for PBI.
uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
uint64_t PredCommon, PredOther, SuccCommon, SuccOther;
bool HasWeights =
extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
SuccTrueWeight, SuccFalseWeight);
if (HasWeights) {
PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
// The weight to CommonDest should be PredCommon * SuccTotal +
// PredOther * SuccCommon.
// The weight to OtherDest should be PredOther * SuccOther.
uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther) +
PredOther * SuccCommon,
PredOther * SuccOther};
// Halve the weights if any of them cannot fit in an uint32_t
FitWeights(NewWeights);
setBranchWeights(PBI, NewWeights[0], NewWeights[1]);
}
// OtherDest may have phi nodes. If so, add an entry from PBI's
// block that are identical to the entries for BI's block.
AddPredecessorToBlock(OtherDest, PBI->getParent(), BB);
// We know that the CommonDest already had an edge from PBI to
// it. If it has PHIs though, the PHIs may have different
// entries for BB and PBI's BB. If so, insert a select to make
// them agree.
for (PHINode &PN : CommonDest->phis()) {
Value *BIV = PN.getIncomingValueForBlock(BB);
unsigned PBBIdx = PN.getBasicBlockIndex(PBI->getParent());
Value *PBIV = PN.getIncomingValue(PBBIdx);
if (BIV != PBIV) {
// Insert a select in PBI to pick the right value.
SelectInst *NV = cast<SelectInst>(
Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName() + ".mux"));
PN.setIncomingValue(PBBIdx, NV);
// Although the select has the same condition as PBI, the original branch
// weights for PBI do not apply to the new select because the select's
// 'logical' edges are incoming edges of the phi that is eliminated, not
// the outgoing edges of PBI.
if (HasWeights) {
uint64_t PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
uint64_t PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
uint64_t SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
uint64_t SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
// The weight to PredCommonDest should be PredCommon * SuccTotal.
// The weight to PredOtherDest should be PredOther * SuccCommon.
uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther),
PredOther * SuccCommon};
FitWeights(NewWeights);
setBranchWeights(NV, NewWeights[0], NewWeights[1]);
}
}
}
LLVM_DEBUG(dbgs() << "INTO: " << *PBI->getParent());
LLVM_DEBUG(dbgs() << *PBI->getParent()->getParent());
// This basic block is probably dead. We know it has at least
// one fewer predecessor.
return true;
}
// Simplifies a terminator by replacing it with a branch to TrueBB if Cond is
// true or to FalseBB if Cond is false.
// Takes care of updating the successors and removing the old terminator.
// Also makes sure not to introduce new successors by assuming that edges to
// non-successor TrueBBs and FalseBBs aren't reachable.
static bool SimplifyTerminatorOnSelect(Instruction *OldTerm, Value *Cond,
BasicBlock *TrueBB, BasicBlock *FalseBB,
uint32_t TrueWeight,
uint32_t FalseWeight) {
// Remove any superfluous successor edges from the CFG.
// First, figure out which successors to preserve.
// If TrueBB and FalseBB are equal, only try to preserve one copy of that
// successor.
BasicBlock *KeepEdge1 = TrueBB;
BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : nullptr;
// Then remove the rest.
for (BasicBlock *Succ : successors(OldTerm)) {
// Make sure only to keep exactly one copy of each edge.
if (Succ == KeepEdge1)
KeepEdge1 = nullptr;
else if (Succ == KeepEdge2)
KeepEdge2 = nullptr;
else
Succ->removePredecessor(OldTerm->getParent(),
/*DontDeleteUselessPHIs=*/true);
}
IRBuilder<> Builder(OldTerm);
Builder.SetCurrentDebugLocation(OldTerm->getDebugLoc());
// Insert an appropriate new terminator.
if (!KeepEdge1 && !KeepEdge2) {
if (TrueBB == FalseBB)
// We were only looking for one successor, and it was present.
// Create an unconditional branch to it.
Builder.CreateBr(TrueBB);
else {
// We found both of the successors we were looking for.
// Create a conditional branch sharing the condition of the select.
BranchInst *NewBI = Builder.CreateCondBr(Cond, TrueBB, FalseBB);
if (TrueWeight != FalseWeight)
setBranchWeights(NewBI, TrueWeight, FalseWeight);
}
} else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
// Neither of the selected blocks were successors, so this
// terminator must be unreachable.
new UnreachableInst(OldTerm->getContext(), OldTerm);
} else {
// One of the selected values was a successor, but the other wasn't.
// Insert an unconditional branch to the one that was found;
// the edge to the one that wasn't must be unreachable.
if (!KeepEdge1)
// Only TrueBB was found.
Builder.CreateBr(TrueBB);
else
// Only FalseBB was found.
Builder.CreateBr(FalseBB);
}
EraseTerminatorAndDCECond(OldTerm);
return true;
}
// Replaces
// (switch (select cond, X, Y)) on constant X, Y
// with a branch - conditional if X and Y lead to distinct BBs,
// unconditional otherwise.
static bool SimplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select) {
// Check for constant integer values in the select.
ConstantInt *TrueVal = dyn_cast<ConstantInt>(Select->getTrueValue());
ConstantInt *FalseVal = dyn_cast<ConstantInt>(Select->getFalseValue());
if (!TrueVal || !FalseVal)
return false;
// Find the relevant condition and destinations.
Value *Condition = Select->getCondition();
BasicBlock *TrueBB = SI->findCaseValue(TrueVal)->getCaseSuccessor();
BasicBlock *FalseBB = SI->findCaseValue(FalseVal)->getCaseSuccessor();
// Get weight for TrueBB and FalseBB.
uint32_t TrueWeight = 0, FalseWeight = 0;
SmallVector<uint64_t, 8> Weights;
bool HasWeights = HasBranchWeights(SI);
if (HasWeights) {
GetBranchWeights(SI, Weights);
if (Weights.size() == 1 + SI->getNumCases()) {
TrueWeight =
(uint32_t)Weights[SI->findCaseValue(TrueVal)->getSuccessorIndex()];
FalseWeight =
(uint32_t)Weights[SI->findCaseValue(FalseVal)->getSuccessorIndex()];
}
}
// Perform the actual simplification.
return SimplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB, TrueWeight,
FalseWeight);
}
// Replaces
// (indirectbr (select cond, blockaddress(@fn, BlockA),
// blockaddress(@fn, BlockB)))
// with
// (br cond, BlockA, BlockB).
static bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI) {
// Check that both operands of the select are block addresses.
BlockAddress *TBA = dyn_cast<BlockAddress>(SI->getTrueValue());
BlockAddress *FBA = dyn_cast<BlockAddress>(SI->getFalseValue());
if (!TBA || !FBA)
return false;
// Extract the actual blocks.
BasicBlock *TrueBB = TBA->getBasicBlock();
BasicBlock *FalseBB = FBA->getBasicBlock();
// Perform the actual simplification.
return SimplifyTerminatorOnSelect(IBI, SI->getCondition(), TrueBB, FalseBB, 0,
0);
}
/// This is called when we find an icmp instruction
/// (a seteq/setne with a constant) as the only instruction in a
/// block that ends with an uncond branch. We are looking for a very specific
/// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In
/// this case, we merge the first two "or's of icmp" into a switch, but then the
/// default value goes to an uncond block with a seteq in it, we get something
/// like:
///
/// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
/// DEFAULT:
/// %tmp = icmp eq i8 %A, 92
/// br label %end
/// end:
/// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
///
/// We prefer to split the edge to 'end' so that there is a true/false entry to
/// the PHI, merging the third icmp into the switch.
bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
ICmpInst *ICI, IRBuilder<> &Builder) {
BasicBlock *BB = ICI->getParent();
// If the block has any PHIs in it or the icmp has multiple uses, it is too
// complex.
if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse())
return false;
Value *V = ICI->getOperand(0);
ConstantInt *Cst = cast<ConstantInt>(ICI->getOperand(1));
// The pattern we're looking for is where our only predecessor is a switch on
// 'V' and this block is the default case for the switch. In this case we can
// fold the compared value into the switch to simplify things.
BasicBlock *Pred = BB->getSinglePredecessor();
if (!Pred || !isa<SwitchInst>(Pred->getTerminator()))
return false;
SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
if (SI->getCondition() != V)
return false;
// If BB is reachable on a non-default case, then we simply know the value of
// V in this block. Substitute it and constant fold the icmp instruction
// away.
if (SI->getDefaultDest() != BB) {
ConstantInt *VVal = SI->findCaseDest(BB);
assert(VVal && "Should have a unique destination value");
ICI->setOperand(0, VVal);
if (Value *V = SimplifyInstruction(ICI, {DL, ICI})) {
ICI->replaceAllUsesWith(V);
ICI->eraseFromParent();
}
// BB is now empty, so it is likely to simplify away.
return requestResimplify();
}
// Ok, the block is reachable from the default dest. If the constant we're
// comparing exists in one of the other edges, then we can constant fold ICI
// and zap it.
if (SI->findCaseValue(Cst) != SI->case_default()) {
Value *V;
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
V = ConstantInt::getFalse(BB->getContext());
else
V = ConstantInt::getTrue(BB->getContext());
ICI->replaceAllUsesWith(V);
ICI->eraseFromParent();
// BB is now empty, so it is likely to simplify away.
return requestResimplify();
}
// The use of the icmp has to be in the 'end' block, by the only PHI node in
// the block.
BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
PHINode *PHIUse = dyn_cast<PHINode>(ICI->user_back());
if (PHIUse == nullptr || PHIUse != &SuccBlock->front() ||
isa<PHINode>(++BasicBlock::iterator(PHIUse)))
return false;
// If the icmp is a SETEQ, then the default dest gets false, the new edge gets
// true in the PHI.
Constant *DefaultCst = ConstantInt::getTrue(BB->getContext());
Constant *NewCst = ConstantInt::getFalse(BB->getContext());
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
std::swap(DefaultCst, NewCst);
// Replace ICI (which is used by the PHI for the default value) with true or
// false depending on if it is EQ or NE.
ICI->replaceAllUsesWith(DefaultCst);
ICI->eraseFromParent();
// Okay, the switch goes to this block on a default value. Add an edge from
// the switch to the merge point on the compared value.
BasicBlock *NewBB =
BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB);
SmallVector<uint64_t, 8> Weights;
bool HasWeights = HasBranchWeights(SI);
if (HasWeights) {
GetBranchWeights(SI, Weights);
if (Weights.size() == 1 + SI->getNumCases()) {
// Split weight for default case to case for "Cst".
Weights[0] = (Weights[0] + 1) >> 1;
Weights.push_back(Weights[0]);
SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
setBranchWeights(SI, MDWeights);
}
}
SI->addCase(Cst, NewBB);
// NewBB branches to the phi block, add the uncond branch and the phi entry.
Builder.SetInsertPoint(NewBB);
Builder.SetCurrentDebugLocation(SI->getDebugLoc());
Builder.CreateBr(SuccBlock);
PHIUse->addIncoming(NewCst, NewBB);
return true;
}
/// The specified branch is a conditional branch.
/// Check to see if it is branching on an or/and chain of icmp instructions, and
/// fold it into a switch instruction if so.
static bool SimplifyBranchOnICmpChain(BranchInst *BI, IRBuilder<> &Builder,
const DataLayout &DL) {
Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
if (!Cond)
return false;
// Change br (X == 0 | X == 1), T, F into a switch instruction.
// If this is a bunch of seteq's or'd together, or if it's a bunch of
// 'setne's and'ed together, collect them.
// Try to gather values from a chain of and/or to be turned into a switch
ConstantComparesGatherer ConstantCompare(Cond, DL);
// Unpack the result
SmallVectorImpl<ConstantInt *> &Values = ConstantCompare.Vals;
Value *CompVal = ConstantCompare.CompValue;
unsigned UsedICmps = ConstantCompare.UsedICmps;
Value *ExtraCase = ConstantCompare.Extra;
// If we didn't have a multiply compared value, fail.
if (!CompVal)
return false;
// Avoid turning single icmps into a switch.
if (UsedICmps <= 1)
return false;
bool TrueWhenEqual = (Cond->getOpcode() == Instruction::Or);
// There might be duplicate constants in the list, which the switch
// instruction can't handle, remove them now.
array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate);
Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
// If Extra was used, we require at least two switch values to do the
// transformation. A switch with one value is just a conditional branch.
if (ExtraCase && Values.size() < 2)
return false;
// TODO: Preserve branch weight metadata, similarly to how
// FoldValueComparisonIntoPredecessors preserves it.
