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llvm-mirror/lib/Transforms/Scalar/JumpThreading.cpp
Thomas Preud'homme 365b05fe29 Make FindAvailableLoadedValue TBAA aware 
FindAvailableLoadedValue() relies on FindAvailablePtrLoadStore() to run
the alias analysis when searching for an equivalent value. However,
FindAvailablePtrLoadStore() calls the alias analysis framework with a
memory location for the load constructed from an address and a size,
which thus lacks TBAA metadata info. This commit modifies
FindAvailablePtrLoadStore() to accept an optional memory location as
parameter to allow FindAvailableLoadedValue() to create it based on the
load instruction, which would then have TBAA metadata info attached.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D99206
2021-03-24 17:20:26 +00:00

3071 lines
118 KiB
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//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements the Jump Threading pass.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/JumpThreading.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LazyValueInfo.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.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/PassManager.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/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <memory>
#include <utility>
using namespace llvm;
using namespace jumpthreading;
#define DEBUG_TYPE "jump-threading"
STATISTIC(NumThreads, "Number of jumps threaded");
STATISTIC(NumFolds, "Number of terminators folded");
STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
static cl::opt<unsigned>
BBDuplicateThreshold("jump-threading-threshold",
cl::desc("Max block size to duplicate for jump threading"),
cl::init(6), cl::Hidden);
static cl::opt<unsigned>
ImplicationSearchThreshold(
"jump-threading-implication-search-threshold",
cl::desc("The number of predecessors to search for a stronger "
"condition to use to thread over a weaker condition"),
cl::init(3), cl::Hidden);
static cl::opt<bool> PrintLVIAfterJumpThreading(
"print-lvi-after-jump-threading",
cl::desc("Print the LazyValueInfo cache after JumpThreading"), cl::init(false),
cl::Hidden);
static cl::opt<bool> JumpThreadingFreezeSelectCond(
"jump-threading-freeze-select-cond",
cl::desc("Freeze the condition when unfolding select"), cl::init(false),
cl::Hidden);
static cl::opt<bool> ThreadAcrossLoopHeaders(
"jump-threading-across-loop-headers",
cl::desc("Allow JumpThreading to thread across loop headers, for testing"),
cl::init(false), cl::Hidden);
namespace {
/// This pass performs 'jump threading', which looks at blocks that have
/// multiple predecessors and multiple successors. If one or more of the
/// predecessors of the block can be proven to always jump to one of the
/// successors, we forward the edge from the predecessor to the successor by
/// duplicating the contents of this block.
///
/// An example of when this can occur is code like this:
///
/// if () { ...
/// X = 4;
/// }
/// if (X < 3) {
///
/// In this case, the unconditional branch at the end of the first if can be
/// revectored to the false side of the second if.
class JumpThreading : public FunctionPass {
JumpThreadingPass Impl;
public:
static char ID; // Pass identification
JumpThreading(bool InsertFreezeWhenUnfoldingSelect = false, int T = -1)
: FunctionPass(ID), Impl(InsertFreezeWhenUnfoldingSelect, T) {
initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<LazyValueInfoWrapperPass>();
AU.addPreserved<LazyValueInfoWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
}
void releaseMemory() override { Impl.releaseMemory(); }
};
} // end anonymous namespace
char JumpThreading::ID = 0;
INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
"Jump Threading", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(JumpThreading, "jump-threading",
"Jump Threading", false, false)
// Public interface to the Jump Threading pass
FunctionPass *llvm::createJumpThreadingPass(bool InsertFr, int Threshold) {
return new JumpThreading(InsertFr, Threshold);
}
JumpThreadingPass::JumpThreadingPass(bool InsertFr, int T) {
InsertFreezeWhenUnfoldingSelect = JumpThreadingFreezeSelectCond | InsertFr;
DefaultBBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
}
// Update branch probability information according to conditional
// branch probability. This is usually made possible for cloned branches
// in inline instances by the context specific profile in the caller.
// For instance,
//
// [Block PredBB]
// [Branch PredBr]
// if (t) {
// Block A;
// } else {
// Block B;
// }
//
// [Block BB]
// cond = PN([true, %A], [..., %B]); // PHI node
// [Branch CondBr]
// if (cond) {
// ... // P(cond == true) = 1%
// }
//
// Here we know that when block A is taken, cond must be true, which means
// P(cond == true | A) = 1
//
// Given that P(cond == true) = P(cond == true | A) * P(A) +
// P(cond == true | B) * P(B)
// we get:
// P(cond == true ) = P(A) + P(cond == true | B) * P(B)
//
// which gives us:
// P(A) is less than P(cond == true), i.e.
// P(t == true) <= P(cond == true)
//
// In other words, if we know P(cond == true) is unlikely, we know
// that P(t == true) is also unlikely.
//
static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
if (!CondBr)
return;
uint64_t TrueWeight, FalseWeight;
if (!CondBr->extractProfMetadata(TrueWeight, FalseWeight))
return;
if (TrueWeight + FalseWeight == 0)
// Zero branch_weights do not give a hint for getting branch probabilities.
// Technically it would result in division by zero denominator, which is
// TrueWeight + FalseWeight.
return;
// Returns the outgoing edge of the dominating predecessor block
// that leads to the PhiNode's incoming block:
auto GetPredOutEdge =
[](BasicBlock *IncomingBB,
BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> {
auto *PredBB = IncomingBB;
auto *SuccBB = PhiBB;
SmallPtrSet<BasicBlock *, 16> Visited;
while (true) {
BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
if (PredBr && PredBr->isConditional())
return {PredBB, SuccBB};
Visited.insert(PredBB);
auto *SinglePredBB = PredBB->getSinglePredecessor();
if (!SinglePredBB)
return {nullptr, nullptr};
// Stop searching when SinglePredBB has been visited. It means we see
// an unreachable loop.
if (Visited.count(SinglePredBB))
return {nullptr, nullptr};
SuccBB = PredBB;
PredBB = SinglePredBB;
}
};
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *PhiOpnd = PN->getIncomingValue(i);
ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
if (!CI || !CI->getType()->isIntegerTy(1))
continue;
BranchProbability BP =
(CI->isOne() ? BranchProbability::getBranchProbability(
TrueWeight, TrueWeight + FalseWeight)
: BranchProbability::getBranchProbability(
FalseWeight, TrueWeight + FalseWeight));
auto PredOutEdge = GetPredOutEdge(PN->getIncomingBlock(i), BB);
if (!PredOutEdge.first)
return;
BasicBlock *PredBB = PredOutEdge.first;
BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
if (!PredBr)
return;
uint64_t PredTrueWeight, PredFalseWeight;
// FIXME: We currently only set the profile data when it is missing.
// With PGO, this can be used to refine even existing profile data with
// context information. This needs to be done after more performance
// testing.
if (PredBr->extractProfMetadata(PredTrueWeight, PredFalseWeight))
continue;
// We can not infer anything useful when BP >= 50%, because BP is the
// upper bound probability value.
if (BP >= BranchProbability(50, 100))
continue;
SmallVector<uint32_t, 2> Weights;
if (PredBr->getSuccessor(0) == PredOutEdge.second) {
Weights.push_back(BP.getNumerator());
Weights.push_back(BP.getCompl().getNumerator());
} else {
Weights.push_back(BP.getCompl().getNumerator());
Weights.push_back(BP.getNumerator());
}
PredBr->setMetadata(LLVMContext::MD_prof,
MDBuilder(PredBr->getParent()->getContext())
.createBranchWeights(Weights));
}
}
/// runOnFunction - Toplevel algorithm.
bool JumpThreading::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
auto TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
// Jump Threading has no sense for the targets with divergent CF
if (TTI->hasBranchDivergence())
return false;
auto TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
auto DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
auto AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
DomTreeUpdater DTU(*DT, DomTreeUpdater::UpdateStrategy::Lazy);
std::unique_ptr<BlockFrequencyInfo> BFI;
std::unique_ptr<BranchProbabilityInfo> BPI;
if (F.hasProfileData()) {
LoopInfo LI{DominatorTree(F)};
BPI.reset(new BranchProbabilityInfo(F, LI, TLI));
BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
}
bool Changed = Impl.runImpl(F, TLI, LVI, AA, &DTU, F.hasProfileData(),
std::move(BFI), std::move(BPI));
if (PrintLVIAfterJumpThreading) {
dbgs() << "LVI for function '" << F.getName() << "':\n";
LVI->printLVI(F, DTU.getDomTree(), dbgs());
}
return Changed;
}
PreservedAnalyses JumpThreadingPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
// Jump Threading has no sense for the targets with divergent CF
if (TTI.hasBranchDivergence())
return PreservedAnalyses::all();
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &LVI = AM.getResult<LazyValueAnalysis>(F);
auto &AA = AM.getResult<AAManager>(F);
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
std::unique_ptr<BlockFrequencyInfo> BFI;
std::unique_ptr<BranchProbabilityInfo> BPI;
if (F.hasProfileData()) {
LoopInfo LI{DominatorTree(F)};
BPI.reset(new BranchProbabilityInfo(F, LI, &TLI));
BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
}
bool Changed = runImpl(F, &TLI, &LVI, &AA, &DTU, F.hasProfileData(),
std::move(BFI), std::move(BPI));
if (PrintLVIAfterJumpThreading) {
dbgs() << "LVI for function '" << F.getName() << "':\n";
LVI.printLVI(F, DTU.getDomTree(), dbgs());
}
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserve<GlobalsAA>();
PA.preserve<DominatorTreeAnalysis>();
PA.preserve<LazyValueAnalysis>();
return PA;
}
bool JumpThreadingPass::runImpl(Function &F, TargetLibraryInfo *TLI_,
LazyValueInfo *LVI_, AliasAnalysis *AA_,
DomTreeUpdater *DTU_, bool HasProfileData_,
std::unique_ptr<BlockFrequencyInfo> BFI_,
std::unique_ptr<BranchProbabilityInfo> BPI_) {
LLVM_DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
TLI = TLI_;
LVI = LVI_;
AA = AA_;
DTU = DTU_;
BFI.reset();
BPI.reset();
// When profile data is available, we need to update edge weights after
// successful jump threading, which requires both BPI and BFI being available.
HasProfileData = HasProfileData_;
auto *GuardDecl = F.getParent()->getFunction(
Intrinsic::getName(Intrinsic::experimental_guard));
HasGuards = GuardDecl && !GuardDecl->use_empty();
if (HasProfileData) {
BPI = std::move(BPI_);
BFI = std::move(BFI_);
}
// Reduce the number of instructions duplicated when optimizing strictly for
// size.
if (BBDuplicateThreshold.getNumOccurrences())
BBDupThreshold = BBDuplicateThreshold;
else if (F.hasFnAttribute(Attribute::MinSize))
BBDupThreshold = 3;
else
BBDupThreshold = DefaultBBDupThreshold;
// JumpThreading must not processes blocks unreachable from entry. It's a
// waste of compute time and can potentially lead to hangs.
SmallPtrSet<BasicBlock *, 16> Unreachable;
assert(DTU && "DTU isn't passed into JumpThreading before using it.");
assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to proceed.");
DominatorTree &DT = DTU->getDomTree();
for (auto &BB : F)
if (!DT.isReachableFromEntry(&BB))
Unreachable.insert(&BB);
if (!ThreadAcrossLoopHeaders)
findLoopHeaders(F);
bool EverChanged = false;
bool Changed;
do {
Changed = false;
for (auto &BB : F) {
if (Unreachable.count(&BB))
continue;
while (processBlock(&BB)) // Thread all of the branches we can over BB.