// Figure out which block is which destination.
BasicBlock *DefaultBB = BI->getSuccessor(1);
BasicBlock *EdgeBB = BI->getSuccessor(0);
if (!TrueWhenEqual)
std::swap(DefaultBB, EdgeBB);
BasicBlock *BB = BI->getParent();
LLVM_DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
<< " cases into SWITCH. BB is:\n"
<< *BB);
// If there are any extra values that couldn't be folded into the switch
// then we evaluate them with an explicit branch first. Split the block
// right before the condbr to handle it.
if (ExtraCase) {
BasicBlock *NewBB =
BB->splitBasicBlock(BI->getIterator(), "switch.early.test");
// Remove the uncond branch added to the old block.
Instruction *OldTI = BB->getTerminator();
Builder.SetInsertPoint(OldTI);
if (TrueWhenEqual)
Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB);
else
Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB);
OldTI->eraseFromParent();
// If there are PHI nodes in EdgeBB, then we need to add a new entry to them
// for the edge we just added.
AddPredecessorToBlock(EdgeBB, BB, NewBB);
LLVM_DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase
<< "\nEXTRABB = " << *BB);
BB = NewBB;
}
Builder.SetInsertPoint(BI);
// Convert pointer to int before we switch.
if (CompVal->getType()->isPointerTy()) {
CompVal = Builder.CreatePtrToInt(
CompVal, DL.getIntPtrType(CompVal->getType()), "magicptr");
}
// Create the new switch instruction now.
SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
// Add all of the 'cases' to the switch instruction.
for (unsigned i = 0, e = Values.size(); i != e; ++i)
New->addCase(Values[i], EdgeBB);
// We added edges from PI to the EdgeBB. As such, if there were any
// PHI nodes in EdgeBB, they need entries to be added corresponding to
// the number of edges added.
for (BasicBlock::iterator BBI = EdgeBB->begin(); isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
Value *InVal = PN->getIncomingValueForBlock(BB);
for (unsigned i = 0, e = Values.size() - 1; i != e; ++i)
PN->addIncoming(InVal, BB);
}
// Erase the old branch instruction.
EraseTerminatorAndDCECond(BI);
LLVM_DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n');
return true;
}
bool SimplifyCFGOpt::SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder) {
if (isa<PHINode>(RI->getValue()))
return SimplifyCommonResume(RI);
else if (isa<LandingPadInst>(RI->getParent()->getFirstNonPHI()) &&
RI->getValue() == RI->getParent()->getFirstNonPHI())
// The resume must unwind the exception that caused control to branch here.
return SimplifySingleResume(RI);
return false;
}
// Simplify resume that is shared by several landing pads (phi of landing pad).
bool SimplifyCFGOpt::SimplifyCommonResume(ResumeInst *RI) {
BasicBlock *BB = RI->getParent();
// Check that there are no other instructions except for debug intrinsics
// between the phi of landing pads (RI->getValue()) and resume instruction.
BasicBlock::iterator I = cast<Instruction>(RI->getValue())->getIterator(),
E = RI->getIterator();
while (++I != E)
if (!isa<DbgInfoIntrinsic>(I))
return false;
SmallSetVector<BasicBlock *, 4> TrivialUnwindBlocks;
auto *PhiLPInst = cast<PHINode>(RI->getValue());
// Check incoming blocks to see if any of them are trivial.
for (unsigned Idx = 0, End = PhiLPInst->getNumIncomingValues(); Idx != End;
Idx++) {
auto *IncomingBB = PhiLPInst->getIncomingBlock(Idx);
auto *IncomingValue = PhiLPInst->getIncomingValue(Idx);
// If the block has other successors, we can not delete it because
// it has other dependents.
if (IncomingBB->getUniqueSuccessor() != BB)
continue;
auto *LandingPad = dyn_cast<LandingPadInst>(IncomingBB->getFirstNonPHI());
// Not the landing pad that caused the control to branch here.
if (IncomingValue != LandingPad)
continue;
bool isTrivial = true;
I = IncomingBB->getFirstNonPHI()->getIterator();
E = IncomingBB->getTerminator()->getIterator();
while (++I != E)
if (!isa<DbgInfoIntrinsic>(I)) {
isTrivial = false;
break;
}
if (isTrivial)
TrivialUnwindBlocks.insert(IncomingBB);
}
// If no trivial unwind blocks, don't do any simplifications.
if (TrivialUnwindBlocks.empty())
return false;
// Turn all invokes that unwind here into calls.
for (auto *TrivialBB : TrivialUnwindBlocks) {
// Blocks that will be simplified should be removed from the phi node.
// Note there could be multiple edges to the resume block, and we need
// to remove them all.
while (PhiLPInst->getBasicBlockIndex(TrivialBB) != -1)
BB->removePredecessor(TrivialBB, true);
for (pred_iterator PI = pred_begin(TrivialBB), PE = pred_end(TrivialBB);
PI != PE;) {
BasicBlock *Pred = *PI++;
removeUnwindEdge(Pred);
}
// In each SimplifyCFG run, only the current processed block can be erased.
// Otherwise, it will break the iteration of SimplifyCFG pass. So instead
// of erasing TrivialBB, we only remove the branch to the common resume
// block so that we can later erase the resume block since it has no
// predecessors.
TrivialBB->getTerminator()->eraseFromParent();
new UnreachableInst(RI->getContext(), TrivialBB);
}
// Delete the resume block if all its predecessors have been removed.
if (pred_empty(BB))
BB->eraseFromParent();
return !TrivialUnwindBlocks.empty();
}
// Simplify resume that is only used by a single (non-phi) landing pad.
bool SimplifyCFGOpt::SimplifySingleResume(ResumeInst *RI) {
BasicBlock *BB = RI->getParent();
LandingPadInst *LPInst = dyn_cast<LandingPadInst>(BB->getFirstNonPHI());
assert(RI->getValue() == LPInst &&
"Resume must unwind the exception that caused control to here");
// Check that there are no other instructions except for debug intrinsics.
BasicBlock::iterator I = LPInst->getIterator(), E = RI->getIterator();
while (++I != E)
if (!isa<DbgInfoIntrinsic>(I))
return false;
// Turn all invokes that unwind here into calls and delete the basic block.
for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE;) {
BasicBlock *Pred = *PI++;
removeUnwindEdge(Pred);
}
// The landingpad is now unreachable. Zap it.
if (LoopHeaders)
LoopHeaders->erase(BB);
BB->eraseFromParent();
return true;
}
static bool removeEmptyCleanup(CleanupReturnInst *RI) {
// If this is a trivial cleanup pad that executes no instructions, it can be
// eliminated. If the cleanup pad continues to the caller, any predecessor
// that is an EH pad will be updated to continue to the caller and any
// predecessor that terminates with an invoke instruction will have its invoke
// instruction converted to a call instruction. If the cleanup pad being
// simplified does not continue to the caller, each predecessor will be
// updated to continue to the unwind destination of the cleanup pad being
// simplified.
BasicBlock *BB = RI->getParent();
CleanupPadInst *CPInst = RI->getCleanupPad();
if (CPInst->getParent() != BB)
// This isn't an empty cleanup.
return false;
// We cannot kill the pad if it has multiple uses. This typically arises
// from unreachable basic blocks.
if (!CPInst->hasOneUse())
return false;
// Check that there are no other instructions except for benign intrinsics.
BasicBlock::iterator I = CPInst->getIterator(), E = RI->getIterator();
while (++I != E) {
auto *II = dyn_cast<IntrinsicInst>(I);
if (!II)
return false;
Intrinsic::ID IntrinsicID = II->getIntrinsicID();
switch (IntrinsicID) {
case Intrinsic::dbg_declare:
case Intrinsic::dbg_value:
case Intrinsic::dbg_label:
case Intrinsic::lifetime_end:
break;
default:
return false;
}
}
// If the cleanup return we are simplifying unwinds to the caller, this will
// set UnwindDest to nullptr.
BasicBlock *UnwindDest = RI->getUnwindDest();
Instruction *DestEHPad = UnwindDest ? UnwindDest->getFirstNonPHI() : nullptr;
// We're about to remove BB from the control flow. Before we do, sink any
// PHINodes into the unwind destination. Doing this before changing the
// control flow avoids some potentially slow checks, since we can currently
// be certain that UnwindDest and BB have no common predecessors (since they
// are both EH pads).
if (UnwindDest) {
// First, go through the PHI nodes in UnwindDest and update any nodes that
// reference the block we are removing
for (BasicBlock::iterator I = UnwindDest->begin(),
IE = DestEHPad->getIterator();
I != IE; ++I) {
PHINode *DestPN = cast<PHINode>(I);
int Idx = DestPN->getBasicBlockIndex(BB);
// Since BB unwinds to UnwindDest, it has to be in the PHI node.
assert(Idx != -1);
// This PHI node has an incoming value that corresponds to a control
// path through the cleanup pad we are removing. If the incoming
// value is in the cleanup pad, it must be a PHINode (because we
// verified above that the block is otherwise empty). Otherwise, the
// value is either a constant or a value that dominates the cleanup
// pad being removed.
//
// Because BB and UnwindDest are both EH pads, all of their
// predecessors must unwind to these blocks, and since no instruction
// can have multiple unwind destinations, there will be no overlap in
// incoming blocks between SrcPN and DestPN.
Value *SrcVal = DestPN->getIncomingValue(Idx);
PHINode *SrcPN = dyn_cast<PHINode>(SrcVal);
// Remove the entry for the block we are deleting.
DestPN->removeIncomingValue(Idx, false);
if (SrcPN && SrcPN->getParent() == BB) {
// If the incoming value was a PHI node in the cleanup pad we are
// removing, we need to merge that PHI node's incoming values into
// DestPN.
for (unsigned SrcIdx = 0, SrcE = SrcPN->getNumIncomingValues();
SrcIdx != SrcE; ++SrcIdx) {
DestPN->addIncoming(SrcPN->getIncomingValue(SrcIdx),
SrcPN->getIncomingBlock(SrcIdx));
}
} else {
// Otherwise, the incoming value came from above BB and
// so we can just reuse it. We must associate all of BB's
// predecessors with this value.
for (auto *pred : predecessors(BB)) {
DestPN->addIncoming(SrcVal, pred);
}
}
}
// Sink any remaining PHI nodes directly into UnwindDest.
Instruction *InsertPt = DestEHPad;
for (BasicBlock::iterator I = BB->begin(),
IE = BB->getFirstNonPHI()->getIterator();
I != IE;) {
// The iterator must be incremented here because the instructions are
// being moved to another block.
PHINode *PN = cast<PHINode>(I++);
if (PN->use_empty())
// If the PHI node has no uses, just leave it. It will be erased
// when we erase BB below.
continue;
// Otherwise, sink this PHI node into UnwindDest.
// Any predecessors to UnwindDest which are not already represented
// must be back edges which inherit the value from the path through
// BB. In this case, the PHI value must reference itself.
for (auto *pred : predecessors(UnwindDest))
if (pred != BB)
PN->addIncoming(PN, pred);
PN->moveBefore(InsertPt);
}
}
for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE;) {
// The iterator must be updated here because we are removing this pred.
BasicBlock *PredBB = *PI++;
if (UnwindDest == nullptr) {
removeUnwindEdge(PredBB);
} else {
Instruction *TI = PredBB->getTerminator();
TI->replaceUsesOfWith(BB, UnwindDest);
}
}
// The cleanup pad is now unreachable. Zap it.
BB->eraseFromParent();
return true;
}
// Try to merge two cleanuppads together.
static bool mergeCleanupPad(CleanupReturnInst *RI) {
// Skip any cleanuprets which unwind to caller, there is nothing to merge
// with.
BasicBlock *UnwindDest = RI->getUnwindDest();
if (!UnwindDest)
return false;
// This cleanupret isn't the only predecessor of this cleanuppad, it wouldn't
// be safe to merge without code duplication.
if (UnwindDest->getSinglePredecessor() != RI->getParent())
return false;
// Verify that our cleanuppad's unwind destination is another cleanuppad.
auto *SuccessorCleanupPad = dyn_cast<CleanupPadInst>(&UnwindDest->front());
if (!SuccessorCleanupPad)
return false;
CleanupPadInst *PredecessorCleanupPad = RI->getCleanupPad();
// Replace any uses of the successor cleanupad with the predecessor pad
// The only cleanuppad uses should be this cleanupret, it's cleanupret and
// funclet bundle operands.
SuccessorCleanupPad->replaceAllUsesWith(PredecessorCleanupPad);
// Remove the old cleanuppad.
SuccessorCleanupPad->eraseFromParent();
// Now, we simply replace the cleanupret with a branch to the unwind
// destination.
BranchInst::Create(UnwindDest, RI->getParent());
RI->eraseFromParent();
return true;
}
bool SimplifyCFGOpt::SimplifyCleanupReturn(CleanupReturnInst *RI) {
// It is possible to transiantly have an undef cleanuppad operand because we
// have deleted some, but not all, dead blocks.
// Eventually, this block will be deleted.
if (isa<UndefValue>(RI->getOperand(0)))
return false;
if (mergeCleanupPad(RI))
return true;
if (removeEmptyCleanup(RI))
return true;
return false;
}
bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) {
BasicBlock *BB = RI->getParent();
if (!BB->getFirstNonPHIOrDbg()->isTerminator())
return false;
// Find predecessors that end with branches.
SmallVector<BasicBlock *, 8> UncondBranchPreds;
SmallVector<BranchInst *, 8> CondBranchPreds;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *P = *PI;
Instruction *PTI = P->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) {
if (BI->isUnconditional())
UncondBranchPreds.push_back(P);
else
CondBranchPreds.push_back(BI);
}
}
// If we found some, do the transformation!
if (!UncondBranchPreds.empty() && DupRet) {
while (!UncondBranchPreds.empty()) {
BasicBlock *Pred = UncondBranchPreds.pop_back_val();
LLVM_DEBUG(dbgs() << "FOLDING: " << *BB
<< "INTO UNCOND BRANCH PRED: " << *Pred);
(void)FoldReturnIntoUncondBranch(RI, BB, Pred);
}
// If we eliminated all predecessors of the block, delete the block now.
if (pred_empty(BB)) {
// We know there are no successors, so just nuke the block.
if (LoopHeaders)
LoopHeaders->erase(BB);
BB->eraseFromParent();
}
return true;
}
// Check out all of the conditional branches going to this return
// instruction. If any of them just select between returns, change the
// branch itself into a select/return pair.
while (!CondBranchPreds.empty()) {
BranchInst *BI = CondBranchPreds.pop_back_val();
// Check to see if the non-BB successor is also a return block.
if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) &&
isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) &&
SimplifyCondBranchToTwoReturns(BI, Builder))
return true;
}
return false;
}
bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
BasicBlock *BB = UI->getParent();
bool Changed = false;
// If there are any instructions immediately before the unreachable that can
// be removed, do so.
while (UI->getIterator() != BB->begin()) {
BasicBlock::iterator BBI = UI->getIterator();
--BBI;
// Do not delete instructions that can have side effects which might cause
// the unreachable to not be reachable; specifically, calls and volatile
// operations may have this effect.
if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI))
break;
if (BBI->mayHaveSideEffects()) {
if (auto *SI = dyn_cast<StoreInst>(BBI)) {
if (SI->isVolatile())
break;
} else if (auto *LI = dyn_cast<LoadInst>(BBI)) {
if (LI->isVolatile())
break;
} else if (auto *RMWI = dyn_cast<AtomicRMWInst>(BBI)) {
if (RMWI->isVolatile())
break;
} else if (auto *CXI = dyn_cast<AtomicCmpXchgInst>(BBI)) {
if (CXI->isVolatile())
break;
} else if (isa<CatchPadInst>(BBI)) {
// A catchpad may invoke exception object constructors and such, which
// in some languages can be arbitrary code, so be conservative by
// default.
// For CoreCLR, it just involves a type test, so can be removed.
if (classifyEHPersonality(BB->getParent()->getPersonalityFn()) !=
EHPersonality::CoreCLR)
break;
} else if (!isa<FenceInst>(BBI) && !isa<VAArgInst>(BBI) &&
!isa<LandingPadInst>(BBI)) {
break;
}
// Note that deleting LandingPad's here is in fact okay, although it
// involves a bit of subtle reasoning. If this inst is a LandingPad,
// all the predecessors of this block will be the unwind edges of Invokes,
// and we can therefore guarantee this block will be erased.
}
// Delete this instruction (any uses are guaranteed to be dead)
if (!BBI->use_empty())
BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
BBI->eraseFromParent();
Changed = true;
}
// If the unreachable instruction is the first in the block, take a gander
// at all of the predecessors of this instruction, and simplify them.
if (&BB->front() != UI)
return Changed;
SmallVector<BasicBlock *, 8> Preds(pred_begin(BB), pred_end(BB));
for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
Instruction *TI = Preds[i]->getTerminator();
IRBuilder<> Builder(TI);
if (auto *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isUnconditional()) {
if (BI->getSuccessor(0) == BB) {
new UnreachableInst(TI->getContext(), TI);
TI->eraseFromParent();
Changed = true;
}
} else {
if (BI->getSuccessor(0) == BB) {
Builder.CreateBr(BI->getSuccessor(1));
EraseTerminatorAndDCECond(BI);
} else if (BI->getSuccessor(1) == BB) {
Builder.CreateBr(BI->getSuccessor(0));
EraseTerminatorAndDCECond(BI);
Changed = true;
}
}
} else if (auto *SI = dyn_cast<SwitchInst>(TI)) {
for (auto i = SI->case_begin(), e = SI->case_end(); i != e;) {
if (i->getCaseSuccessor() != BB) {
++i;
continue;
}
BB->removePredecessor(SI->getParent());
i = SI->removeCase(i);
e = SI->case_end();
Changed = true;
}
} else if (auto *II = dyn_cast<InvokeInst>(TI)) {
if (II->getUnwindDest() == BB) {
removeUnwindEdge(TI->getParent());
Changed = true;
}
} else if (auto *CSI = dyn_cast<CatchSwitchInst>(TI)) {
if (CSI->getUnwindDest() == BB) {
removeUnwindEdge(TI->getParent());
Changed = true;
continue;
}
for (CatchSwitchInst::handler_iterator I = CSI->handler_begin(),
E = CSI->handler_end();
I != E; ++I) {
if (*I == BB) {
CSI->removeHandler(I);
--I;
--E;
Changed = true;
}
}
if (CSI->getNumHandlers() == 0) {
BasicBlock *CatchSwitchBB = CSI->getParent();
if (CSI->hasUnwindDest()) {
// Redirect preds to the unwind dest
CatchSwitchBB->replaceAllUsesWith(CSI->getUnwindDest());
} else {
// Rewrite all preds to unwind to caller (or from invoke to call).
SmallVector<BasicBlock *, 8> EHPreds(predecessors(CatchSwitchBB));
for (BasicBlock *EHPred : EHPreds)
removeUnwindEdge(EHPred);
}
// The catchswitch is no longer reachable.
new UnreachableInst(CSI->getContext(), CSI);
CSI->eraseFromParent();
Changed = true;
}
} else if (isa<CleanupReturnInst>(TI)) {
new UnreachableInst(TI->getContext(), TI);
TI->eraseFromParent();
Changed = true;
}
}
// If this block is now dead, remove it.
if (pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) {
// We know there are no successors, so just nuke the block.
if (LoopHeaders)
LoopHeaders->erase(BB);
BB->eraseFromParent();
return true;
}
return Changed;
}
static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
assert(Cases.size() >= 1);
array_pod_sort(Cases.begin(), Cases.end(), ConstantIntSortPredicate);
for (size_t I = 1, E = Cases.size(); I != E; ++I) {
if (Cases[I - 1]->getValue() != Cases[I]->getValue() + 1)
return false;
}
return true;
}
/// Turn a switch with two reachable destinations into an integer range
/// comparison and branch.
static bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder) {
assert(SI->getNumCases() > 1 && "Degenerate switch?");
bool HasDefault =
!isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
// Partition the cases into two sets with different destinations.
BasicBlock *DestA = HasDefault ? SI->getDefaultDest() : nullptr;
BasicBlock *DestB = nullptr;
SmallVector<ConstantInt *, 16> CasesA;
SmallVector<ConstantInt *, 16> CasesB;
for (auto Case : SI->cases()) {
BasicBlock *Dest = Case.getCaseSuccessor();
if (!DestA)
DestA = Dest;
if (Dest == DestA) {
CasesA.push_back(Case.getCaseValue());
continue;
}
if (!DestB)
DestB = Dest;
if (Dest == DestB) {
CasesB.push_back(Case.getCaseValue());
continue;
}
return false; // More than two destinations.
}
assert(DestA && DestB &&
"Single-destination switch should have been folded.");
assert(DestA != DestB);
assert(DestB != SI->getDefaultDest());
assert(!CasesB.empty() && "There must be non-default cases.");
assert(!CasesA.empty() || HasDefault);
// Figure out if one of the sets of cases form a contiguous range.
SmallVectorImpl<ConstantInt *> *ContiguousCases = nullptr;
BasicBlock *ContiguousDest = nullptr;
BasicBlock *OtherDest = nullptr;
if (!CasesA.empty() && CasesAreContiguous(CasesA)) {
ContiguousCases = &CasesA;
ContiguousDest = DestA;
OtherDest = DestB;
} else if (CasesAreContiguous(CasesB)) {
ContiguousCases = &CasesB;
ContiguousDest = DestB;
OtherDest = DestA;
} else
return false;
// Start building the compare and branch.
Constant *Offset = ConstantExpr::getNeg(ContiguousCases->back());
Constant *NumCases =
ConstantInt::get(Offset->getType(), ContiguousCases->size());
Value *Sub = SI->getCondition();
if (!Offset->isNullValue())
Sub = Builder.CreateAdd(Sub, Offset, Sub->getName() + ".off");
Value *Cmp;
// If NumCases overflowed, then all possible values jump to the successor.
if (NumCases->isNullValue() && !ContiguousCases->empty())
Cmp = ConstantInt::getTrue(SI->getContext());
else
Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
BranchInst *NewBI = Builder.CreateCondBr(Cmp, ContiguousDest, OtherDest);
// Update weight for the newly-created conditional branch.
if (HasBranchWeights(SI)) {
SmallVector<uint64_t, 8> Weights;
GetBranchWeights(SI, Weights);
if (Weights.size() == 1 + SI->getNumCases()) {
uint64_t TrueWeight = 0;
uint64_t FalseWeight = 0;
for (size_t I = 0, E = Weights.size(); I != E; ++I) {
if (SI->getSuccessor(I) == ContiguousDest)
TrueWeight += Weights[I];
else
FalseWeight += Weights[I];
}
while (TrueWeight > UINT32_MAX || FalseWeight > UINT32_MAX) {
TrueWeight /= 2;
FalseWeight /= 2;
}
setBranchWeights(NewBI, TrueWeight, FalseWeight);
}
}
// Prune obsolete incoming values off the successors' PHI nodes.
for (auto BBI = ContiguousDest->begin(); isa<PHINode>(BBI); ++BBI) {
unsigned PreviousEdges = ContiguousCases->size();
if (ContiguousDest == SI->getDefaultDest())
++PreviousEdges;
for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
}
for (auto BBI = OtherDest->begin(); isa<PHINode>(BBI); ++BBI) {
unsigned PreviousEdges = SI->getNumCases() - ContiguousCases->size();
if (OtherDest == SI->getDefaultDest())
++PreviousEdges;
for (unsigned I = 0, E = PreviousEdges - 1; I != E; ++I)
cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
}
// Drop the switch.