Changed = true;
// Jump threading may have introduced redundant debug values into BB
// which should be removed.
// Remove redundant pseudo probes as well.
if (Changed)
RemoveRedundantDbgInstrs(&BB, true);
// Stop processing BB if it's the entry or is now deleted. The following
// routines attempt to eliminate BB and locating a suitable replacement
// for the entry is non-trivial.
if (&BB == &F.getEntryBlock() || DTU->isBBPendingDeletion(&BB))
continue;
if (pred_empty(&BB)) {
// When processBlock makes BB unreachable it doesn't bother to fix up
// the instructions in it. We must remove BB to prevent invalid IR.
LLVM_DEBUG(dbgs() << " JT: Deleting dead block '" << BB.getName()
<< "' with terminator: " << *BB.getTerminator()
<< '\n');
LoopHeaders.erase(&BB);
LVI->eraseBlock(&BB);
DeleteDeadBlock(&BB, DTU);
Changed = true;
continue;
}
// processBlock doesn't thread BBs with unconditional TIs. However, if BB
// is "almost empty", we attempt to merge BB with its sole successor.
auto *BI = dyn_cast<BranchInst>(BB.getTerminator());
if (BI && BI->isUnconditional()) {
BasicBlock *Succ = BI->getSuccessor(0);
if (
// The terminator must be the only non-phi instruction in BB.
BB.getFirstNonPHIOrDbg(true)->isTerminator() &&
// Don't alter Loop headers and latches to ensure another pass can
// detect and transform nested loops later.
!LoopHeaders.count(&BB) && !LoopHeaders.count(Succ) &&
TryToSimplifyUncondBranchFromEmptyBlock(&BB, DTU)) {
RemoveRedundantDbgInstrs(Succ, true);
// BB is valid for cleanup here because we passed in DTU. F remains
// BB's parent until a DTU->getDomTree() event.
LVI->eraseBlock(&BB);
Changed = true;
}
}
}
EverChanged |= Changed;
} while (Changed);
LoopHeaders.clear();
return EverChanged;
}
// Replace uses of Cond with ToVal when safe to do so. If all uses are
// replaced, we can remove Cond. We cannot blindly replace all uses of Cond
// because we may incorrectly replace uses when guards/assumes are uses of
// of `Cond` and we used the guards/assume to reason about the `Cond` value
// at the end of block. RAUW unconditionally replaces all uses
// including the guards/assumes themselves and the uses before the
// guard/assume.
static void replaceFoldableUses(Instruction *Cond, Value *ToVal) {
assert(Cond->getType() == ToVal->getType());
auto *BB = Cond->getParent();
// We can unconditionally replace all uses in non-local blocks (i.e. uses
// strictly dominated by BB), since LVI information is true from the
// terminator of BB.
replaceNonLocalUsesWith(Cond, ToVal);
for (Instruction &I : reverse(*BB)) {
// Reached the Cond whose uses we are trying to replace, so there are no
// more uses.
if (&I == Cond)
break;
// We only replace uses in instructions that are guaranteed to reach the end
// of BB, where we know Cond is ToVal.
if (!isGuaranteedToTransferExecutionToSuccessor(&I))
break;
I.replaceUsesOfWith(Cond, ToVal);
}
if (Cond->use_empty() && !Cond->mayHaveSideEffects())
Cond->eraseFromParent();
}
/// Return the cost of duplicating a piece of this block from first non-phi
/// and before StopAt instruction to thread across it. Stop scanning the block
/// when exceeding the threshold. If duplication is impossible, returns ~0U.
static unsigned getJumpThreadDuplicationCost(BasicBlock *BB,
Instruction *StopAt,
unsigned Threshold) {
assert(StopAt->getParent() == BB && "Not an instruction from proper BB?");
/// Ignore PHI nodes, these will be flattened when duplication happens.
BasicBlock::const_iterator I(BB->getFirstNonPHI());
// FIXME: THREADING will delete values that are just used to compute the
// branch, so they shouldn't count against the duplication cost.
unsigned Bonus = 0;
if (BB->getTerminator() == StopAt) {
// Threading through a switch statement is particularly profitable. If this
// block ends in a switch, decrease its cost to make it more likely to
// happen.
if (isa<SwitchInst>(StopAt))
Bonus = 6;
// The same holds for indirect branches, but slightly more so.
if (isa<IndirectBrInst>(StopAt))
Bonus = 8;
}
// Bump the threshold up so the early exit from the loop doesn't skip the
// terminator-based Size adjustment at the end.
Threshold += Bonus;
// Sum up the cost of each instruction until we get to the terminator. Don't
// include the terminator because the copy won't include it.
unsigned Size = 0;
for (; &*I != StopAt; ++I) {
// Stop scanning the block if we've reached the threshold.
if (Size > Threshold)
return Size;
// Debugger intrinsics don't incur code size.
if (isa<DbgInfoIntrinsic>(I)) continue;
// Pseudo-probes don't incur code size.
if (isa<PseudoProbeInst>(I))
continue;
// If this is a pointer->pointer bitcast, it is free.
if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
continue;
// Freeze instruction is free, too.
if (isa<FreezeInst>(I))
continue;
// Bail out if this instruction gives back a token type, it is not possible
// to duplicate it if it is used outside this BB.
if (I->getType()->isTokenTy() && I->isUsedOutsideOfBlock(BB))
return ~0U;
// All other instructions count for at least one unit.
++Size;
// Calls are more expensive. If they are non-intrinsic calls, we model them
// as having cost of 4. If they are a non-vector intrinsic, we model them
// as having cost of 2 total, and if they are a vector intrinsic, we model
// them as having cost 1.
if (const CallInst *CI = dyn_cast<CallInst>(I)) {
if (CI->cannotDuplicate() || CI->isConvergent())
// Blocks with NoDuplicate are modelled as having infinite cost, so they
// are never duplicated.
return ~0U;
else if (!isa<IntrinsicInst>(CI))
Size += 3;
else if (!CI->getType()->isVectorTy())
Size += 1;
}
}
return Size > Bonus ? Size - Bonus : 0;
}
/// findLoopHeaders - We do not want jump threading to turn proper loop
/// structures into irreducible loops. Doing this breaks up the loop nesting
/// hierarchy and pessimizes later transformations. To prevent this from
/// happening, we first have to find the loop headers. Here we approximate this
/// by finding targets of backedges in the CFG.
///
/// Note that there definitely are cases when we want to allow threading of
/// edges across a loop header. For example, threading a jump from outside the
/// loop (the preheader) to an exit block of the loop is definitely profitable.
/// It is also almost always profitable to thread backedges from within the loop
/// to exit blocks, and is often profitable to thread backedges to other blocks
/// within the loop (forming a nested loop). This simple analysis is not rich
/// enough to track all of these properties and keep it up-to-date as the CFG
/// mutates, so we don't allow any of these transformations.
void JumpThreadingPass::findLoopHeaders(Function &F) {
SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
FindFunctionBackedges(F, Edges);
for (const auto &Edge : Edges)
LoopHeaders.insert(Edge.second);
}
/// getKnownConstant - Helper method to determine if we can thread over a
/// terminator with the given value as its condition, and if so what value to
/// use for that. What kind of value this is depends on whether we want an
/// integer or a block address, but an undef is always accepted.
/// Returns null if Val is null or not an appropriate constant.
static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
if (!Val)
return nullptr;
// Undef is "known" enough.
if (UndefValue *U = dyn_cast<UndefValue>(Val))
return U;
if (Preference == WantBlockAddress)
return dyn_cast<BlockAddress>(Val->stripPointerCasts());
return dyn_cast<ConstantInt>(Val);
}
/// computeValueKnownInPredecessors - Given a basic block BB and a value V, see
/// if we can infer that the value is a known ConstantInt/BlockAddress or undef
/// in any of our predecessors. If so, return the known list of value and pred
/// BB in the result vector.
///
/// This returns true if there were any known values.
bool JumpThreadingPass::computeValueKnownInPredecessorsImpl(
Value *V, BasicBlock *BB, PredValueInfo &Result,
ConstantPreference Preference, DenseSet<Value *> &RecursionSet,
Instruction *CxtI) {
// This method walks up use-def chains recursively. Because of this, we could
// get into an infinite loop going around loops in the use-def chain. To
// prevent this, keep track of what (value, block) pairs we've already visited
// and terminate the search if we loop back to them
if (!RecursionSet.insert(V).second)
return false;
// If V is a constant, then it is known in all predecessors.
if (Constant *KC = getKnownConstant(V, Preference)) {
for (BasicBlock *Pred : predecessors(BB))
Result.emplace_back(KC, Pred);
return !Result.empty();
}
// If V is a non-instruction value, or an instruction in a different block,
// then it can't be derived from a PHI.
Instruction *I = dyn_cast<Instruction>(V);
if (!I || I->getParent() != BB) {
// Okay, if this is a live-in value, see if it has a known value at the end
// of any of our predecessors.
//
// FIXME: This should be an edge property, not a block end property.
/// TODO: Per PR2563, we could infer value range information about a
/// predecessor based on its terminator.
//
// FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
// "I" is a non-local compare-with-a-constant instruction. This would be
// able to handle value inequalities better, for example if the compare is
// "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
// Perhaps getConstantOnEdge should be smart enough to do this?
for (BasicBlock *P : predecessors(BB)) {
// If the value is known by LazyValueInfo to be a constant in a
// predecessor, use that information to try to thread this block.
Constant *PredCst = LVI->getConstantOnEdge(V, P, BB, CxtI);
if (Constant *KC = getKnownConstant(PredCst, Preference))
Result.emplace_back(KC, P);
}
return !Result.empty();
}
/// If I is a PHI node, then we know the incoming values for any constants.
if (PHINode *PN = dyn_cast<PHINode>(I)) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *InVal = PN->getIncomingValue(i);
if (Constant *KC = getKnownConstant(InVal, Preference)) {
Result.emplace_back(KC, PN->getIncomingBlock(i));
} else {
Constant *CI = LVI->getConstantOnEdge(InVal,
PN->getIncomingBlock(i),
BB, CxtI);
if (Constant *KC = getKnownConstant(CI, Preference))
Result.emplace_back(KC, PN->getIncomingBlock(i));
}
}
return !Result.empty();
}
// Handle Cast instructions.
if (CastInst *CI = dyn_cast<CastInst>(I)) {
Value *Source = CI->getOperand(0);
computeValueKnownInPredecessorsImpl(Source, BB, Result, Preference,
RecursionSet, CxtI);
if (Result.empty())
return false;
// Convert the known values.
for (auto &R : Result)
R.first = ConstantExpr::getCast(CI->getOpcode(), R.first, CI->getType());
return true;
}
if (FreezeInst *FI = dyn_cast<FreezeInst>(I)) {
Value *Source = FI->getOperand(0);
computeValueKnownInPredecessorsImpl(Source, BB, Result, Preference,
RecursionSet, CxtI);
erase_if(Result, [](auto &Pair) {
return !isGuaranteedNotToBeUndefOrPoison(Pair.first);
});
return !Result.empty();
}
// Handle some boolean conditions.