SI->eraseFromParent();
return true;
}
/// Compute masked bits for the condition of a switch
/// and use it to remove dead cases.
static bool eliminateDeadSwitchCases(SwitchInst *SI, AssumptionCache *AC,
const DataLayout &DL) {
Value *Cond = SI->getCondition();
unsigned Bits = Cond->getType()->getIntegerBitWidth();
KnownBits Known = computeKnownBits(Cond, DL, 0, AC, SI);
// We can also eliminate cases by determining that their values are outside of
// the limited range of the condition based on how many significant (non-sign)
// bits are in the condition value.
unsigned ExtraSignBits = ComputeNumSignBits(Cond, DL, 0, AC, SI) - 1;
unsigned MaxSignificantBitsInCond = Bits - ExtraSignBits;
// Gather dead cases.
SmallVector<ConstantInt *, 8> DeadCases;
for (auto &Case : SI->cases()) {
const APInt &CaseVal = Case.getCaseValue()->getValue();
if (Known.Zero.intersects(CaseVal) || !Known.One.isSubsetOf(CaseVal) ||
(CaseVal.getMinSignedBits() > MaxSignificantBitsInCond)) {
DeadCases.push_back(Case.getCaseValue());
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal
<< " is dead.\n");
}
}
// If we can prove that the cases must cover all possible values, the
// default destination becomes dead and we can remove it. If we know some
// of the bits in the value, we can use that to more precisely compute the
// number of possible unique case values.
bool HasDefault =
!isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
const unsigned NumUnknownBits =
Bits - (Known.Zero | Known.One).countPopulation();
assert(NumUnknownBits <= Bits);
if (HasDefault && DeadCases.empty() &&
NumUnknownBits < 64 /* avoid overflow */ &&
SI->getNumCases() == (1ULL << NumUnknownBits)) {
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
BasicBlock *NewDefault =
SplitBlockPredecessors(SI->getDefaultDest(), SI->getParent(), "");
SI->setDefaultDest(&*NewDefault);
SplitBlock(&*NewDefault, &NewDefault->front());
auto *OldTI = NewDefault->getTerminator();
new UnreachableInst(SI->getContext(), OldTI);
EraseTerminatorAndDCECond(OldTI);
return true;
}
SmallVector<uint64_t, 8> Weights;
bool HasWeight = HasBranchWeights(SI);
if (HasWeight) {
GetBranchWeights(SI, Weights);
HasWeight = (Weights.size() == 1 + SI->getNumCases());
}
// Remove dead cases from the switch.
for (ConstantInt *DeadCase : DeadCases) {
SwitchInst::CaseIt CaseI = SI->findCaseValue(DeadCase);
assert(CaseI != SI->case_default() &&
"Case was not found. Probably mistake in DeadCases forming.");
if (HasWeight) {
std::swap(Weights[CaseI->getCaseIndex() + 1], Weights.back());
Weights.pop_back();
}
// Prune unused values from PHI nodes.
CaseI->getCaseSuccessor()->removePredecessor(SI->getParent());
SI->removeCase(CaseI);
}
if (HasWeight && Weights.size() >= 2) {
SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
setBranchWeights(SI, MDWeights);
}
return !DeadCases.empty();
}
/// If BB would be eligible for simplification by
/// TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated
/// by an unconditional branch), look at the phi node for BB in the successor
/// block and see if the incoming value is equal to CaseValue. If so, return
/// the phi node, and set PhiIndex to BB's index in the phi node.
static PHINode *FindPHIForConditionForwarding(ConstantInt *CaseValue,
BasicBlock *BB, int *PhiIndex) {
if (BB->getFirstNonPHIOrDbg() != BB->getTerminator())
return nullptr; // BB must be empty to be a candidate for simplification.
if (!BB->getSinglePredecessor())
return nullptr; // BB must be dominated by the switch.
BranchInst *Branch = dyn_cast<BranchInst>(BB->getTerminator());
if (!Branch || !Branch->isUnconditional())
return nullptr; // Terminator must be unconditional branch.
BasicBlock *Succ = Branch->getSuccessor(0);
for (PHINode &PHI : Succ->phis()) {
int Idx = PHI.getBasicBlockIndex(BB);
assert(Idx >= 0 && "PHI has no entry for predecessor?");
Value *InValue = PHI.getIncomingValue(Idx);
if (InValue != CaseValue)
continue;
*PhiIndex = Idx;
return &PHI;
}
return nullptr;
}
/// Try to forward the condition of a switch instruction to a phi node
/// dominated by the switch, if that would mean that some of the destination
/// blocks of the switch can be folded away. Return true if a change is made.
static bool ForwardSwitchConditionToPHI(SwitchInst *SI) {
using ForwardingNodesMap = DenseMap<PHINode *, SmallVector<int, 4>>;
ForwardingNodesMap ForwardingNodes;
BasicBlock *SwitchBlock = SI->getParent();
bool Changed = false;
for (auto &Case : SI->cases()) {
ConstantInt *CaseValue = Case.getCaseValue();
BasicBlock *CaseDest = Case.getCaseSuccessor();
// Replace phi operands in successor blocks that are using the constant case
// value rather than the switch condition variable:
// switchbb:
// switch i32 %x, label %default [
// i32 17, label %succ
// ...
// succ:
// %r = phi i32 ... [ 17, %switchbb ] ...
// -->
// %r = phi i32 ... [ %x, %switchbb ] ...
for (PHINode &Phi : CaseDest->phis()) {
// This only works if there is exactly 1 incoming edge from the switch to
// a phi. If there is >1, that means multiple cases of the switch map to 1
// value in the phi, and that phi value is not the switch condition. Thus,
// this transform would not make sense (the phi would be invalid because
// a phi can't have different incoming values from the same block).
int SwitchBBIdx = Phi.getBasicBlockIndex(SwitchBlock);
if (Phi.getIncomingValue(SwitchBBIdx) == CaseValue &&
count(Phi.blocks(), SwitchBlock) == 1) {
Phi.setIncomingValue(SwitchBBIdx, SI->getCondition());
Changed = true;
}
}
// Collect phi nodes that are indirectly using this switch's case constants.
int PhiIdx;
if (auto *Phi = FindPHIForConditionForwarding(CaseValue, CaseDest, &PhiIdx))
ForwardingNodes[Phi].push_back(PhiIdx);
}
for (auto &ForwardingNode : ForwardingNodes) {
PHINode *Phi = ForwardingNode.first;
SmallVectorImpl<int> &Indexes = ForwardingNode.second;
if (Indexes.size() < 2)
continue;
for (int Index : Indexes)
Phi->setIncomingValue(Index, SI->getCondition());
Changed = true;
}
return Changed;
}
/// Return true if the backend will be able to handle
/// initializing an array of constants like C.
static bool ValidLookupTableConstant(Constant *C, const TargetTransformInfo &TTI) {
if (C->isThreadDependent())
return false;
if (C->isDLLImportDependent())
return false;
if (!isa<ConstantFP>(C) && !isa<ConstantInt>(C) &&
!isa<ConstantPointerNull>(C) && !isa<GlobalValue>(C) &&
!isa<UndefValue>(C) && !isa<ConstantExpr>(C))
return false;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
if (!CE->isGEPWithNoNotionalOverIndexing())
return false;
if (!ValidLookupTableConstant(CE->getOperand(0), TTI))
return false;
}
if (!TTI.shouldBuildLookupTablesForConstant(C))
return false;
return true;
}
/// If V is a Constant, return it. Otherwise, try to look up
/// its constant value in ConstantPool, returning 0 if it's not there.
static Constant *
LookupConstant(Value *V,
const SmallDenseMap<Value *, Constant *> &ConstantPool) {
if (Constant *C = dyn_cast<Constant>(V))
return C;
return ConstantPool.lookup(V);
}
/// Try to fold instruction I into a constant. This works for
/// simple instructions such as binary operations where both operands are
/// constant or can be replaced by constants from the ConstantPool. Returns the
/// resulting constant on success, 0 otherwise.
static Constant *
ConstantFold(Instruction *I, const DataLayout &DL,
const SmallDenseMap<Value *, Constant *> &ConstantPool) {
if (SelectInst *Select = dyn_cast<SelectInst>(I)) {
Constant *A = LookupConstant(Select->getCondition(), ConstantPool);
if (!A)
return nullptr;
if (A->isAllOnesValue())
return LookupConstant(Select->getTrueValue(), ConstantPool);
if (A->isNullValue())
return LookupConstant(Select->getFalseValue(), ConstantPool);
return nullptr;
}
SmallVector<Constant *, 4> COps;
for (unsigned N = 0, E = I->getNumOperands(); N != E; ++N) {
if (Constant *A = LookupConstant(I->getOperand(N), ConstantPool))
COps.push_back(A);
else
return nullptr;
}
if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
return ConstantFoldCompareInstOperands(Cmp->getPredicate(), COps[0],
COps[1], DL);
}
return ConstantFoldInstOperands(I, COps, DL);
}
/// Try to determine the resulting constant values in phi nodes
/// at the common destination basic block, *CommonDest, for one of the case
/// destionations CaseDest corresponding to value CaseVal (0 for the default
/// case), of a switch instruction SI.
static bool
GetCaseResults(SwitchInst *SI, ConstantInt *CaseVal, BasicBlock *CaseDest,
BasicBlock **CommonDest,
SmallVectorImpl<std::pair<PHINode *, Constant *>> &Res,
const DataLayout &DL, const TargetTransformInfo &TTI) {
// The block from which we enter the common destination.
BasicBlock *Pred = SI->getParent();
// If CaseDest is empty except for some side-effect free instructions through
// which we can constant-propagate the CaseVal, continue to its successor.
SmallDenseMap<Value *, Constant *> ConstantPool;
ConstantPool.insert(std::make_pair(SI->getCondition(), CaseVal));
for (Instruction &I :CaseDest->instructionsWithoutDebug()) {
if (I.isTerminator()) {
// If the terminator is a simple branch, continue to the next block.
if (I.getNumSuccessors() != 1 || I.isExceptionalTerminator())
return false;
Pred = CaseDest;
CaseDest = I.getSuccessor(0);
} else if (Constant *C = ConstantFold(&I, DL, ConstantPool)) {
// Instruction is side-effect free and constant.
// If the instruction has uses outside this block or a phi node slot for
// the block, it is not safe to bypass the instruction since it would then
// no longer dominate all its uses.
for (auto &Use : I.uses()) {
User *User = Use.getUser();
if (Instruction *I = dyn_cast<Instruction>(User))
if (I->getParent() == CaseDest)
continue;
if (PHINode *Phi = dyn_cast<PHINode>(User))
if (Phi->getIncomingBlock(Use) == CaseDest)
continue;
return false;
}
ConstantPool.insert(std::make_pair(&I, C));
} else {
break;
}
}
// If we did not have a CommonDest before, use the current one.
if (!*CommonDest)
*CommonDest = CaseDest;
// If the destination isn't the common one, abort.
if (CaseDest != *CommonDest)
return false;
// Get the values for this case from phi nodes in the destination block.
for (PHINode &PHI : (*CommonDest)->phis()) {
int Idx = PHI.getBasicBlockIndex(Pred);
if (Idx == -1)
continue;
Constant *ConstVal =
LookupConstant(PHI.getIncomingValue(Idx), ConstantPool);
if (!ConstVal)
return false;
// Be conservative about which kinds of constants we support.
if (!ValidLookupTableConstant(ConstVal, TTI))
return false;
Res.push_back(std::make_pair(&PHI, ConstVal));
}
return Res.size() > 0;
}
// Helper function used to add CaseVal to the list of cases that generate
// Result. Returns the updated number of cases that generate this result.
static uintptr_t MapCaseToResult(ConstantInt *CaseVal,
SwitchCaseResultVectorTy &UniqueResults,
Constant *Result) {
for (auto &I : UniqueResults) {
if (I.first == Result) {
I.second.push_back(CaseVal);
return I.second.size();
}
}
UniqueResults.push_back(
std::make_pair(Result, SmallVector<ConstantInt *, 4>(1, CaseVal)));
return 1;
}
// Helper function that initializes a map containing
// results for the PHI node of the common destination block for a switch
// instruction. Returns false if multiple PHI nodes have been found or if
// there is not a common destination block for the switch.
static bool
InitializeUniqueCases(SwitchInst *SI, PHINode *&PHI, BasicBlock *&CommonDest,
SwitchCaseResultVectorTy &UniqueResults,
Constant *&DefaultResult, const DataLayout &DL,
const TargetTransformInfo &TTI,
uintptr_t MaxUniqueResults, uintptr_t MaxCasesPerResult) {
for (auto &I : SI->cases()) {
ConstantInt *CaseVal = I.getCaseValue();
// Resulting value at phi nodes for this case value.
SwitchCaseResultsTy Results;
if (!GetCaseResults(SI, CaseVal, I.getCaseSuccessor(), &CommonDest, Results,
DL, TTI))
return false;
// Only one value per case is permitted.
if (Results.size() > 1)
return false;
// Add the case->result mapping to UniqueResults.
const uintptr_t NumCasesForResult =
MapCaseToResult(CaseVal, UniqueResults, Results.begin()->second);
// Early out if there are too many cases for this result.
if (NumCasesForResult > MaxCasesPerResult)
return false;
// Early out if there are too many unique results.
if (UniqueResults.size() > MaxUniqueResults)
return false;
// Check the PHI consistency.
if (!PHI)
PHI = Results[0].first;
else if (PHI != Results[0].first)
return false;
}
// Find the default result value.
SmallVector<std::pair<PHINode *, Constant *>, 1> DefaultResults;
BasicBlock *DefaultDest = SI->getDefaultDest();
GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest, DefaultResults,
DL, TTI);
// If the default value is not found abort unless the default destination
// is unreachable.
DefaultResult =
DefaultResults.size() == 1 ? DefaultResults.begin()->second : nullptr;
if ((!DefaultResult &&
!isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg())))
return false;
return true;
}
// Helper function that checks if it is possible to transform a switch with only
// two cases (or two cases + default) that produces a result into a select.
// Example:
// switch (a) {
// case 10: %0 = icmp eq i32 %a, 10
// return 10; %1 = select i1 %0, i32 10, i32 4
// case 20: ----> %2 = icmp eq i32 %a, 20
// return 2; %3 = select i1 %2, i32 2, i32 %1
// default:
// return 4;
// }
static Value *ConvertTwoCaseSwitch(const SwitchCaseResultVectorTy &ResultVector,
Constant *DefaultResult, Value *Condition,
IRBuilder<> &Builder) {
assert(ResultVector.size() == 2 &&
"We should have exactly two unique results at this point");
// If we are selecting between only two cases transform into a simple
// select or a two-way select if default is possible.
if (ResultVector[0].second.size() == 1 &&
ResultVector[1].second.size() == 1) {
ConstantInt *const FirstCase = ResultVector[0].second[0];
ConstantInt *const SecondCase = ResultVector[1].second[0];
bool DefaultCanTrigger = DefaultResult;
Value *SelectValue = ResultVector[1].first;
if (DefaultCanTrigger) {
Value *const ValueCompare =
Builder.CreateICmpEQ(Condition, SecondCase, "switch.selectcmp");
SelectValue = Builder.CreateSelect(ValueCompare, ResultVector[1].first,
DefaultResult, "switch.select");
}
Value *const ValueCompare =
Builder.CreateICmpEQ(Condition, FirstCase, "switch.selectcmp");
return Builder.CreateSelect(ValueCompare, ResultVector[0].first,
SelectValue, "switch.select");
}
return nullptr;
}
// Helper function to cleanup a switch instruction that has been converted into
// a select, fixing up PHI nodes and basic blocks.
static void RemoveSwitchAfterSelectConversion(SwitchInst *SI, PHINode *PHI,
Value *SelectValue,
IRBuilder<> &Builder) {
BasicBlock *SelectBB = SI->getParent();
while (PHI->getBasicBlockIndex(SelectBB) >= 0)
PHI->removeIncomingValue(SelectBB);
PHI->addIncoming(SelectValue, SelectBB);
Builder.CreateBr(PHI->getParent());
// Remove the switch.
for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
BasicBlock *Succ = SI->getSuccessor(i);
if (Succ == PHI->getParent())
continue;
Succ->removePredecessor(SelectBB);
}
SI->eraseFromParent();
}
/// If the switch is only used to initialize one or more
/// phi nodes in a common successor block with only two different
/// constant values, replace the switch with select.
static bool switchToSelect(SwitchInst *SI, IRBuilder<> &Builder,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
Value *const Cond = SI->getCondition();
PHINode *PHI = nullptr;
BasicBlock *CommonDest = nullptr;
Constant *DefaultResult;
SwitchCaseResultVectorTy UniqueResults;
// Collect all the cases that will deliver the same value from the switch.
if (!InitializeUniqueCases(SI, PHI, CommonDest, UniqueResults, DefaultResult,
DL, TTI, 2, 1))
return false;
// Selects choose between maximum two values.
if (UniqueResults.size() != 2)
return false;
assert(PHI != nullptr && "PHI for value select not found");
Builder.SetInsertPoint(SI);
Value *SelectValue =
ConvertTwoCaseSwitch(UniqueResults, DefaultResult, Cond, Builder);
if (SelectValue) {
RemoveSwitchAfterSelectConversion(SI, PHI, SelectValue, Builder);
return true;
}
// The switch couldn't be converted into a select.
return false;
}
namespace {
/// This class represents a lookup table that can be used to replace a switch.
class SwitchLookupTable {
public:
/// Create a lookup table to use as a switch replacement with the contents
/// of Values, using DefaultValue to fill any holes in the table.
SwitchLookupTable(
Module &M, uint64_t TableSize, ConstantInt *Offset,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName);
/// Build instructions with Builder to retrieve the value at
/// the position given by Index in the lookup table.
Value *BuildLookup(Value *Index, IRBuilder<> &Builder);
/// Return true if a table with TableSize elements of
/// type ElementType would fit in a target-legal register.
static bool WouldFitInRegister(const DataLayout &DL, uint64_t TableSize,
Type *ElementType);
private:
// Depending on the contents of the table, it can be represented in
// different ways.
enum {
// For tables where each element contains the same value, we just have to
// store that single value and return it for each lookup.
SingleValueKind,
// For tables where there is a linear relationship between table index
// and values. We calculate the result with a simple multiplication
// and addition instead of a table lookup.
LinearMapKind,
// For small tables with integer elements, we can pack them into a bitmap
// that fits into a target-legal register. Values are retrieved by
// shift and mask operations.
BitMapKind,
// The table is stored as an array of values. Values are retrieved by load
// instructions from the table.
ArrayKind
} Kind;
// For SingleValueKind, this is the single value.
Constant *SingleValue = nullptr;
// For BitMapKind, this is the bitmap.
ConstantInt *BitMap = nullptr;
IntegerType *BitMapElementTy = nullptr;
// For LinearMapKind, these are the constants used to derive the value.
ConstantInt *LinearOffset = nullptr;
ConstantInt *LinearMultiplier = nullptr;
// For ArrayKind, this is the array.
GlobalVariable *Array = nullptr;
};
} // end anonymous namespace
SwitchLookupTable::SwitchLookupTable(
Module &M, uint64_t TableSize, ConstantInt *Offset,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values,
Constant *DefaultValue, const DataLayout &DL, const StringRef &FuncName) {
assert(Values.size() && "Can't build lookup table without values!");
assert(TableSize >= Values.size() && "Can't fit values in table!");
// If all values in the table are equal, this is that value.
SingleValue = Values.begin()->second;
Type *ValueType = Values.begin()->second->getType();
// Build up the table contents.
SmallVector<Constant *, 64> TableContents(TableSize);
for (size_t I = 0, E = Values.size(); I != E; ++I) {
ConstantInt *CaseVal = Values[I].first;
Constant *CaseRes = Values[I].second;
assert(CaseRes->getType() == ValueType);
uint64_t Idx = (CaseVal->getValue() - Offset->getValue()).getLimitedValue();
TableContents[Idx] = CaseRes;
if (CaseRes != SingleValue)
SingleValue = nullptr;
}
// Fill in any holes in the table with the default result.
if (Values.size() < TableSize) {
assert(DefaultValue &&
"Need a default value to fill the lookup table holes.");
assert(DefaultValue->getType() == ValueType);
for (uint64_t I = 0; I < TableSize; ++I) {
if (!TableContents[I])
TableContents[I] = DefaultValue;
}
if (DefaultValue != SingleValue)
SingleValue = nullptr;
}
// If each element in the table contains the same value, we only need to store
// that single value.
if (SingleValue) {
Kind = SingleValueKind;
return;
}
// Check if we can derive the value with a linear transformation from the
// table index.
if (isa<IntegerType>(ValueType)) {
bool LinearMappingPossible = true;
APInt PrevVal;
APInt DistToPrev;
assert(TableSize >= 2 && "Should be a SingleValue table.");
// Check if there is the same distance between two consecutive values.
for (uint64_t I = 0; I < TableSize; ++I) {
ConstantInt *ConstVal = dyn_cast<ConstantInt>(TableContents[I]);
if (!ConstVal) {
// This is an undef. We could deal with it, but undefs in lookup tables
// are very seldom. It's probably not worth the additional complexity.
LinearMappingPossible = false;
break;
}
const APInt &Val = ConstVal->getValue();
if (I != 0) {
APInt Dist = Val - PrevVal;
if (I == 1) {
DistToPrev = Dist;
} else if (Dist != DistToPrev) {
LinearMappingPossible = false;
break;
}
}
PrevVal = Val;
}
if (LinearMappingPossible) {
LinearOffset = cast<ConstantInt>(TableContents[0]);
LinearMultiplier = ConstantInt::get(M.getContext(), DistToPrev);
Kind = LinearMapKind;
++NumLinearMaps;
return;
}
}
// If the type is integer and the table fits in a register, build a bitmap.
if (WouldFitInRegister(DL, TableSize, ValueType)) {
IntegerType *IT = cast<IntegerType>(ValueType);
APInt TableInt(TableSize * IT->getBitWidth(), 0);
for (uint64_t I = TableSize; I > 0; --I) {
TableInt <<= IT->getBitWidth();
// Insert values into the bitmap. Undef values are set to zero.
if (!isa<UndefValue>(TableContents[I - 1])) {
ConstantInt *Val = cast<ConstantInt>(TableContents[I - 1]);
TableInt |= Val->getValue().zext(TableInt.getBitWidth());
}
}
BitMap = ConstantInt::get(M.getContext(), TableInt);
BitMapElementTy = IT;
Kind = BitMapKind;
++NumBitMaps;
return;
}
// Store the table in an array.
ArrayType *ArrayTy = ArrayType::get(ValueType, TableSize);
Constant *Initializer = ConstantArray::get(ArrayTy, TableContents);
Array = new GlobalVariable(M, ArrayTy, /*constant=*/true,
GlobalVariable::PrivateLinkage, Initializer,
"switch.table." + FuncName);
Array->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
// Set the alignment to that of an array items. We will be only loading one
// value out of it.
Array->setAlignment(DL.getPrefTypeAlignment(ValueType));
Kind = ArrayKind;
}
Value *SwitchLookupTable::BuildLookup(Value *Index, IRBuilder<> &Builder) {
switch (Kind) {
case SingleValueKind:
return SingleValue;
case LinearMapKind: {
// Derive the result value from the input value.
Value *Result = Builder.CreateIntCast(Index, LinearMultiplier->getType(),
false, "switch.idx.cast");
if (!LinearMultiplier->isOne())
Result = Builder.CreateMul(Result, LinearMultiplier, "switch.idx.mult");
if (!LinearOffset->isZero())
Result = Builder.CreateAdd(Result, LinearOffset, "switch.offset");
return Result;
}
case BitMapKind: {
// Type of the bitmap (e.g. i59).
IntegerType *MapTy = BitMap->getType();
// Cast Index to the same type as the bitmap.
// Note: The Index is <= the number of elements in the table, so
// truncating it to the width of the bitmask is safe.
Value *ShiftAmt = Builder.CreateZExtOrTrunc(Index, MapTy, "switch.cast");
// Multiply the shift amount by the element width.
ShiftAmt = Builder.CreateMul(
ShiftAmt, ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()),
"switch.shiftamt");
// Shift down.
Value *DownShifted =
Builder.CreateLShr(BitMap, ShiftAmt, "switch.downshift");
// Mask off.
return Builder.CreateTrunc(DownShifted, BitMapElementTy, "switch.masked");
}
case ArrayKind: {
// Make sure the table index will not overflow when treated as signed.