if (I->getType()->getPrimitiveSizeInBits() == 1) {
using namespace PatternMatch;
assert(Preference == WantInteger && "One-bit non-integer type?");
// X | true -> true
// X & false -> false
Value *Op0, *Op1;
if (match(I, m_LogicalOr(m_Value(Op0), m_Value(Op1))) ||
match(I, m_LogicalAnd(m_Value(Op0), m_Value(Op1)))) {
PredValueInfoTy LHSVals, RHSVals;
computeValueKnownInPredecessorsImpl(Op0, BB, LHSVals, WantInteger,
RecursionSet, CxtI);
computeValueKnownInPredecessorsImpl(Op1, BB, RHSVals, WantInteger,
RecursionSet, CxtI);
if (LHSVals.empty() && RHSVals.empty())
return false;
ConstantInt *InterestingVal;
if (match(I, m_LogicalOr()))
InterestingVal = ConstantInt::getTrue(I->getContext());
else
InterestingVal = ConstantInt::getFalse(I->getContext());
SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
// Scan for the sentinel. If we find an undef, force it to the
// interesting value: x|undef -> true and x&undef -> false.
for (const auto &LHSVal : LHSVals)
if (LHSVal.first == InterestingVal || isa<UndefValue>(LHSVal.first)) {
Result.emplace_back(InterestingVal, LHSVal.second);
LHSKnownBBs.insert(LHSVal.second);
}
for (const auto &RHSVal : RHSVals)
if (RHSVal.first == InterestingVal || isa<UndefValue>(RHSVal.first)) {
// If we already inferred a value for this block on the LHS, don't
// re-add it.
if (!LHSKnownBBs.count(RHSVal.second))
Result.emplace_back(InterestingVal, RHSVal.second);
}
return !Result.empty();
}
// Handle the NOT form of XOR.
if (I->getOpcode() == Instruction::Xor &&
isa<ConstantInt>(I->getOperand(1)) &&
cast<ConstantInt>(I->getOperand(1))->isOne()) {
computeValueKnownInPredecessorsImpl(I->getOperand(0), BB, Result,
WantInteger, RecursionSet, CxtI);
if (Result.empty())
return false;
// Invert the known values.
for (auto &R : Result)
R.first = ConstantExpr::getNot(R.first);
return true;
}
// Try to simplify some other binary operator values.
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
assert(Preference != WantBlockAddress
&& "A binary operator creating a block address?");
if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
PredValueInfoTy LHSVals;
computeValueKnownInPredecessorsImpl(BO->getOperand(0), BB, LHSVals,
WantInteger, RecursionSet, CxtI);
// Try to use constant folding to simplify the binary operator.
for (const auto &LHSVal : LHSVals) {
Constant *V = LHSVal.first;
Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.emplace_back(KC, LHSVal.second);
}
}
return !Result.empty();
}
// Handle compare with phi operand, where the PHI is defined in this block.
if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
assert(Preference == WantInteger && "Compares only produce integers");
Type *CmpType = Cmp->getType();
Value *CmpLHS = Cmp->getOperand(0);
Value *CmpRHS = Cmp->getOperand(1);
CmpInst::Predicate Pred = Cmp->getPredicate();
PHINode *PN = dyn_cast<PHINode>(CmpLHS);
if (!PN)
PN = dyn_cast<PHINode>(CmpRHS);
if (PN && PN->getParent() == BB) {
const DataLayout &DL = PN->getModule()->getDataLayout();
// We can do this simplification if any comparisons fold to true or false.
// See if any do.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
Value *LHS, *RHS;
if (PN == CmpLHS) {
LHS = PN->getIncomingValue(i);
RHS = CmpRHS->DoPHITranslation(BB, PredBB);
} else {
LHS = CmpLHS->DoPHITranslation(BB, PredBB);
RHS = PN->getIncomingValue(i);
}
Value *Res = SimplifyCmpInst(Pred, LHS, RHS, {DL});
if (!Res) {
if (!isa<Constant>(RHS))
continue;
// getPredicateOnEdge call will make no sense if LHS is defined in BB.
auto LHSInst = dyn_cast<Instruction>(LHS);
if (LHSInst && LHSInst->getParent() == BB)
continue;
LazyValueInfo::Tristate
ResT = LVI->getPredicateOnEdge(Pred, LHS,
cast<Constant>(RHS), PredBB, BB,
CxtI ? CxtI : Cmp);
if (ResT == LazyValueInfo::Unknown)
continue;
Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
}
if (Constant *KC = getKnownConstant(Res, WantInteger))
Result.emplace_back(KC, PredBB);
}
return !Result.empty();
}
// If comparing a live-in value against a constant, see if we know the
// live-in value on any predecessors.
if (isa<Constant>(CmpRHS) && !CmpType->isVectorTy()) {
Constant *CmpConst = cast<Constant>(CmpRHS);
if (!isa<Instruction>(CmpLHS) ||
cast<Instruction>(CmpLHS)->getParent() != BB) {
for (BasicBlock *P : predecessors(BB)) {
// If the value is known by LazyValueInfo to be a constant in a
// predecessor, use that information to try to thread this block.
LazyValueInfo::Tristate Res =
LVI->getPredicateOnEdge(Pred, CmpLHS,
CmpConst, P, BB, CxtI ? CxtI : Cmp);
if (Res == LazyValueInfo::Unknown)
continue;
Constant *ResC = ConstantInt::get(CmpType, Res);
Result.emplace_back(ResC, P);
}
return !Result.empty();
}
// InstCombine can fold some forms of constant range checks into
// (icmp (add (x, C1)), C2). See if we have we have such a thing with
// x as a live-in.
{
using namespace PatternMatch;
Value *AddLHS;
ConstantInt *AddConst;
if (isa<ConstantInt>(CmpConst) &&
match(CmpLHS, m_Add(m_Value(AddLHS), m_ConstantInt(AddConst)))) {
if (!isa<Instruction>(AddLHS) ||
cast<Instruction>(AddLHS)->getParent() != BB) {
for (BasicBlock *P : predecessors(BB)) {
// If the value is known by LazyValueInfo to be a ConstantRange in
// a predecessor, use that information to try to thread this
// block.
ConstantRange CR = LVI->getConstantRangeOnEdge(
AddLHS, P, BB, CxtI ? CxtI : cast<Instruction>(CmpLHS));
// Propagate the range through the addition.
CR = CR.add(AddConst->getValue());
// Get the range where the compare returns true.
ConstantRange CmpRange = ConstantRange::makeExactICmpRegion(
Pred, cast<ConstantInt>(CmpConst)->getValue());
Constant *ResC;
if (CmpRange.contains(CR))
ResC = ConstantInt::getTrue(CmpType);
else if (CmpRange.inverse().contains(CR))
ResC = ConstantInt::getFalse(CmpType);
else
continue;
Result.emplace_back(ResC, P);
}
return !Result.empty();
}
}
}
// Try to find a constant value for the LHS of a comparison,
// and evaluate it statically if we can.
PredValueInfoTy LHSVals;
computeValueKnownInPredecessorsImpl(I->getOperand(0), BB, LHSVals,
WantInteger, RecursionSet, CxtI);
for (const auto &LHSVal : LHSVals) {
Constant *V = LHSVal.first;
Constant *Folded = ConstantExpr::getCompare(Pred, V, CmpConst);
if (Constant *KC = getKnownConstant(Folded, WantInteger))
Result.emplace_back(KC, LHSVal.second);
}
return !Result.empty();
}
}
if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
// Handle select instructions where at least one operand is a known constant
// and we can figure out the condition value for any predecessor block.
Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
PredValueInfoTy Conds;
if ((TrueVal || FalseVal) &&
computeValueKnownInPredecessorsImpl(SI->getCondition(), BB, Conds,
WantInteger, RecursionSet, CxtI)) {
for (auto &C : Conds) {
Constant *Cond = C.first;
// Figure out what value to use for the condition.
bool KnownCond;
if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
// A known boolean.
KnownCond = CI->isOne();
} else {
assert(isa<UndefValue>(Cond) && "Unexpected condition value");
// Either operand will do, so be sure to pick the one that's a known
// constant.
// FIXME: Do this more cleverly if both values are known constants?
KnownCond = (TrueVal != nullptr);
}
// See if the select has a known constant value for this predecessor.
if (Constant *Val = KnownCond ? TrueVal : FalseVal)
Result.emplace_back(Val, C.second);
}
return !Result.empty();
}
}
// If all else fails, see if LVI can figure out a constant value for us.
assert(CxtI->getParent() == BB && "CxtI should be in BB");
Constant *CI = LVI->getConstant(V, CxtI);
if (Constant *KC = getKnownConstant(CI, Preference)) {
for (BasicBlock *Pred : predecessors(BB))
Result.emplace_back(KC, Pred);
}
return !Result.empty();
}
/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
/// in an undefined jump, decide which block is best to revector to.
///
/// Since we can pick an arbitrary destination, we pick the successor with the
/// fewest predecessors. This should reduce the in-degree of the others.
static unsigned getBestDestForJumpOnUndef(BasicBlock *BB) {
Instruction *BBTerm = BB->getTerminator();
unsigned MinSucc = 0;
BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
// Compute the successor with the minimum number of predecessors.
unsigned MinNumPreds = pred_size(TestBB);
for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
TestBB = BBTerm->getSuccessor(i);
unsigned NumPreds = pred_size(TestBB);
if (NumPreds < MinNumPreds) {
MinSucc = i;
MinNumPreds = NumPreds;
}
}
return MinSucc;
}
static bool hasAddressTakenAndUsed(BasicBlock *BB) {
if (!BB->hasAddressTaken()) return false;
// If the block has its address taken, it may be a tree of dead constants
// hanging off of it. These shouldn't keep the block alive.
BlockAddress *BA = BlockAddress::get(BB);
BA->removeDeadConstantUsers();
return !BA->use_empty();
}
/// processBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreadingPass::processBlock(BasicBlock *BB) {
// If the block is trivially dead, just return and let the caller nuke it.
// This simplifies other transformations.
if (DTU->isBBPendingDeletion(BB) ||
(pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()))
return false;
// If this block has a single predecessor, and if that pred has a single
// successor, merge the blocks. This encourages recursive jump threading
// because now the condition in this block can be threaded through
// predecessors of our predecessor block.
if (maybeMergeBasicBlockIntoOnlyPred(BB))
return true;
if (tryToUnfoldSelectInCurrBB(BB))
return true;
// Look if we can propagate guards to predecessors.
if (HasGuards && processGuards(BB))
return true;
// What kind of constant we're looking for.
ConstantPreference Preference = WantInteger;
// Look to see if the terminator is a conditional branch, switch or indirect
// branch, if not we can't thread it.
Value *Condition;
Instruction *Terminator = BB->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
// Can't thread an unconditional jump.
if (BI->isUnconditional()) return false;
Condition = BI->getCondition();
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
Condition = SI->getCondition();
} else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
// Can't thread indirect branch with no successors.
if (IB->getNumSuccessors() == 0) return false;
Condition = IB->getAddress()->stripPointerCasts();
Preference = WantBlockAddress;
} else {
return false; // Must be an invoke or callbr.
}
// Keep track if we constant folded the condition in this invocation.
bool ConstantFolded = false;
// Run constant folding to see if we can reduce the condition to a simple
// constant.
if (Instruction *I = dyn_cast<Instruction>(Condition)) {
Value *SimpleVal =
ConstantFoldInstruction(I, BB->getModule()->getDataLayout(), TLI);
if (SimpleVal) {
I->replaceAllUsesWith(SimpleVal);
if (isInstructionTriviallyDead(I, TLI))
I->eraseFromParent();
Condition = SimpleVal;
ConstantFolded = true;
}
}
// If the terminator is branching on an undef or freeze undef, we can pick any
// of the successors to branch to. Let getBestDestForJumpOnUndef decide.
auto *FI = dyn_cast<FreezeInst>(Condition);
if (isa<UndefValue>(Condition) ||
(FI && isa<UndefValue>(FI->getOperand(0)) && FI->hasOneUse())) {
unsigned BestSucc = getBestDestForJumpOnUndef(BB);
std::vector<DominatorTree::UpdateType> Updates;
// Fold the branch/switch.
Instruction *BBTerm = BB->getTerminator();
Updates.reserve(BBTerm->getNumSuccessors());
for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
if (i == BestSucc) continue;
BasicBlock *Succ = BBTerm->getSuccessor(i);
Succ->removePredecessor(BB, true);
Updates.push_back({DominatorTree::Delete, BB, Succ});
}
LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
<< "' folding undef terminator: " << *BBTerm << '\n');
BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
BBTerm->eraseFromParent();
DTU->applyUpdatesPermissive(Updates);
if (FI)
FI->eraseFromParent();
return true;
}
// If the terminator of this block is branching on a constant, simplify the
// terminator to an unconditional branch. This can occur due to threading in
// other blocks.
if (getKnownConstant(Condition, Preference)) {
LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
<< "' folding terminator: " << *BB->getTerminator()
<< '\n');
++NumFolds;
ConstantFoldTerminator(BB, true, nullptr, DTU);
if (HasProfileData)
BPI->eraseBlock(BB);
return true;
}
Instruction *CondInst = dyn_cast<Instruction>(Condition);
// All the rest of our checks depend on the condition being an instruction.
if (!CondInst) {
// FIXME: Unify this with code below.
if (processThreadableEdges(Condition, BB, Preference, Terminator))
return true;
return ConstantFolded;
}
if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
// If we're branching on a conditional, LVI might be able to determine
// it's value at the branch instruction. We only handle comparisons
// against a constant at this time.
// TODO: This should be extended to handle switches as well.
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
if (CondBr && CondConst) {
// We should have returned as soon as we turn a conditional branch to
// unconditional. Because its no longer interesting as far as jump
// threading is concerned.
assert(CondBr->isConditional() && "Threading on unconditional terminator");
LazyValueInfo::Tristate Ret =
LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
CondConst, CondBr);
if (Ret != LazyValueInfo::Unknown) {
unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
BasicBlock *ToRemoveSucc = CondBr->getSuccessor(ToRemove);
ToRemoveSucc->removePredecessor(BB, true);
BranchInst *UncondBr =
BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
UncondBr->setDebugLoc(CondBr->getDebugLoc());
CondBr->eraseFromParent();
if (CondCmp->use_empty())
CondCmp->eraseFromParent();
// We can safely replace *some* uses of the CondInst if it has
// exactly one value as returned by LVI. RAUW is incorrect in the
// presence of guards and assumes, that have the `Cond` as the use. This
// is because we use the guards/assume to reason about the `Cond` value
// at the end of block, but RAUW unconditionally replaces all uses
// including the guards/assumes themselves and the uses before the
// guard/assume.
else if (CondCmp->getParent() == BB) {
auto *CI = Ret == LazyValueInfo::True ?
ConstantInt::getTrue(CondCmp->getType()) :
ConstantInt::getFalse(CondCmp->getType());
replaceFoldableUses(CondCmp, CI);
}
DTU->applyUpdatesPermissive(
{{DominatorTree::Delete, BB, ToRemoveSucc}});
if (HasProfileData)
BPI->eraseBlock(BB);
return true;
}
// We did not manage to simplify this branch, try to see whether
// CondCmp depends on a known phi-select pattern.
if (tryToUnfoldSelect(CondCmp, BB))
return true;
}
}
if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
if (tryToUnfoldSelect(SI, BB))
return true;
// Check for some cases that are worth simplifying. Right now we want to look
// for loads that are used by a switch or by the condition for the branch. If
// we see one, check to see if it's partially redundant. If so, insert a PHI
// which can then be used to thread the values.
Value *SimplifyValue = CondInst;
if (auto *FI = dyn_cast<FreezeInst>(SimplifyValue))
// Look into freeze's operand
SimplifyValue = FI->getOperand(0);
if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
if (isa<Constant>(CondCmp->getOperand(1)))
SimplifyValue = CondCmp->getOperand(0);
// TODO: There are other places where load PRE would be profitable, such as
// more complex comparisons.
if (LoadInst *LoadI = dyn_cast<LoadInst>(SimplifyValue))
if (simplifyPartiallyRedundantLoad(LoadI))
return true;
// Before threading, try to propagate profile data backwards:
if (PHINode *PN = dyn_cast<PHINode>(CondInst))
if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
updatePredecessorProfileMetadata(PN, BB);
// Handle a variety of cases where we are branching on something derived from
// a PHI node in the current block. If we can prove that any predecessors
// compute a predictable value based on a PHI node, thread those predecessors.
if (processThreadableEdges(CondInst, BB, Preference, Terminator))
return true;
// If this is an otherwise-unfoldable branch on a phi node or freeze(phi) in
// the current block, see if we can simplify.
PHINode *PN = dyn_cast<PHINode>(
isa<FreezeInst>(CondInst) ? cast<FreezeInst>(CondInst)->getOperand(0)
: CondInst);
if (PN && PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return processBranchOnPHI(PN);
// If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
if (CondInst->getOpcode() == Instruction::Xor &&
CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return processBranchOnXOR(cast<BinaryOperator>(CondInst));
// Search for a stronger dominating condition that can be used to simplify a
// conditional branch leaving BB.
if (processImpliedCondition(BB))
return true;
return false;
}
bool JumpThreadingPass::processImpliedCondition(BasicBlock *BB) {
auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional())
return false;
Value *Cond = BI->getCondition();
BasicBlock *CurrentBB = BB;
BasicBlock *CurrentPred = BB->getSinglePredecessor();
unsigned Iter = 0;
auto &DL = BB->getModule()->getDataLayout();
while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator());
if (!PBI || !PBI->isConditional())
return false;
if (PBI->getSuccessor(0) != CurrentBB && PBI->getSuccessor(1) != CurrentBB)
return false;
bool CondIsTrue = PBI->getSuccessor(0) == CurrentBB;
Optional<bool> Implication =
isImpliedCondition(PBI->getCondition(), Cond, DL, CondIsTrue);
if (Implication) {
BasicBlock *KeepSucc = BI->getSuccessor(*Implication ? 0 : 1);
BasicBlock *RemoveSucc = BI->getSuccessor(*Implication ? 1 : 0);
RemoveSucc->removePredecessor(BB);
BranchInst *UncondBI = BranchInst::Create(KeepSucc, BI);
UncondBI->setDebugLoc(BI->getDebugLoc());
BI->eraseFromParent();
DTU->applyUpdatesPermissive({{DominatorTree::Delete, BB, RemoveSucc}});
if (HasProfileData)
BPI->eraseBlock(BB);
return true;
}
CurrentBB = CurrentPred;
CurrentPred = CurrentBB->getSinglePredecessor();
}
return false;
}
/// Return true if Op is an instruction defined in the given block.
static bool isOpDefinedInBlock(Value *Op, BasicBlock *BB) {
if (Instruction *OpInst = dyn_cast<Instruction>(Op))
if (OpInst->getParent() == BB)
return true;
return false;
}
/// simplifyPartiallyRedundantLoad - If LoadI is an obviously partially
/// redundant load instruction, eliminate it by replacing it with a PHI node.
/// This is an important optimization that encourages jump threading, and needs
/// to be run interlaced with other jump threading tasks.
bool JumpThreadingPass::simplifyPartiallyRedundantLoad(LoadInst *LoadI) {
// Don't hack volatile and ordered loads.
if (!LoadI->isUnordered()) return false;
// If the load is defined in a block with exactly one predecessor, it can't be
// partially redundant.
BasicBlock *LoadBB = LoadI->getParent();
if (LoadBB->getSinglePredecessor())
return false;
// If the load is defined in an EH pad, it can't be partially redundant,
// because the edges between the invoke and the EH pad cannot have other
// instructions between them.
if (LoadBB->isEHPad())
return false;
Value *LoadedPtr = LoadI->getOperand(0);
// If the loaded operand is defined in the LoadBB and its not a phi,
// it can't be available in predecessors.
if (isOpDefinedInBlock(LoadedPtr, LoadBB) && !isa<PHINode>(LoadedPtr))
return false;
// Scan a few instructions up from the load, to see if it is obviously live at
// the entry to its block.
BasicBlock::iterator BBIt(LoadI);
bool IsLoadCSE;
if (Value *AvailableVal = FindAvailableLoadedValue(
LoadI, LoadBB, BBIt, DefMaxInstsToScan, AA, &IsLoadCSE)) {
// If the value of the load is locally available within the block, just use
// it. This frequently occurs for reg2mem'd allocas.
if (IsLoadCSE) {
LoadInst *NLoadI = cast<LoadInst>(AvailableVal);
combineMetadataForCSE(NLoadI, LoadI, false);
};
// If the returned value is the load itself, replace with an undef. This can
// only happen in dead loops.
if (AvailableVal == LoadI)
AvailableVal = UndefValue::get(LoadI->getType());
if (AvailableVal->getType() != LoadI->getType())
AvailableVal = CastInst::CreateBitOrPointerCast(
AvailableVal, LoadI->getType(), "", LoadI);
LoadI->replaceAllUsesWith(AvailableVal);
LoadI->eraseFromParent();
return true;
}
// Otherwise, if we scanned the whole block and got to the top of the block,
// we know the block is locally transparent to the load. If not, something
// might clobber its value.
if (BBIt != LoadBB->begin())
return false;
// If all of the loads and stores that feed the value have the same AA tags,
// then we can propagate them onto any newly inserted loads.
AAMDNodes AATags;
LoadI->getAAMetadata(AATags);
SmallPtrSet<BasicBlock*, 8> PredsScanned;
using AvailablePredsTy = SmallVector<std::pair<BasicBlock *, Value *>, 8>;
AvailablePredsTy AvailablePreds;
BasicBlock *OneUnavailablePred = nullptr;
SmallVector<LoadInst*, 8> CSELoads;
// If we got here, the loaded value is transparent through to the start of the
// block. Check to see if it is available in any of the predecessor blocks.
for (BasicBlock *PredBB : predecessors(LoadBB)) {
// If we already scanned this predecessor, skip it.
if (!PredsScanned.insert(PredBB).second)
continue;
BBIt = PredBB->end();
unsigned NumScanedInst = 0;
Value *PredAvailable = nullptr;
// NOTE: We don't CSE load that is volatile or anything stronger than
// unordered, that should have been checked when we entered the function.
assert(LoadI->isUnordered() &&
"Attempting to CSE volatile or atomic loads");
// If this is a load on a phi pointer, phi-translate it and search
// for available load/store to the pointer in predecessors.
Type *AccessTy = LoadI->getType();
const auto &DL = LoadI->getModule()->getDataLayout();
MemoryLocation Loc(LoadedPtr->DoPHITranslation(LoadBB, PredBB),
LocationSize::precise(DL.getTypeStoreSize(AccessTy)),
AATags);
PredAvailable = findAvailablePtrLoadStore(Loc, AccessTy, LoadI->isAtomic(),
PredBB, BBIt, DefMaxInstsToScan,
AA, &IsLoadCSE, &NumScanedInst);
// If PredBB has a single predecessor, continue scanning through the
// single predecessor.