IntegerType *IT = cast<IntegerType>(Index->getType());
uint64_t TableSize =
Array->getInitializer()->getType()->getArrayNumElements();
if (TableSize > (1ULL << (IT->getBitWidth() - 1)))
Index = Builder.CreateZExt(
Index, IntegerType::get(IT->getContext(), IT->getBitWidth() + 1),
"switch.tableidx.zext");
Value *GEPIndices[] = {Builder.getInt32(0), Index};
Value *GEP = Builder.CreateInBoundsGEP(Array->getValueType(), Array,
GEPIndices, "switch.gep");
return Builder.CreateLoad(GEP, "switch.load");
}
}
llvm_unreachable("Unknown lookup table kind!");
}
bool SwitchLookupTable::WouldFitInRegister(const DataLayout &DL,
uint64_t TableSize,
Type *ElementType) {
auto *IT = dyn_cast<IntegerType>(ElementType);
if (!IT)
return false;
// FIXME: If the type is wider than it needs to be, e.g. i8 but all values
// are <= 15, we could try to narrow the type.
// Avoid overflow, fitsInLegalInteger uses unsigned int for the width.
if (TableSize >= UINT_MAX / IT->getBitWidth())
return false;
return DL.fitsInLegalInteger(TableSize * IT->getBitWidth());
}
/// Determine whether a lookup table should be built for this switch, based on
/// the number of cases, size of the table, and the types of the results.
static bool
ShouldBuildLookupTable(SwitchInst *SI, uint64_t TableSize,
const TargetTransformInfo &TTI, const DataLayout &DL,
const SmallDenseMap<PHINode *, Type *> &ResultTypes) {
if (SI->getNumCases() > TableSize || TableSize >= UINT64_MAX / 10)
return false; // TableSize overflowed, or mul below might overflow.
bool AllTablesFitInRegister = true;
bool HasIllegalType = false;
for (const auto &I : ResultTypes) {
Type *Ty = I.second;
// Saturate this flag to true.
HasIllegalType = HasIllegalType || !TTI.isTypeLegal(Ty);
// Saturate this flag to false.
AllTablesFitInRegister =
AllTablesFitInRegister &&
SwitchLookupTable::WouldFitInRegister(DL, TableSize, Ty);
// If both flags saturate, we're done. NOTE: This *only* works with
// saturating flags, and all flags have to saturate first due to the
// non-deterministic behavior of iterating over a dense map.
if (HasIllegalType && !AllTablesFitInRegister)
break;
}
// If each table would fit in a register, we should build it anyway.
if (AllTablesFitInRegister)
return true;
// Don't build a table that doesn't fit in-register if it has illegal types.
if (HasIllegalType)
return false;
// The table density should be at least 40%. This is the same criterion as for
// jump tables, see SelectionDAGBuilder::handleJTSwitchCase.
// FIXME: Find the best cut-off.
return SI->getNumCases() * 10 >= TableSize * 4;
}
/// Try to reuse the switch table index compare. Following pattern:
/// \code
/// if (idx < tablesize)
/// r = table[idx]; // table does not contain default_value
/// else
/// r = default_value;
/// if (r != default_value)
/// ...
/// \endcode
/// Is optimized to:
/// \code
/// cond = idx < tablesize;
/// if (cond)
/// r = table[idx];
/// else
/// r = default_value;
/// if (cond)
/// ...
/// \endcode
/// Jump threading will then eliminate the second if(cond).
static void reuseTableCompare(
User *PhiUser, BasicBlock *PhiBlock, BranchInst *RangeCheckBranch,
Constant *DefaultValue,
const SmallVectorImpl<std::pair<ConstantInt *, Constant *>> &Values) {
ICmpInst *CmpInst = dyn_cast<ICmpInst>(PhiUser);
if (!CmpInst)
return;
// We require that the compare is in the same block as the phi so that jump
// threading can do its work afterwards.
if (CmpInst->getParent() != PhiBlock)
return;
Constant *CmpOp1 = dyn_cast<Constant>(CmpInst->getOperand(1));
if (!CmpOp1)
return;
Value *RangeCmp = RangeCheckBranch->getCondition();
Constant *TrueConst = ConstantInt::getTrue(RangeCmp->getType());
Constant *FalseConst = ConstantInt::getFalse(RangeCmp->getType());
// Check if the compare with the default value is constant true or false.
Constant *DefaultConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
DefaultValue, CmpOp1, true);
if (DefaultConst != TrueConst && DefaultConst != FalseConst)
return;
// Check if the compare with the case values is distinct from the default
// compare result.
for (auto ValuePair : Values) {
Constant *CaseConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
ValuePair.second, CmpOp1, true);
if (!CaseConst || CaseConst == DefaultConst || isa<UndefValue>(CaseConst))
return;
assert((CaseConst == TrueConst || CaseConst == FalseConst) &&
"Expect true or false as compare result.");
}
// Check if the branch instruction dominates the phi node. It's a simple
// dominance check, but sufficient for our needs.
// Although this check is invariant in the calling loops, it's better to do it
// at this late stage. Practically we do it at most once for a switch.
BasicBlock *BranchBlock = RangeCheckBranch->getParent();
for (auto PI = pred_begin(PhiBlock), E = pred_end(PhiBlock); PI != E; ++PI) {
BasicBlock *Pred = *PI;
if (Pred != BranchBlock && Pred->getUniquePredecessor() != BranchBlock)
return;
}
if (DefaultConst == FalseConst) {
// The compare yields the same result. We can replace it.
CmpInst->replaceAllUsesWith(RangeCmp);
++NumTableCmpReuses;
} else {
// The compare yields the same result, just inverted. We can replace it.
Value *InvertedTableCmp = BinaryOperator::CreateXor(
RangeCmp, ConstantInt::get(RangeCmp->getType(), 1), "inverted.cmp",
RangeCheckBranch);
CmpInst->replaceAllUsesWith(InvertedTableCmp);
++NumTableCmpReuses;
}
}
/// If the switch is only used to initialize one or more phi nodes in a common
/// successor block with different constant values, replace the switch with
/// lookup tables.
static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
assert(SI->getNumCases() > 1 && "Degenerate switch?");
Function *Fn = SI->getParent()->getParent();
// Only build lookup table when we have a target that supports it or the
// attribute is not set.
if (!TTI.shouldBuildLookupTables() ||
(Fn->getFnAttribute("no-jump-tables").getValueAsString() == "true"))
return false;
// FIXME: If the switch is too sparse for a lookup table, perhaps we could
// split off a dense part and build a lookup table for that.
// FIXME: This creates arrays of GEPs to constant strings, which means each
// GEP needs a runtime relocation in PIC code. We should just build one big
// string and lookup indices into that.
// Ignore switches with less than three cases. Lookup tables will not make
// them faster, so we don't analyze them.
if (SI->getNumCases() < 3)
return false;
// Figure out the corresponding result for each case value and phi node in the
// common destination, as well as the min and max case values.
assert(!empty(SI->cases()));
SwitchInst::CaseIt CI = SI->case_begin();
ConstantInt *MinCaseVal = CI->getCaseValue();
ConstantInt *MaxCaseVal = CI->getCaseValue();
BasicBlock *CommonDest = nullptr;
using ResultListTy = SmallVector<std::pair<ConstantInt *, Constant *>, 4>;
SmallDenseMap<PHINode *, ResultListTy> ResultLists;
SmallDenseMap<PHINode *, Constant *> DefaultResults;
SmallDenseMap<PHINode *, Type *> ResultTypes;
SmallVector<PHINode *, 4> PHIs;
for (SwitchInst::CaseIt E = SI->case_end(); CI != E; ++CI) {
ConstantInt *CaseVal = CI->getCaseValue();
if (CaseVal->getValue().slt(MinCaseVal->getValue()))
MinCaseVal = CaseVal;
if (CaseVal->getValue().sgt(MaxCaseVal->getValue()))
MaxCaseVal = CaseVal;
// Resulting value at phi nodes for this case value.
using ResultsTy = SmallVector<std::pair<PHINode *, Constant *>, 4>;
ResultsTy Results;
if (!GetCaseResults(SI, CaseVal, CI->getCaseSuccessor(), &CommonDest,
Results, DL, TTI))
return false;
// Append the result from this case to the list for each phi.
for (const auto &I : Results) {
PHINode *PHI = I.first;
Constant *Value = I.second;
if (!ResultLists.count(PHI))
PHIs.push_back(PHI);
ResultLists[PHI].push_back(std::make_pair(CaseVal, Value));
}
}
// Keep track of the result types.
for (PHINode *PHI : PHIs) {
ResultTypes[PHI] = ResultLists[PHI][0].second->getType();
}
uint64_t NumResults = ResultLists[PHIs[0]].size();
APInt RangeSpread = MaxCaseVal->getValue() - MinCaseVal->getValue();
uint64_t TableSize = RangeSpread.getLimitedValue() + 1;
bool TableHasHoles = (NumResults < TableSize);
// If the table has holes, we need a constant result for the default case
// or a bitmask that fits in a register.
SmallVector<std::pair<PHINode *, Constant *>, 4> DefaultResultsList;
bool HasDefaultResults =
GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest,
DefaultResultsList, DL, TTI);
bool NeedMask = (TableHasHoles && !HasDefaultResults);
if (NeedMask) {
// As an extra penalty for the validity test we require more cases.
if (SI->getNumCases() < 4) // FIXME: Find best threshold value (benchmark).
return false;
if (!DL.fitsInLegalInteger(TableSize))
return false;
}
for (const auto &I : DefaultResultsList) {
PHINode *PHI = I.first;
Constant *Result = I.second;
DefaultResults[PHI] = Result;
}
if (!ShouldBuildLookupTable(SI, TableSize, TTI, DL, ResultTypes))
return false;
// Create the BB that does the lookups.
Module &Mod = *CommonDest->getParent()->getParent();
BasicBlock *LookupBB = BasicBlock::Create(
Mod.getContext(), "switch.lookup", CommonDest->getParent(), CommonDest);
// Compute the table index value.
Builder.SetInsertPoint(SI);
Value *TableIndex;
if (MinCaseVal->isNullValue())
TableIndex = SI->getCondition();
else
TableIndex = Builder.CreateSub(SI->getCondition(), MinCaseVal,
"switch.tableidx");
// Compute the maximum table size representable by the integer type we are
// switching upon.
unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
uint64_t MaxTableSize = CaseSize > 63 ? UINT64_MAX : 1ULL << CaseSize;
assert(MaxTableSize >= TableSize &&
"It is impossible for a switch to have more entries than the max "
"representable value of its input integer type's size.");
// If the default destination is unreachable, or if the lookup table covers
// all values of the conditional variable, branch directly to the lookup table
// BB. Otherwise, check that the condition is within the case range.
const bool DefaultIsReachable =
!isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
BranchInst *RangeCheckBranch = nullptr;
if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
Builder.CreateBr(LookupBB);
// Note: We call removeProdecessor later since we need to be able to get the
// PHI value for the default case in case we're using a bit mask.
} else {
Value *Cmp = Builder.CreateICmpULT(
TableIndex, ConstantInt::get(MinCaseVal->getType(), TableSize));
RangeCheckBranch =
Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
}
// Populate the BB that does the lookups.
Builder.SetInsertPoint(LookupBB);
if (NeedMask) {
// Before doing the lookup, we do the hole check. The LookupBB is therefore
// re-purposed to do the hole check, and we create a new LookupBB.
BasicBlock *MaskBB = LookupBB;
MaskBB->setName("switch.hole_check");
LookupBB = BasicBlock::Create(Mod.getContext(), "switch.lookup",
CommonDest->getParent(), CommonDest);
// Make the mask's bitwidth at least 8-bit and a power-of-2 to avoid
// unnecessary illegal types.
uint64_t TableSizePowOf2 = NextPowerOf2(std::max(7ULL, TableSize - 1ULL));
APInt MaskInt(TableSizePowOf2, 0);
APInt One(TableSizePowOf2, 1);
// Build bitmask; fill in a 1 bit for every case.
const ResultListTy &ResultList = ResultLists[PHIs[0]];
for (size_t I = 0, E = ResultList.size(); I != E; ++I) {
uint64_t Idx = (ResultList[I].first->getValue() - MinCaseVal->getValue())
.getLimitedValue();
MaskInt |= One << Idx;
}
ConstantInt *TableMask = ConstantInt::get(Mod.getContext(), MaskInt);
// Get the TableIndex'th bit of the bitmask.