BasicBlock *SinglePredBB = PredBB;
while (!PredAvailable && SinglePredBB && BBIt == SinglePredBB->begin() &&
NumScanedInst < DefMaxInstsToScan) {
SinglePredBB = SinglePredBB->getSinglePredecessor();
if (SinglePredBB) {
BBIt = SinglePredBB->end();
PredAvailable = findAvailablePtrLoadStore(
Loc, AccessTy, LoadI->isAtomic(), SinglePredBB, BBIt,
(DefMaxInstsToScan - NumScanedInst), AA, &IsLoadCSE,
&NumScanedInst);
}
}
if (!PredAvailable) {
OneUnavailablePred = PredBB;
continue;
}
if (IsLoadCSE)
CSELoads.push_back(cast<LoadInst>(PredAvailable));
// If so, this load is partially redundant. Remember this info so that we
// can create a PHI node.
AvailablePreds.emplace_back(PredBB, PredAvailable);
}
// If the loaded value isn't available in any predecessor, it isn't partially
// redundant.
if (AvailablePreds.empty()) return false;
// Okay, the loaded value is available in at least one (and maybe all!)
// predecessors. If the value is unavailable in more than one unique
// predecessor, we want to insert a merge block for those common predecessors.
// This ensures that we only have to insert one reload, thus not increasing
// code size.
BasicBlock *UnavailablePred = nullptr;
// If the value is unavailable in one of predecessors, we will end up
// inserting a new instruction into them. It is only valid if all the
// instructions before LoadI are guaranteed to pass execution to its
// successor, or if LoadI is safe to speculate.
// TODO: If this logic becomes more complex, and we will perform PRE insertion
// farther than to a predecessor, we need to reuse the code from GVN's PRE.
// It requires domination tree analysis, so for this simple case it is an
// overkill.
if (PredsScanned.size() != AvailablePreds.size() &&
!isSafeToSpeculativelyExecute(LoadI))
for (auto I = LoadBB->begin(); &*I != LoadI; ++I)
if (!isGuaranteedToTransferExecutionToSuccessor(&*I))
return false;
// If there is exactly one predecessor where the value is unavailable, the
// already computed 'OneUnavailablePred' block is it. If it ends in an
// unconditional branch, we know that it isn't a critical edge.
if (PredsScanned.size() == AvailablePreds.size()+1 &&
OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
UnavailablePred = OneUnavailablePred;
} else if (PredsScanned.size() != AvailablePreds.size()) {
// Otherwise, we had multiple unavailable predecessors or we had a critical
// edge from the one.
SmallVector<BasicBlock*, 8> PredsToSplit;
SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
for (const auto &AvailablePred : AvailablePreds)
AvailablePredSet.insert(AvailablePred.first);
// Add all the unavailable predecessors to the PredsToSplit list.
for (BasicBlock *P : predecessors(LoadBB)) {
// If the predecessor is an indirect goto, we can't split the edge.
// Same for CallBr.
if (isa<IndirectBrInst>(P->getTerminator()) ||
isa<CallBrInst>(P->getTerminator()))
return false;
if (!AvailablePredSet.count(P))
PredsToSplit.push_back(P);
}
// Split them out to their own block.
UnavailablePred = splitBlockPreds(LoadBB, PredsToSplit, "thread-pre-split");
}
// If the value isn't available in all predecessors, then there will be
// exactly one where it isn't available. Insert a load on that edge and add
// it to the AvailablePreds list.
if (UnavailablePred) {
assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
"Can't handle critical edge here!");
LoadInst *NewVal = new LoadInst(
LoadI->getType(), LoadedPtr->DoPHITranslation(LoadBB, UnavailablePred),
LoadI->getName() + ".pr", false, LoadI->getAlign(),
LoadI->getOrdering(), LoadI->getSyncScopeID(),
UnavailablePred->getTerminator());
NewVal->setDebugLoc(LoadI->getDebugLoc());
if (AATags)
NewVal->setAAMetadata(AATags);
AvailablePreds.emplace_back(UnavailablePred, NewVal);
}
// Now we know that each predecessor of this block has a value in
// AvailablePreds, sort them for efficient access as we're walking the preds.
array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
// Create a PHI node at the start of the block for the PRE'd load value.
pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
PHINode *PN = PHINode::Create(LoadI->getType(), std::distance(PB, PE), "",
&LoadBB->front());
PN->takeName(LoadI);
PN->setDebugLoc(LoadI->getDebugLoc());
// Insert new entries into the PHI for each predecessor. A single block may
// have multiple entries here.
for (pred_iterator PI = PB; PI != PE; ++PI) {
BasicBlock *P = *PI;
AvailablePredsTy::iterator I =
llvm::lower_bound(AvailablePreds, std::make_pair(P, (Value *)nullptr));
assert(I != AvailablePreds.end() && I->first == P &&
"Didn't find entry for predecessor!");
// If we have an available predecessor but it requires casting, insert the
// cast in the predecessor and use the cast. Note that we have to update the
// AvailablePreds vector as we go so that all of the PHI entries for this
// predecessor use the same bitcast.
Value *&PredV = I->second;
if (PredV->getType() != LoadI->getType())
PredV = CastInst::CreateBitOrPointerCast(PredV, LoadI->getType(), "",
P->getTerminator());
PN->addIncoming(PredV, I->first);
}
for (LoadInst *PredLoadI : CSELoads) {
combineMetadataForCSE(PredLoadI, LoadI, true);
}
LoadI->replaceAllUsesWith(PN);
LoadI->eraseFromParent();
return true;
}
/// findMostPopularDest - The specified list contains multiple possible
/// threadable destinations. Pick the one that occurs the most frequently in
/// the list.
static BasicBlock *
findMostPopularDest(BasicBlock *BB,
const SmallVectorImpl<std::pair<BasicBlock *,
BasicBlock *>> &PredToDestList) {
assert(!PredToDestList.empty());
// Determine popularity. If there are multiple possible destinations, we
// explicitly choose to ignore 'undef' destinations. We prefer to thread
// blocks with known and real destinations to threading undef. We'll handle
// them later if interesting.
MapVector<BasicBlock *, unsigned> DestPopularity;
// Populate DestPopularity with the successors in the order they appear in the
// successor list. This way, we ensure determinism by iterating it in the
// same order in std::max_element below. We map nullptr to 0 so that we can
// return nullptr when PredToDestList contains nullptr only.
DestPopularity[nullptr] = 0;
for (auto *SuccBB : successors(BB))
DestPopularity[SuccBB] = 0;
for (const auto &PredToDest : PredToDestList)
if (PredToDest.second)
DestPopularity[PredToDest.second]++;
// Find the most popular dest.
using VT = decltype(DestPopularity)::value_type;
auto MostPopular = std::max_element(
DestPopularity.begin(), DestPopularity.end(),
[](const VT &L, const VT &R) { return L.second < R.second; });
// Okay, we have finally picked the most popular destination.
return MostPopular->first;
}
// Try to evaluate the value of V when the control flows from PredPredBB to
// BB->getSinglePredecessor() and then on to BB.
Constant *JumpThreadingPass::evaluateOnPredecessorEdge(BasicBlock *BB,
BasicBlock *PredPredBB,
Value *V) {
BasicBlock *PredBB = BB->getSinglePredecessor();
assert(PredBB && "Expected a single predecessor");
if (Constant *Cst = dyn_cast<Constant>(V)) {
return Cst;
}
// Consult LVI if V is not an instruction in BB or PredBB.
Instruction *I = dyn_cast<Instruction>(V);
if (!I || (I->getParent() != BB && I->getParent() != PredBB)) {
return LVI->getConstantOnEdge(V, PredPredBB, PredBB, nullptr);
}
// Look into a PHI argument.
if (PHINode *PHI = dyn_cast<PHINode>(V)) {
if (PHI->getParent() == PredBB)
return dyn_cast<Constant>(PHI->getIncomingValueForBlock(PredPredBB));
return nullptr;
}
// If we have a CmpInst, try to fold it for each incoming edge into PredBB.
if (CmpInst *CondCmp = dyn_cast<CmpInst>(V)) {
if (CondCmp->getParent() == BB) {
Constant *Op0 =
evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(0));
Constant *Op1 =
evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(1));
if (Op0 && Op1) {
return ConstantExpr::getCompare(CondCmp->getPredicate(), Op0, Op1);
}
}
return nullptr;
}
return nullptr;
}
bool JumpThreadingPass::processThreadableEdges(Value *Cond, BasicBlock *BB,
ConstantPreference Preference,
Instruction *CxtI) {
// If threading this would thread across a loop header, don't even try to
// thread the edge.
if (LoopHeaders.count(BB))
return false;
PredValueInfoTy PredValues;
if (!computeValueKnownInPredecessors(Cond, BB, PredValues, Preference,
CxtI)) {
// We don't have known values in predecessors. See if we can thread through
// BB and its sole predecessor.
return maybethreadThroughTwoBasicBlocks(BB, Cond);
}
assert(!PredValues.empty() &&
"computeValueKnownInPredecessors returned true with no values");
LLVM_DEBUG(dbgs() << "IN BB: " << *BB;
for (const auto &PredValue : PredValues) {
dbgs() << " BB '" << BB->getName()
<< "': FOUND condition = " << *PredValue.first
<< " for pred '" << PredValue.second->getName() << "'.\n";
});
// Decide what we want to thread through. Convert our list of known values to
// a list of known destinations for each pred. This also discards duplicate
// predecessors and keeps track of the undefined inputs (which are represented
// as a null dest in the PredToDestList).
SmallPtrSet<BasicBlock*, 16> SeenPreds;
SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
BasicBlock *OnlyDest = nullptr;
BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
Constant *OnlyVal = nullptr;
Constant *MultipleVal = (Constant *)(intptr_t)~0ULL;
for (const auto &PredValue : PredValues) {
BasicBlock *Pred = PredValue.second;
if (!SeenPreds.insert(Pred).second)
continue; // Duplicate predecessor entry.
Constant *Val = PredValue.first;
BasicBlock *DestBB;
if (isa<UndefValue>(Val))
DestBB = nullptr;
else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
DestBB = SI->findCaseValue(cast<ConstantInt>(Val))->getCaseSuccessor();
} else {
assert(isa<IndirectBrInst>(BB->getTerminator())
&& "Unexpected terminator");
assert(isa<BlockAddress>(Val) && "Expecting a constant blockaddress");
DestBB = cast<BlockAddress>(Val)->getBasicBlock();
}
// If we have exactly one destination, remember it for efficiency below.
if (PredToDestList.empty()) {
OnlyDest = DestBB;
OnlyVal = Val;
} else {
if (OnlyDest != DestBB)
OnlyDest = MultipleDestSentinel;
// It possible we have same destination, but different value, e.g. default
// case in switchinst.
if (Val != OnlyVal)
OnlyVal = MultipleVal;
}
// If the predecessor ends with an indirect goto, we can't change its
// destination. Same for CallBr.
if (isa<IndirectBrInst>(Pred->getTerminator()) ||
isa<CallBrInst>(Pred->getTerminator()))
continue;
PredToDestList.emplace_back(Pred, DestBB);
}
// If all edges were unthreadable, we fail.
if (PredToDestList.empty())
return false;
// If all the predecessors go to a single known successor, we want to fold,
// not thread. By doing so, we do not need to duplicate the current block and
// also miss potential opportunities in case we dont/cant duplicate.
if (OnlyDest && OnlyDest != MultipleDestSentinel) {
if (BB->hasNPredecessors(PredToDestList.size())) {
bool SeenFirstBranchToOnlyDest = false;
std::vector <DominatorTree::UpdateType> Updates;
Updates.reserve(BB->getTerminator()->getNumSuccessors() - 1);
for (BasicBlock *SuccBB : successors(BB)) {
if (SuccBB == OnlyDest && !SeenFirstBranchToOnlyDest) {
SeenFirstBranchToOnlyDest = true; // Don't modify the first branch.
} else {
SuccBB->removePredecessor(BB, true); // This is unreachable successor.
Updates.push_back({DominatorTree::Delete, BB, SuccBB});
}
}
// Finally update the terminator.
Instruction *Term = BB->getTerminator();
BranchInst::Create(OnlyDest, Term);
Term->eraseFromParent();
DTU->applyUpdatesPermissive(Updates);
if (HasProfileData)
BPI->eraseBlock(BB);
// If the condition is now dead due to the removal of the old terminator,
// erase it.
if (auto *CondInst = dyn_cast<Instruction>(Cond)) {
if (CondInst->use_empty() && !CondInst->mayHaveSideEffects())
CondInst->eraseFromParent();
// We can safely replace *some* uses of the CondInst if it has
// exactly one value as returned by LVI. RAUW is incorrect in the
// presence of guards and assumes, that have the `Cond` as the use. This
// is because we use the guards/assume to reason about the `Cond` value
// at the end of block, but RAUW unconditionally replaces all uses
// including the guards/assumes themselves and the uses before the
// guard/assume.
else if (OnlyVal && OnlyVal != MultipleVal &&
CondInst->getParent() == BB)
replaceFoldableUses(CondInst, OnlyVal);
}
return true;
}
}
// Determine which is the most common successor. If we have many inputs and
// this block is a switch, we want to start by threading the batch that goes
// to the most popular destination first. If we only know about one
// threadable destination (the common case) we can avoid this.
BasicBlock *MostPopularDest = OnlyDest;
if (MostPopularDest == MultipleDestSentinel) {
// Remove any loop headers from the Dest list, threadEdge conservatively
// won't process them, but we might have other destination that are eligible
// and we still want to process.
erase_if(PredToDestList,
[&](const std::pair<BasicBlock *, BasicBlock *> &PredToDest) {
return LoopHeaders.contains(PredToDest.second);
});
if (PredToDestList.empty())
return false;
MostPopularDest = findMostPopularDest(BB, PredToDestList);
}
// Now that we know what the most popular destination is, factor all
// predecessors that will jump to it into a single predecessor.
SmallVector<BasicBlock*, 16> PredsToFactor;
for (const auto &PredToDest : PredToDestList)
if (PredToDest.second == MostPopularDest) {
BasicBlock *Pred = PredToDest.first;
// This predecessor may be a switch or something else that has multiple
// edges to the block. Factor each of these edges by listing them
// according to # occurrences in PredsToFactor.
for (BasicBlock *Succ : successors(Pred))
if (Succ == BB)
PredsToFactor.push_back(Pred);
}
// If the threadable edges are branching on an undefined value, we get to pick
// the destination that these predecessors should get to.
if (!MostPopularDest)
MostPopularDest = BB->getTerminator()->
getSuccessor(getBestDestForJumpOnUndef(BB));
// Ok, try to thread it!
return tryThreadEdge(BB, PredsToFactor, MostPopularDest);
}
/// processBranchOnPHI - We have an otherwise unthreadable conditional branch on
/// a PHI node (or freeze PHI) in the current block. See if there are any
/// simplifications we can do based on inputs to the phi node.
bool JumpThreadingPass::processBranchOnPHI(PHINode *PN) {
BasicBlock *BB = PN->getParent();
// TODO: We could make use of this to do it once for blocks with common PHI
// values.
SmallVector<BasicBlock*, 1> PredBBs;
PredBBs.resize(1);
// If any of the predecessor blocks end in an unconditional branch, we can
// *duplicate* the conditional branch into that block in order to further
// encourage jump threading and to eliminate cases where we have branch on a
// phi of an icmp (branch on icmp is much better).
// This is still beneficial when a frozen phi is used as the branch condition
// because it allows CodeGenPrepare to further canonicalize br(freeze(icmp))
// to br(icmp(freeze ...)).
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
BasicBlock *PredBB = PN->getIncomingBlock(i);
if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
if (PredBr->isUnconditional()) {
PredBBs[0] = PredBB;
// Try to duplicate BB into PredBB.
if (duplicateCondBranchOnPHIIntoPred(BB, PredBBs))
return true;
}
}
return false;
}
/// processBranchOnXOR - We have an otherwise unthreadable conditional branch on
/// a xor instruction in the current block. See if there are any
/// simplifications we can do based on inputs to the xor.
bool JumpThreadingPass::processBranchOnXOR(BinaryOperator *BO) {
BasicBlock *BB = BO->getParent();
// If either the LHS or RHS of the xor is a constant, don't do this
// optimization.
if (isa<ConstantInt>(BO->getOperand(0)) ||
isa<ConstantInt>(BO->getOperand(1)))
return false;
// If the first instruction in BB isn't a phi, we won't be able to infer
// anything special about any particular predecessor.
if (!isa<PHINode>(BB->front()))
return false;
// If this BB is a landing pad, we won't be able to split the edge into it.
if (BB->isEHPad())
return false;
// If we have a xor as the branch input to this block, and we know that the
// LHS or RHS of the xor in any predecessor is true/false, then we can clone
// the condition into the predecessor and fix that value to true, saving some
// logical ops on that path and encouraging other paths to simplify.
//
// This copies something like this:
//
// BB:
// %X = phi i1 [1], [%X']
// %Y = icmp eq i32 %A, %B
// %Z = xor i1 %X, %Y
// br i1 %Z, ...
//
// Into:
// BB':
// %Y = icmp ne i32 %A, %B
// br i1 %Y, ...
PredValueInfoTy XorOpValues;
bool isLHS = true;
if (!computeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
WantInteger, BO)) {
assert(XorOpValues.empty());
if (!computeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
WantInteger, BO))
return false;
isLHS = false;
}
assert(!XorOpValues.empty() &&
"computeValueKnownInPredecessors returned true with no values");
// Scan the information to see which is most popular: true or false. The
// predecessors can be of the set true, false, or undef.
unsigned NumTrue = 0, NumFalse = 0;
for (const auto &XorOpValue : XorOpValues) {
if (isa<UndefValue>(XorOpValue.first))
// Ignore undefs for the count.
continue;
if (cast<ConstantInt>(XorOpValue.first)->isZero())
++NumFalse;
else
++NumTrue;
}
// Determine which value to split on, true, false, or undef if neither.
ConstantInt *SplitVal = nullptr;
if (NumTrue > NumFalse)
SplitVal = ConstantInt::getTrue(BB->getContext());
else if (NumTrue != 0 || NumFalse != 0)
SplitVal = ConstantInt::getFalse(BB->getContext());
// Collect all of the blocks that this can be folded into so that we can
// factor this once and clone it once.
SmallVector<BasicBlock*, 8> BlocksToFoldInto;
for (const auto &XorOpValue : XorOpValues) {
if (XorOpValue.first != SplitVal && !isa<UndefValue>(XorOpValue.first))
continue;
BlocksToFoldInto.push_back(XorOpValue.second);
}
// If we inferred a value for all of the predecessors, then duplication won't
// help us. However, we can just replace the LHS or RHS with the constant.
if (BlocksToFoldInto.size() ==
cast<PHINode>(BB->front()).getNumIncomingValues()) {
if (!SplitVal) {
// If all preds provide undef, just nuke the xor, because it is undef too.
BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
BO->eraseFromParent();
} else if (SplitVal->isZero()) {
// If all preds provide 0, replace the xor with the other input.
BO->replaceAllUsesWith(BO->getOperand(isLHS));
BO->eraseFromParent();
} else {
// If all preds provide 1, set the computed value to 1.
BO->setOperand(!isLHS, SplitVal);
}
return true;
}
// If any of predecessors end with an indirect goto, we can't change its
// destination. Same for CallBr.
if (any_of(BlocksToFoldInto, [](BasicBlock *Pred) {
return isa<IndirectBrInst>(Pred->getTerminator()) ||
isa<CallBrInst>(Pred->getTerminator());
}))
return false;
// Try to duplicate BB into PredBB.
return duplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
}
/// addPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
/// predecessor to the PHIBB block. If it has PHI nodes, add entries for
/// NewPred using the entries from OldPred (suitably mapped).
static void addPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
BasicBlock *OldPred,
BasicBlock *NewPred,
DenseMap<Instruction*, Value*> &ValueMap) {
for (PHINode &PN : PHIBB->phis()) {
// Ok, we have a PHI node. Figure out what the incoming value was for the
// DestBlock.
Value *IV = PN.getIncomingValueForBlock(OldPred);
// Remap the value if necessary.
if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
if (I != ValueMap.end())
IV = I->second;
}
PN.addIncoming(IV, NewPred);
}
}
/// Merge basic block BB into its sole predecessor if possible.
bool JumpThreadingPass::maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB) {
BasicBlock *SinglePred = BB->getSinglePredecessor();
if (!SinglePred)
return false;
const Instruction *TI = SinglePred->getTerminator();
if (TI->isExceptionalTerminator() || TI->getNumSuccessors() != 1 ||
SinglePred == BB || hasAddressTakenAndUsed(BB))
return false;
// If SinglePred was a loop header, BB becomes one.
if (LoopHeaders.erase(SinglePred))
LoopHeaders.insert(BB);
LVI->eraseBlock(SinglePred);
MergeBasicBlockIntoOnlyPred(BB, DTU);
// Now that BB is merged into SinglePred (i.e. SinglePred code followed by
// BB code within one basic block `BB`), we need to invalidate the LVI
// information associated with BB, because the LVI information need not be
// true for all of BB after the merge. For example,
// Before the merge, LVI info and code is as follows:
// SinglePred: <LVI info1 for %p val>
// %y = use of %p
// call @exit() // need not transfer execution to successor.
// assume(%p) // from this point on %p is true
// br label %BB
// BB: <LVI info2 for %p val, i.e. %p is true>
// %x = use of %p
// br label exit
//
// Note that this LVI info for blocks BB and SinglPred is correct for %p
// (info2 and info1 respectively). After the merge and the deletion of the
// LVI info1 for SinglePred. We have the following code:
// BB: <LVI info2 for %p val>
// %y = use of %p
// call @exit()
// assume(%p)
// %x = use of %p <-- LVI info2 is correct from here onwards.
// br label exit
// LVI info2 for BB is incorrect at the beginning of BB.
// Invalidate LVI information for BB if the LVI is not provably true for
// all of BB.
if (!isGuaranteedToTransferExecutionToSuccessor(BB))
LVI->eraseBlock(BB);
return true;
}
/// Update the SSA form. NewBB contains instructions that are copied from BB.
/// ValueMapping maps old values in BB to new ones in NewBB.
void JumpThreadingPass::updateSSA(
BasicBlock *BB, BasicBlock *NewBB,
DenseMap<Instruction *, Value *> &ValueMapping) {
// If there were values defined in BB that are used outside the block, then we
// now have to update all uses of the value to use either the original value,
// the cloned value, or some PHI derived value. This can require arbitrary
// PHI insertion, of which we are prepared to do, clean these up now.