// If this bit is 0 (meaning hole) jump to the default destination,
// else continue with table lookup.
IntegerType *MapTy = TableMask->getType();
Value *MaskIndex =
Builder.CreateZExtOrTrunc(TableIndex, MapTy, "switch.maskindex");
Value *Shifted = Builder.CreateLShr(TableMask, MaskIndex, "switch.shifted");
Value *LoBit = Builder.CreateTrunc(
Shifted, Type::getInt1Ty(Mod.getContext()), "switch.lobit");
Builder.CreateCondBr(LoBit, LookupBB, SI->getDefaultDest());
Builder.SetInsertPoint(LookupBB);
AddPredecessorToBlock(SI->getDefaultDest(), MaskBB, SI->getParent());
}
if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
// We cached PHINodes in PHIs. To avoid accessing deleted PHINodes later,
// do not delete PHINodes here.
SI->getDefaultDest()->removePredecessor(SI->getParent(),
/*DontDeleteUselessPHIs=*/true);
}
bool ReturnedEarly = false;
for (PHINode *PHI : PHIs) {
const ResultListTy &ResultList = ResultLists[PHI];
// If using a bitmask, use any value to fill the lookup table holes.
Constant *DV = NeedMask ? ResultLists[PHI][0].second : DefaultResults[PHI];
StringRef FuncName = Fn->getName();
SwitchLookupTable Table(Mod, TableSize, MinCaseVal, ResultList, DV, DL,
FuncName);
Value *Result = Table.BuildLookup(TableIndex, Builder);
// If the result is used to return immediately from the function, we want to
// do that right here.
if (PHI->hasOneUse() && isa<ReturnInst>(*PHI->user_begin()) &&
PHI->user_back() == CommonDest->getFirstNonPHIOrDbg()) {
Builder.CreateRet(Result);
ReturnedEarly = true;
break;
}
// Do a small peephole optimization: re-use the switch table compare if
// possible.
if (!TableHasHoles && HasDefaultResults && RangeCheckBranch) {
BasicBlock *PhiBlock = PHI->getParent();
// Search for compare instructions which use the phi.
for (auto *User : PHI->users()) {
reuseTableCompare(User, PhiBlock, RangeCheckBranch, DV, ResultList);
}
}
PHI->addIncoming(Result, LookupBB);
}
if (!ReturnedEarly)
Builder.CreateBr(CommonDest);
// Remove the switch.
for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
BasicBlock *Succ = SI->getSuccessor(i);
if (Succ == SI->getDefaultDest())
continue;
Succ->removePredecessor(SI->getParent());
}
SI->eraseFromParent();
++NumLookupTables;
if (NeedMask)
++NumLookupTablesHoles;
return true;
}
static bool isSwitchDense(ArrayRef<int64_t> Values) {
// See also SelectionDAGBuilder::isDense(), which this function was based on.
uint64_t Diff = (uint64_t)Values.back() - (uint64_t)Values.front();
uint64_t Range = Diff + 1;
uint64_t NumCases = Values.size();
// 40% is the default density for building a jump table in optsize/minsize mode.
uint64_t MinDensity = 40;
return NumCases * 100 >= Range * MinDensity;
}
/// Try to transform a switch that has "holes" in it to a contiguous sequence
/// of cases.
///
/// A switch such as: switch(i) {case 5: case 9: case 13: case 17:} can be
/// range-reduced to: switch ((i-5) / 4) {case 0: case 1: case 2: case 3:}.
///
/// This converts a sparse switch into a dense switch which allows better
/// lowering and could also allow transforming into a lookup table.
static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
const DataLayout &DL,
const TargetTransformInfo &TTI) {
auto *CondTy = cast<IntegerType>(SI->getCondition()->getType());
if (CondTy->getIntegerBitWidth() > 64 ||
!DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
return false;
// Only bother with this optimization if there are more than 3 switch cases;
// SDAG will only bother creating jump tables for 4 or more cases.
if (SI->getNumCases() < 4)
return false;
// This transform is agnostic to the signedness of the input or case values. We
// can treat the case values as signed or unsigned. We can optimize more common
// cases such as a sequence crossing zero {-4,0,4,8} if we interpret case values
// as signed.
SmallVector<int64_t,4> Values;
for (auto &C : SI->cases())
Values.push_back(C.getCaseValue()->getValue().getSExtValue());
llvm::sort(Values);
// If the switch is already dense, there's nothing useful to do here.
if (isSwitchDense(Values))
return false;
// First, transform the values such that they start at zero and ascend.
int64_t Base = Values[0];
for (auto &V : Values)
V -= (uint64_t)(Base);
// Now we have signed numbers that have been shifted so that, given enough
// precision, there are no negative values. Since the rest of the transform
// is bitwise only, we switch now to an unsigned representation.
uint64_t GCD = 0;
for (auto &V : Values)
GCD = GreatestCommonDivisor64(GCD, (uint64_t)V);
// This transform can be done speculatively because it is so cheap - it results
// in a single rotate operation being inserted. This can only happen if the
// factor extracted is a power of 2.
// FIXME: If the GCD is an odd number we can multiply by the multiplicative
// inverse of GCD and then perform this transform.
// FIXME: It's possible that optimizing a switch on powers of two might also
// be beneficial - flag values are often powers of two and we could use a CLZ
// as the key function.
if (GCD <= 1 || !isPowerOf2_64(GCD))
// No common divisor found or too expensive to compute key function.
return false;
unsigned Shift = Log2_64(GCD);
for (auto &V : Values)
V = (int64_t)((uint64_t)V >> Shift);
if (!isSwitchDense(Values))
// Transform didn't create a dense switch.
return false;
// The obvious transform is to shift the switch condition right and emit a
// check that the condition actually cleanly divided by GCD, i.e.
// C & (1 << Shift - 1) == 0
// inserting a new CFG edge to handle the case where it didn't divide cleanly.
//
// A cheaper way of doing this is a simple ROTR(C, Shift). This performs the
// shift and puts the shifted-off bits in the uppermost bits. If any of these
// are nonzero then the switch condition will be very large and will hit the
// default case.
auto *Ty = cast<IntegerType>(SI->getCondition()->getType());
Builder.SetInsertPoint(SI);
auto *ShiftC = ConstantInt::get(Ty, Shift);
auto *Sub = Builder.CreateSub(SI->getCondition(), ConstantInt::get(Ty, Base));
auto *LShr = Builder.CreateLShr(Sub, ShiftC);
auto *Shl = Builder.CreateShl(Sub, Ty->getBitWidth() - Shift);
auto *Rot = Builder.CreateOr(LShr, Shl);
SI->replaceUsesOfWith(SI->getCondition(), Rot);
for (auto Case : SI->cases()) {
auto *Orig = Case.getCaseValue();
auto Sub = Orig->getValue() - APInt(Ty->getBitWidth(), Base);
Case.setValue(
cast<ConstantInt>(ConstantInt::get(Ty, Sub.lshr(ShiftC->getValue()))));
}
return true;
}
bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
BasicBlock *BB = SI->getParent();
if (isValueEqualityComparison(SI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder))
return requestResimplify();
Value *Cond = SI->getCondition();
if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
if (SimplifySwitchOnSelect(SI, Select))
return requestResimplify();
// If the block only contains the switch, see if we can fold the block
// away into any preds.
if (SI == &*BB->instructionsWithoutDebug().begin())
if (FoldValueComparisonIntoPredecessors(SI, Builder))
return requestResimplify();
}
// Try to transform the switch into an icmp and a branch.
if (TurnSwitchRangeIntoICmp(SI, Builder))
return requestResimplify();
// Remove unreachable cases.
if (eliminateDeadSwitchCases(SI, Options.AC, DL))
return requestResimplify();
if (switchToSelect(SI, Builder, DL, TTI))
return requestResimplify();
if (Options.ForwardSwitchCondToPhi && ForwardSwitchConditionToPHI(SI))
return requestResimplify();
// The conversion from switch to lookup tables results in difficult-to-analyze
// code and makes pruning branches much harder. This is a problem if the
// switch expression itself can still be restricted as a result of inlining or
// CVP. Therefore, only apply this transformation during late stages of the
// optimisation pipeline.
if (Options.ConvertSwitchToLookupTable &&
SwitchToLookupTable(SI, Builder, DL, TTI))
return requestResimplify();
if (ReduceSwitchRange(SI, Builder, DL, TTI))
return requestResimplify();
return false;
}
bool SimplifyCFGOpt::SimplifyIndirectBr(IndirectBrInst *IBI) {
BasicBlock *BB = IBI->getParent();
bool Changed = false;
// Eliminate redundant destinations.
SmallPtrSet<Value *, 8> Succs;
for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
BasicBlock *Dest = IBI->getDestination(i);
if (!Dest->hasAddressTaken() || !Succs.insert(Dest).second) {
Dest->removePredecessor(BB);
IBI->removeDestination(i);
--i;
--e;
Changed = true;
}
}
if (IBI->getNumDestinations() == 0) {
// If the indirectbr has no successors, change it to unreachable.
new UnreachableInst(IBI->getContext(), IBI);
EraseTerminatorAndDCECond(IBI);
return true;
}
if (IBI->getNumDestinations() == 1) {
// If the indirectbr has one successor, change it to a direct branch.
BranchInst::Create(IBI->getDestination(0), IBI);
EraseTerminatorAndDCECond(IBI);
return true;
}
if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
if (SimplifyIndirectBrOnSelect(IBI, SI))
return requestResimplify();
}
return Changed;
}
/// Given an block with only a single landing pad and a unconditional branch
/// try to find another basic block which this one can be merged with. This
/// handles cases where we have multiple invokes with unique landing pads, but
/// a shared handler.
///
/// We specifically choose to not worry about merging non-empty blocks
/// here. That is a PRE/scheduling problem and is best solved elsewhere. In
/// practice, the optimizer produces empty landing pad blocks quite frequently
/// when dealing with exception dense code. (see: instcombine, gvn, if-else
/// sinking in this file)
///
/// This is primarily a code size optimization. We need to avoid performing
/// any transform which might inhibit optimization (such as our ability to
/// specialize a particular handler via tail commoning). We do this by not
/// merging any blocks which require us to introduce a phi. Since the same
/// values are flowing through both blocks, we don't lose any ability to
/// specialize. If anything, we make such specialization more likely.
///
/// TODO - This transformation could remove entries from a phi in the target
/// block when the inputs in the phi are the same for the two blocks being
/// merged. In some cases, this could result in removal of the PHI entirely.
static bool TryToMergeLandingPad(LandingPadInst *LPad, BranchInst *BI,
BasicBlock *BB) {
auto Succ = BB->getUniqueSuccessor();
assert(Succ);
// If there's a phi in the successor block, we'd likely have to introduce
// a phi into the merged landing pad block.
if (isa<PHINode>(*Succ->begin()))
return false;
for (BasicBlock *OtherPred : predecessors(Succ)) {
if (BB == OtherPred)
continue;
BasicBlock::iterator I = OtherPred->begin();
LandingPadInst *LPad2 = dyn_cast<LandingPadInst>(I);
if (!LPad2 || !LPad2->isIdenticalTo(LPad))
continue;
for (++I; isa<DbgInfoIntrinsic>(I); ++I)
;
BranchInst *BI2 = dyn_cast<BranchInst>(I);
if (!BI2 || !BI2->isIdenticalTo(BI))
continue;
// We've found an identical block. Update our predecessors to take that
// path instead and make ourselves dead.