SSAUpdater SSAUpdate;
SmallVector<Use *, 16> UsesToRename;
for (Instruction &I : *BB) {
// Scan all uses of this instruction to see if it is used outside of its
// block, and if so, record them in UsesToRename.
for (Use &U : I.uses()) {
Instruction *User = cast<Instruction>(U.getUser());
if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
if (UserPN->getIncomingBlock(U) == BB)
continue;
} else if (User->getParent() == BB)
continue;
UsesToRename.push_back(&U);
}
// If there are no uses outside the block, we're done with this instruction.
if (UsesToRename.empty())
continue;
LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
// We found a use of I outside of BB. Rename all uses of I that are outside
// its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
// with the two values we know.
SSAUpdate.Initialize(I.getType(), I.getName());
SSAUpdate.AddAvailableValue(BB, &I);
SSAUpdate.AddAvailableValue(NewBB, ValueMapping[&I]);
while (!UsesToRename.empty())
SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
LLVM_DEBUG(dbgs() << "\n");
}
}
/// Clone instructions in range [BI, BE) to NewBB. For PHI nodes, we only clone
/// arguments that come from PredBB. Return the map from the variables in the
/// source basic block to the variables in the newly created basic block.
DenseMap<Instruction *, Value *>
JumpThreadingPass::cloneInstructions(BasicBlock::iterator BI,
BasicBlock::iterator BE, BasicBlock *NewBB,
BasicBlock *PredBB) {
// We are going to have to map operands from the source basic block to the new
// copy of the block 'NewBB'. If there are PHI nodes in the source basic
// block, evaluate them to account for entry from PredBB.
DenseMap<Instruction *, Value *> ValueMapping;
// Clone the phi nodes of the source basic block into NewBB. The resulting
// phi nodes are trivial since NewBB only has one predecessor, but SSAUpdater
// might need to rewrite the operand of the cloned phi.
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
PHINode *NewPN = PHINode::Create(PN->getType(), 1, PN->getName(), NewBB);
NewPN->addIncoming(PN->getIncomingValueForBlock(PredBB), PredBB);
ValueMapping[PN] = NewPN;
}
// Clone noalias scope declarations in the threaded block. When threading a
// loop exit, we would otherwise end up with two idential scope declarations
// visible at the same time.
SmallVector<MDNode *> NoAliasScopes;
DenseMap<MDNode *, MDNode *> ClonedScopes;
LLVMContext &Context = PredBB->getContext();
identifyNoAliasScopesToClone(BI, BE, NoAliasScopes);
cloneNoAliasScopes(NoAliasScopes, ClonedScopes, "thread", Context);
// Clone the non-phi instructions of the source basic block into NewBB,
// keeping track of the mapping and using it to remap operands in the cloned
// instructions.
for (; BI != BE; ++BI) {
Instruction *New = BI->clone();
New->setName(BI->getName());
NewBB->getInstList().push_back(New);
ValueMapping[&*BI] = New;
adaptNoAliasScopes(New, ClonedScopes, Context);
// Remap operands to patch up intra-block references.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
DenseMap<Instruction *, Value *>::iterator I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
New->setOperand(i, I->second);
}
}
return ValueMapping;
}
/// Attempt to thread through two successive basic blocks.
bool JumpThreadingPass::maybethreadThroughTwoBasicBlocks(BasicBlock *BB,
Value *Cond) {
// Consider:
//
// PredBB:
// %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
// %tobool = icmp eq i32 %cond, 0
// br i1 %tobool, label %BB, label ...
//
// BB:
// %cmp = icmp eq i32* %var, null
// br i1 %cmp, label ..., label ...
//
// We don't know the value of %var at BB even if we know which incoming edge
// we take to BB. However, once we duplicate PredBB for each of its incoming
// edges (say, PredBB1 and PredBB2), we know the value of %var in each copy of
// PredBB. Then we can thread edges PredBB1->BB and PredBB2->BB through BB.
// Require that BB end with a Branch for simplicity.
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
if (!CondBr)
return false;
// BB must have exactly one predecessor.
BasicBlock *PredBB = BB->getSinglePredecessor();
if (!PredBB)
return false;
// Require that PredBB end with a conditional Branch. If PredBB ends with an
// unconditional branch, we should be merging PredBB and BB instead. For
// simplicity, we don't deal with a switch.
BranchInst *PredBBBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
if (!PredBBBranch || PredBBBranch->isUnconditional())
return false;
// If PredBB has exactly one incoming edge, we don't gain anything by copying
// PredBB.
if (PredBB->getSinglePredecessor())
return false;
// Don't thread through PredBB if it contains a successor edge to itself, in
// which case we would infinite loop. Suppose we are threading an edge from
// PredPredBB through PredBB and BB to SuccBB with PredBB containing a
// successor edge to itself. If we allowed jump threading in this case, we
// could duplicate PredBB and BB as, say, PredBB.thread and BB.thread. Since
// PredBB.thread has a successor edge to PredBB, we would immediately come up
// with another jump threading opportunity from PredBB.thread through PredBB
// and BB to SuccBB. This jump threading would repeatedly occur. That is, we
// would keep peeling one iteration from PredBB.
if (llvm::is_contained(successors(PredBB), PredBB))
return false;
// Don't thread across a loop header.
if (LoopHeaders.count(PredBB))
return false;
// Avoid complication with duplicating EH pads.
if (PredBB->isEHPad())
return false;
// Find a predecessor that we can thread. For simplicity, we only consider a
// successor edge out of BB to which we thread exactly one incoming edge into
// PredBB.
unsigned ZeroCount = 0;
unsigned OneCount = 0;
BasicBlock *ZeroPred = nullptr;
BasicBlock *OnePred = nullptr;
for (BasicBlock *P : predecessors(PredBB)) {
if (ConstantInt *CI = dyn_cast_or_null<ConstantInt>(
evaluateOnPredecessorEdge(BB, P, Cond))) {
if (CI->isZero()) {
ZeroCount++;
ZeroPred = P;
} else if (CI->isOne()) {
OneCount++;
OnePred = P;
}
}
}
// Disregard complicated cases where we have to thread multiple edges.
BasicBlock *PredPredBB;
if (ZeroCount == 1) {
PredPredBB = ZeroPred;
} else if (OneCount == 1) {
PredPredBB = OnePred;
} else {
return false;
}
BasicBlock *SuccBB = CondBr->getSuccessor(PredPredBB == ZeroPred);
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
<< "' - would thread to self!\n");
return false;
}
// If threading this would thread across a loop header, don't thread the edge.
// See the comments above findLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) {
LLVM_DEBUG({
bool BBIsHeader = LoopHeaders.count(BB);
bool SuccIsHeader = LoopHeaders.count(SuccBB);
dbgs() << " Not threading across "
<< (BBIsHeader ? "loop header BB '" : "block BB '")
<< BB->getName() << "' to dest "
<< (SuccIsHeader ? "loop header BB '" : "block BB '")
<< SuccBB->getName()
<< "' - it might create an irreducible loop!\n";
});
return false;
}
// Compute the cost of duplicating BB and PredBB.
unsigned BBCost =
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
unsigned PredBBCost = getJumpThreadDuplicationCost(
PredBB, PredBB->getTerminator(), BBDupThreshold);
// Give up if costs are too high. We need to check BBCost and PredBBCost
// individually before checking their sum because getJumpThreadDuplicationCost
// return (unsigned)~0 for those basic blocks that cannot be duplicated.
if (BBCost > BBDupThreshold || PredBBCost > BBDupThreshold ||
BBCost + PredBBCost > BBDupThreshold) {
LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << PredBBCost
<< " for PredBB, " << BBCost << "for BB\n");
return false;
}
// Now we are ready to duplicate PredBB.
threadThroughTwoBasicBlocks(PredPredBB, PredBB, BB, SuccBB);
return true;
}
void JumpThreadingPass::threadThroughTwoBasicBlocks(BasicBlock *PredPredBB,
BasicBlock *PredBB,
BasicBlock *BB,
BasicBlock *SuccBB) {
LLVM_DEBUG(dbgs() << " Threading through '" << PredBB->getName() << "' and '"
<< BB->getName() << "'\n");
BranchInst *CondBr = cast<BranchInst>(BB->getTerminator());
BranchInst *PredBBBranch = cast<BranchInst>(PredBB->getTerminator());
BasicBlock *NewBB =
BasicBlock::Create(PredBB->getContext(), PredBB->getName() + ".thread",
PredBB->getParent(), PredBB);
NewBB->moveAfter(PredBB);
// Set the block frequency of NewBB.
if (HasProfileData) {
auto NewBBFreq = BFI->getBlockFreq(PredPredBB) *
BPI->getEdgeProbability(PredPredBB, PredBB);
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
}
// We are going to have to map operands from the original BB block to the new
// copy of the block 'NewBB'. If there are PHI nodes in PredBB, evaluate them
// to account for entry from PredPredBB.
DenseMap<Instruction *, Value *> ValueMapping =
cloneInstructions(PredBB->begin(), PredBB->end(), NewBB, PredPredBB);
// Copy the edge probabilities from PredBB to NewBB.
if (HasProfileData)
BPI->copyEdgeProbabilities(PredBB, NewBB);
// Update the terminator of PredPredBB to jump to NewBB instead of PredBB.
// This eliminates predecessors from PredPredBB, which requires us to simplify
// any PHI nodes in PredBB.
Instruction *PredPredTerm = PredPredBB->getTerminator();
for (unsigned i = 0, e = PredPredTerm->getNumSuccessors(); i != e; ++i)
if (PredPredTerm->getSuccessor(i) == PredBB) {
PredBB->removePredecessor(PredPredBB, true);
PredPredTerm->setSuccessor(i, NewBB);
}
addPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(0), PredBB, NewBB,
ValueMapping);
addPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(1), PredBB, NewBB,
ValueMapping);
DTU->applyUpdatesPermissive(
{{DominatorTree::Insert, NewBB, CondBr->getSuccessor(0)},
{DominatorTree::Insert, NewBB, CondBr->getSuccessor(1)},
{DominatorTree::Insert, PredPredBB, NewBB},
{DominatorTree::Delete, PredPredBB, PredBB}});
updateSSA(PredBB, NewBB, ValueMapping);
// Clean up things like PHI nodes with single operands, dead instructions,
// etc.
SimplifyInstructionsInBlock(NewBB, TLI);
SimplifyInstructionsInBlock(PredBB, TLI);
SmallVector<BasicBlock *, 1> PredsToFactor;
PredsToFactor.push_back(NewBB);
threadEdge(BB, PredsToFactor, SuccBB);
}
/// tryThreadEdge - Thread an edge if it's safe and profitable to do so.
bool JumpThreadingPass::tryThreadEdge(
BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
BasicBlock *SuccBB) {
// If threading to the same block as we come from, we would infinite loop.
if (SuccBB == BB) {
LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
<< "' - would thread to self!\n");
return false;
}
// If threading this would thread across a loop header, don't thread the edge.
// See the comments above findLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) {
LLVM_DEBUG({
bool BBIsHeader = LoopHeaders.count(BB);
bool SuccIsHeader = LoopHeaders.count(SuccBB);
dbgs() << " Not threading across "
<< (BBIsHeader ? "loop header BB '" : "block BB '") << BB->getName()
<< "' to dest " << (SuccIsHeader ? "loop header BB '" : "block BB '")
<< SuccBB->getName() << "' - it might create an irreducible loop!\n";
});
return false;
}
unsigned JumpThreadCost =
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
if (JumpThreadCost > BBDupThreshold) {
LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
<< "' - Cost is too high: " << JumpThreadCost << "\n");
return false;
}
threadEdge(BB, PredBBs, SuccBB);
return true;
}
/// threadEdge - We have decided that it is safe and profitable to factor the
/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
/// across BB. Transform the IR to reflect this change.
void JumpThreadingPass::threadEdge(BasicBlock *BB,
const SmallVectorImpl<BasicBlock *> &PredBBs,
BasicBlock *SuccBB) {
assert(SuccBB != BB && "Don't create an infinite loop");
assert(!LoopHeaders.count(BB) && !LoopHeaders.count(SuccBB) &&
"Don't thread across loop headers");
// And finally, do it! Start by factoring the predecessors if needed.