SmallPtrSet<BasicBlock *, 16> Preds;
Preds.insert(pred_begin(BB), pred_end(BB));
for (BasicBlock *Pred : Preds) {
InvokeInst *II = cast<InvokeInst>(Pred->getTerminator());
assert(II->getNormalDest() != BB && II->getUnwindDest() == BB &&
"unexpected successor");
II->setUnwindDest(OtherPred);
}
// The debug info in OtherPred doesn't cover the merged control flow that
// used to go through BB. We need to delete it or update it.
for (auto I = OtherPred->begin(), E = OtherPred->end(); I != E;) {
Instruction &Inst = *I;
I++;
if (isa<DbgInfoIntrinsic>(Inst))
Inst.eraseFromParent();
}
SmallPtrSet<BasicBlock *, 16> Succs;
Succs.insert(succ_begin(BB), succ_end(BB));
for (BasicBlock *Succ : Succs) {
Succ->removePredecessor(BB);
}
IRBuilder<> Builder(BI);
Builder.CreateUnreachable();
BI->eraseFromParent();
return true;
}
return false;
}
bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI,
IRBuilder<> &Builder) {
BasicBlock *BB = BI->getParent();
BasicBlock *Succ = BI->getSuccessor(0);
// If the Terminator is the only non-phi instruction, simplify the block.
// If LoopHeader is provided, check if the block or its successor is a loop
// header. (This is for early invocations before loop simplify and
// vectorization to keep canonical loop forms for nested loops. These blocks
// can be eliminated when the pass is invoked later in the back-end.)
// Note that if BB has only one predecessor then we do not introduce new
// backedge, so we can eliminate BB.
bool NeedCanonicalLoop =
Options.NeedCanonicalLoop &&
(LoopHeaders && BB->hasNPredecessorsOrMore(2) &&
(LoopHeaders->count(BB) || LoopHeaders->count(Succ)));
BasicBlock::iterator I = BB->getFirstNonPHIOrDbg()->getIterator();
if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
!NeedCanonicalLoop && TryToSimplifyUncondBranchFromEmptyBlock(BB))
return true;
// If the only instruction in the block is a seteq/setne comparison against a
// constant, try to simplify the block.
if (ICmpInst *ICI = dyn_cast<ICmpInst>(I))
if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
for (++I; isa<DbgInfoIntrinsic>(I); ++I)
;
if (I->isTerminator() &&
tryToSimplifyUncondBranchWithICmpInIt(ICI, Builder))
return true;
}
// See if we can merge an empty landing pad block with another which is
// equivalent.
if (LandingPadInst *LPad = dyn_cast<LandingPadInst>(I)) {
for (++I; isa<DbgInfoIntrinsic>(I); ++I)
;
if (I->isTerminator() && TryToMergeLandingPad(LPad, BI, BB))
return true;
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and our successor, fold the comparison into the
// predecessor and use logical operations to update the incoming value
// for PHI nodes in common successor.
if (FoldBranchToCommonDest(BI, Options.BonusInstThreshold))
return requestResimplify();
return false;
}
static BasicBlock *allPredecessorsComeFromSameSource(BasicBlock *BB) {
BasicBlock *PredPred = nullptr;
for (auto *P : predecessors(BB)) {
BasicBlock *PPred = P->getSinglePredecessor();
if (!PPred || (PredPred && PredPred != PPred))
return nullptr;
PredPred = PPred;
}
return PredPred;
}
bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
BasicBlock *BB = BI->getParent();
const Function *Fn = BB->getParent();
if (Fn && Fn->hasFnAttribute(Attribute::OptForFuzzing))
return false;
// Conditional branch
if (isValueEqualityComparison(BI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this
// switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder))
return requestResimplify();
// This block must be empty, except for the setcond inst, if it exists.
// Ignore dbg intrinsics.
auto I = BB->instructionsWithoutDebug().begin();
if (&*I == BI) {
if (FoldValueComparisonIntoPredecessors(BI, Builder))
return requestResimplify();
} else if (&*I == cast<Instruction>(BI->getCondition())) {
++I;
if (&*I == BI && FoldValueComparisonIntoPredecessors(BI, Builder))
return requestResimplify();
}
}
// Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction.
if (SimplifyBranchOnICmpChain(BI, Builder, DL))
return true;
// If this basic block has dominating predecessor blocks and the dominating
// blocks' conditions imply BI's condition, we know the direction of BI.
Optional<bool> Imp = isImpliedByDomCondition(BI->getCondition(), BI, DL);
if (Imp) {
// Turn this into a branch on constant.
auto *OldCond = BI->getCondition();
ConstantInt *TorF = *Imp ? ConstantInt::getTrue(BB->getContext())
: ConstantInt::getFalse(BB->getContext());
BI->setCondition(TorF);
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
return requestResimplify();
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.
if (FoldBranchToCommonDest(BI, Options.BonusInstThreshold))
return requestResimplify();
// We have a conditional branch to two blocks that are only reachable
// from BI. We know that the condbr dominates the two blocks, so see if
// there is any identical code in the "then" and "else" blocks. If so, we
// can hoist it up to the branching block.
if (BI->getSuccessor(0)->getSinglePredecessor()) {
if (BI->getSuccessor(1)->getSinglePredecessor()) {
if (HoistThenElseCodeToIf(BI, TTI))
return requestResimplify();
} else {
// If Successor #1 has multiple preds, we may be able to conditionally
// execute Successor #0 if it branches to Successor #1.
Instruction *Succ0TI = BI->getSuccessor(0)->getTerminator();
if (Succ0TI->getNumSuccessors() == 1 &&
Succ0TI->getSuccessor(0) == BI->getSuccessor(1))
if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0), TTI))
return requestResimplify();
}
} else if (BI->getSuccessor(1)->getSinglePredecessor()) {
// If Successor #0 has multiple preds, we may be able to conditionally
// execute Successor #1 if it branches to Successor #0.
Instruction *Succ1TI = BI->getSuccessor(1)->getTerminator();
if (Succ1TI->getNumSuccessors() == 1 &&
Succ1TI->getSuccessor(0) == BI->getSuccessor(0))
if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1), TTI))
return requestResimplify();
}
// If this is a branch on a phi node in the current block, thread control
// through this block if any PHI node entries are constants.
if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
if (PN->getParent() == BI->getParent())
if (FoldCondBranchOnPHI(BI, DL, Options.AC))
return requestResimplify();
// Scan predecessor blocks for conditional branches.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
if (PBI != BI && PBI->isConditional())
if (SimplifyCondBranchToCondBranch(PBI, BI, DL))
return requestResimplify();
// Look for diamond patterns.
if (MergeCondStores)
if (BasicBlock *PrevBB = allPredecessorsComeFromSameSource(BB))
if (BranchInst *PBI = dyn_cast<BranchInst>(PrevBB->getTerminator()))
if (PBI != BI && PBI->isConditional())
if (mergeConditionalStores(PBI, BI, DL))
return requestResimplify();
return false;
}
/// Check if passing a value to an instruction will cause undefined behavior.
static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
Constant *C = dyn_cast<Constant>(V);
if (!C)
return false;
if (I->use_empty())
return false;
if (C->isNullValue() || isa<UndefValue>(C)) {
// Only look at the first use, avoid hurting compile time with long uselists
User *Use = *I->user_begin();
// Now make sure that there are no instructions in between that can alter
// control flow (eg. calls)
for (BasicBlock::iterator
i = ++BasicBlock::iterator(I),
UI = BasicBlock::iterator(dyn_cast<Instruction>(Use));
i != UI; ++i)
if (i == I->getParent()->end() || i->mayHaveSideEffects())
return false;
// Look through GEPs. A load from a GEP derived from NULL is still undefined
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
if (GEP->getPointerOperand() == I)
return passingValueIsAlwaysUndefined(V, GEP);
// Look through bitcasts.
if (BitCastInst *BC = dyn_cast<BitCastInst>(Use))
return passingValueIsAlwaysUndefined(V, BC);
// Load from null is undefined.
if (LoadInst *LI = dyn_cast<LoadInst>(Use))
if (!LI->isVolatile())
return !NullPointerIsDefined(LI->getFunction(),
LI->getPointerAddressSpace());
// Store to null is undefined.
if (StoreInst *SI = dyn_cast<StoreInst>(Use))
if (!SI->isVolatile())
return (!NullPointerIsDefined(SI->getFunction(),
SI->getPointerAddressSpace())) &&
SI->getPointerOperand() == I;
// A call to null is undefined.
if (auto CS = CallSite(Use))
return !NullPointerIsDefined(CS->getFunction()) &&
CS.getCalledValue() == I;
}
return false;
}
/// If BB has an incoming value that will always trigger undefined behavior
/// (eg. null pointer dereference), remove the branch leading here.
static bool removeUndefIntroducingPredecessor(BasicBlock *BB) {
for (PHINode &PHI : BB->phis())
for (unsigned i = 0, e = PHI.getNumIncomingValues(); i != e; ++i)
if (passingValueIsAlwaysUndefined(PHI.getIncomingValue(i), &PHI)) {
Instruction *T = PHI.getIncomingBlock(i)->getTerminator();
IRBuilder<> Builder(T);
if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
BB->removePredecessor(PHI.getIncomingBlock(i));
// Turn uncoditional branches into unreachables and remove the dead
// destination from conditional branches.
if (BI->isUnconditional())
Builder.CreateUnreachable();
else
Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1)
: BI->getSuccessor(0));
BI->eraseFromParent();
return true;
}
// TODO: SwitchInst.
}
return false;
}
bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
bool Changed = false;
assert(BB && BB->getParent() && "Block not embedded in function!");
assert(BB->getTerminator() && "Degenerate basic block encountered!");
// Remove basic blocks that have no predecessors (except the entry block)...
// or that just have themself as a predecessor. These are unreachable.
if ((pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()) ||
BB->getSinglePredecessor() == BB) {
LLVM_DEBUG(dbgs() << "Removing BB: \n" << *BB);
DeleteDeadBlock(BB);
return true;
}
// Check to see if we can constant propagate this terminator instruction
// away...
Changed |= ConstantFoldTerminator(BB, true);
// Check for and eliminate duplicate PHI nodes in this block.
Changed |= EliminateDuplicatePHINodes(BB);
// Check for and remove branches that will always cause undefined behavior.
Changed |= removeUndefIntroducingPredecessor(BB);
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
if (MergeBlockIntoPredecessor(BB))
return true;
if (SinkCommon && Options.SinkCommonInsts)
Changed |= SinkCommonCodeFromPredecessors(BB);
IRBuilder<> Builder(BB);
// If there is a trivial two-entry PHI node in this basic block, and we can
// eliminate it, do so now.
if (auto *PN = dyn_cast<PHINode>(BB->begin()))
if (PN->getNumIncomingValues() == 2)
Changed |= FoldTwoEntryPHINode(PN, TTI, DL);
Builder.SetInsertPoint(BB->getTerminator());
if (auto *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
if (BI->isUnconditional()) {
if (SimplifyUncondBranch(BI, Builder))
return true;
} else {
if (SimplifyCondBranch(BI, Builder))
return true;
}
} else if (auto *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
if (SimplifyReturn(RI, Builder))
return true;
} else if (auto *RI = dyn_cast<ResumeInst>(BB->getTerminator())) {
if (SimplifyResume(RI, Builder))
return true;
} else if (auto *RI = dyn_cast<CleanupReturnInst>(BB->getTerminator())) {
if (SimplifyCleanupReturn(RI))
return true;
} else if (auto *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
if (SimplifySwitch(SI, Builder))
return true;
} else if (auto *UI = dyn_cast<UnreachableInst>(BB->getTerminator())) {
if (SimplifyUnreachable(UI))
return true;
} else if (auto *IBI = dyn_cast<IndirectBrInst>(BB->getTerminator())) {
if (SimplifyIndirectBr(IBI))
return true;
}
return Changed;
}
bool SimplifyCFGOpt::run(BasicBlock *BB) {
bool Changed = false;
// Repeated simplify BB as long as resimplification is requested.
do {
Resimplify = false;
// Perform one round of simplifcation. Resimplify flag will be set if
// another iteration is requested.
Changed |= simplifyOnce(BB);
} while (Resimplify);
return Changed;
}
bool llvm::simplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
const SimplifyCFGOptions &Options,
SmallPtrSetImpl<BasicBlock *> *LoopHeaders) {
return SimplifyCFGOpt(TTI, BB->getModule()->getDataLayout(), LoopHeaders,
Options)
.run(BB);
}
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