BasicBlock *PredBB;
if (PredBBs.size() == 1)
PredBB = PredBBs[0];
else {
LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
<< " common predecessors.\n");
PredBB = splitBlockPreds(BB, PredBBs, ".thr_comm");
}
// And finally, do it!
LLVM_DEBUG(dbgs() << " Threading edge from '" << PredBB->getName()
<< "' to '" << SuccBB->getName()
<< ", across block:\n " << *BB << "\n");
LVI->threadEdge(PredBB, BB, SuccBB);
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
BB->getName()+".thread",
BB->getParent(), BB);
NewBB->moveAfter(PredBB);
// Set the block frequency of NewBB.
if (HasProfileData) {
auto NewBBFreq =
BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
}
// Copy all the instructions from BB to NewBB except the terminator.
DenseMap<Instruction *, Value *> ValueMapping =
cloneInstructions(BB->begin(), std::prev(BB->end()), NewBB, PredBB);
// We didn't copy the terminator from BB over to NewBB, because there is now
// an unconditional jump to SuccBB. Insert the unconditional jump.
BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB);
NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
// Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
// PHI nodes for NewBB now.
addPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
// Update the terminator of PredBB to jump to NewBB instead of BB. This
// eliminates predecessors from BB, which requires us to simplify any PHI
// nodes in BB.
Instruction *PredTerm = PredBB->getTerminator();
for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
if (PredTerm->getSuccessor(i) == BB) {
BB->removePredecessor(PredBB, true);
PredTerm->setSuccessor(i, NewBB);
}
// Enqueue required DT updates.
DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, SuccBB},
{DominatorTree::Insert, PredBB, NewBB},
{DominatorTree::Delete, PredBB, BB}});
updateSSA(BB, NewBB, ValueMapping);
// At this point, the IR is fully up to date and consistent. Do a quick scan
// over the new instructions and zap any that are constants or dead. This
// frequently happens because of phi translation.
SimplifyInstructionsInBlock(NewBB, TLI);
// Update the edge weight from BB to SuccBB, which should be less than before.
updateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB);
// Threaded an edge!
++NumThreads;
}
/// Create a new basic block that will be the predecessor of BB and successor of
/// all blocks in Preds. When profile data is available, update the frequency of
/// this new block.
BasicBlock *JumpThreadingPass::splitBlockPreds(BasicBlock *BB,
ArrayRef<BasicBlock *> Preds,
const char *Suffix) {
SmallVector<BasicBlock *, 2> NewBBs;
// Collect the frequencies of all predecessors of BB, which will be used to
// update the edge weight of the result of splitting predecessors.
DenseMap<BasicBlock *, BlockFrequency> FreqMap;
if (HasProfileData)
for (auto Pred : Preds)
FreqMap.insert(std::make_pair(
Pred, BFI->getBlockFreq(Pred) * BPI->getEdgeProbability(Pred, BB)));
// In the case when BB is a LandingPad block we create 2 new predecessors
// instead of just one.
if (BB->isLandingPad()) {
std::string NewName = std::string(Suffix) + ".split-lp";
SplitLandingPadPredecessors(BB, Preds, Suffix, NewName.c_str(), NewBBs);
} else {
NewBBs.push_back(SplitBlockPredecessors(BB, Preds, Suffix));
}
std::vector<DominatorTree::UpdateType> Updates;
Updates.reserve((2 * Preds.size()) + NewBBs.size());
for (auto NewBB : NewBBs) {
BlockFrequency NewBBFreq(0);
Updates.push_back({DominatorTree::Insert, NewBB, BB});
for (auto Pred : predecessors(NewBB)) {
Updates.push_back({DominatorTree::Delete, Pred, BB});
Updates.push_back({DominatorTree::Insert, Pred, NewBB});
if (HasProfileData) // Update frequencies between Pred -> NewBB.
NewBBFreq += FreqMap.lookup(Pred);
}
if (HasProfileData) // Apply the summed frequency to NewBB.
BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
}
DTU->applyUpdatesPermissive(Updates);
return NewBBs[0];
}
bool JumpThreadingPass::doesBlockHaveProfileData(BasicBlock *BB) {
const Instruction *TI = BB->getTerminator();
assert(TI->getNumSuccessors() > 1 && "not a split");
MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
if (!WeightsNode)
return false;
MDString *MDName = cast<MDString>(WeightsNode->getOperand(0));
if (MDName->getString() != "branch_weights")
return false;
// Ensure there are weights for all of the successors. Note that the first
// operand to the metadata node is a name, not a weight.
return WeightsNode->getNumOperands() == TI->getNumSuccessors() + 1;
}
/// Update the block frequency of BB and branch weight and the metadata on the
/// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
/// Freq(PredBB->BB) / Freq(BB->SuccBB).
void JumpThreadingPass::updateBlockFreqAndEdgeWeight(BasicBlock *PredBB,
BasicBlock *BB,
BasicBlock *NewBB,
BasicBlock *SuccBB) {
if (!HasProfileData)
return;
assert(BFI && BPI && "BFI & BPI should have been created here");
// As the edge from PredBB to BB is deleted, we have to update the block
// frequency of BB.
auto BBOrigFreq = BFI->getBlockFreq(BB);
auto NewBBFreq = BFI->getBlockFreq(NewBB);
auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(BB, SuccBB);
auto BBNewFreq = BBOrigFreq - NewBBFreq;
BFI->setBlockFreq(BB, BBNewFreq.getFrequency());
// Collect updated outgoing edges' frequencies from BB and use them to update
// edge probabilities.
SmallVector<uint64_t, 4> BBSuccFreq;
for (BasicBlock *Succ : successors(BB)) {
auto SuccFreq = (Succ == SuccBB)
? BB2SuccBBFreq - NewBBFreq
: BBOrigFreq * BPI->getEdgeProbability(BB, Succ);
BBSuccFreq.push_back(SuccFreq.getFrequency());
}
uint64_t MaxBBSuccFreq =
*std::max_element(BBSuccFreq.begin(), BBSuccFreq.end());
SmallVector<BranchProbability, 4> BBSuccProbs;
if (MaxBBSuccFreq == 0)
BBSuccProbs.assign(BBSuccFreq.size(),
{1, static_cast<unsigned>(BBSuccFreq.size())});
else {
for (uint64_t Freq : BBSuccFreq)
BBSuccProbs.push_back(
BranchProbability::getBranchProbability(Freq, MaxBBSuccFreq));
// Normalize edge probabilities so that they sum up to one.
BranchProbability::normalizeProbabilities(BBSuccProbs.begin(),
BBSuccProbs.end());
}
// Update edge probabilities in BPI.
BPI->setEdgeProbability(BB, BBSuccProbs);
// Update the profile metadata as well.
//
// Don't do this if the profile of the transformed blocks was statically
// estimated. (This could occur despite the function having an entry
// frequency in completely cold parts of the CFG.)
//
// In this case we don't want to suggest to subsequent passes that the
// calculated weights are fully consistent. Consider this graph:
//
// check_1
// 50% / |
// eq_1 | 50%
// \ |
// check_2
// 50% / |
// eq_2 | 50%
// \ |
// check_3
// 50% / |
// eq_3 | 50%
// \ |
//
// Assuming the blocks check_* all compare the same value against 1, 2 and 3,
// the overall probabilities are inconsistent; the total probability that the
// value is either 1, 2 or 3 is 150%.
//
// As a consequence if we thread eq_1 -> check_2 to check_3, check_2->check_3
// becomes 0%. This is even worse if the edge whose probability becomes 0% is
// the loop exit edge. Then based solely on static estimation we would assume
// the loop was extremely hot.
//
// FIXME this locally as well so that BPI and BFI are consistent as well. We
// shouldn't make edges extremely likely or unlikely based solely on static
// estimation.
if (BBSuccProbs.size() >= 2 && doesBlockHaveProfileData(BB)) {
SmallVector<uint32_t, 4> Weights;
for (auto Prob : BBSuccProbs)
Weights.push_back(Prob.getNumerator());
auto TI = BB->getTerminator();
TI->setMetadata(
LLVMContext::MD_prof,
MDBuilder(TI->getParent()->getContext()).createBranchWeights(Weights));
}
}
/// duplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
/// to BB which contains an i1 PHI node and a conditional branch on that PHI.
/// If we can duplicate the contents of BB up into PredBB do so now, this
/// improves the odds that the branch will be on an analyzable instruction like
/// a compare.
bool JumpThreadingPass::duplicateCondBranchOnPHIIntoPred(
BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs) {
assert(!PredBBs.empty() && "Can't handle an empty set");
// If BB is a loop header, then duplicating this block outside the loop would
// cause us to transform this into an irreducible loop, don't do this.
// See the comments above findLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB)) {
LLVM_DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
<< "' into predecessor block '" << PredBBs[0]->getName()
<< "' - it might create an irreducible loop!\n");
return false;
}
unsigned DuplicationCost =
getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
if (DuplicationCost > BBDupThreshold) {
LLVM_DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
<< "' - Cost is too high: " << DuplicationCost << "\n");
return false;
}
// And finally, do it! Start by factoring the predecessors if needed.
std::vector<DominatorTree::UpdateType> Updates;
BasicBlock *PredBB;
if (PredBBs.size() == 1)
PredBB = PredBBs[0];
else {
LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
<< " common predecessors.\n");
PredBB = splitBlockPreds(BB, PredBBs, ".thr_comm");
}
Updates.push_back({DominatorTree::Delete, PredBB, BB});
// Okay, we decided to do this! Clone all the instructions in BB onto the end
// of PredBB.
LLVM_DEBUG(dbgs() << " Duplicating block '" << BB->getName()
<< "' into end of '" << PredBB->getName()
<< "' to eliminate branch on phi. Cost: "
<< DuplicationCost << " block is:" << *BB << "\n");
// Unless PredBB ends with an unconditional branch, split the edge so that we
// can just clone the bits from BB into the end of the new PredBB.
BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
if (!OldPredBranch || !OldPredBranch->isUnconditional()) {
BasicBlock *OldPredBB = PredBB;
PredBB = SplitEdge(OldPredBB, BB);
Updates.push_back({DominatorTree::Insert, OldPredBB, PredBB});
Updates.push_back({DominatorTree::Insert, PredBB, BB});
Updates.push_back({DominatorTree::Delete, OldPredBB, BB});
OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
}
// We are going to have to map operands from the original BB block into the
// PredBB block. Evaluate PHI nodes in BB.
DenseMap<Instruction*, Value*> ValueMapping;
BasicBlock::iterator BI = BB->begin();
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
// Clone the non-phi instructions of BB into PredBB, keeping track of the
// mapping and using it to remap operands in the cloned instructions.
for (; BI != BB->end(); ++BI) {
Instruction *New = BI->clone();
// Remap operands to patch up intra-block references.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
New->setOperand(i, I->second);
}
// If this instruction can be simplified after the operands are updated,
// just use the simplified value instead. This frequently happens due to
// phi translation.
if (Value *IV = SimplifyInstruction(
New,
{BB->getModule()->getDataLayout(), TLI, nullptr, nullptr, New})) {
ValueMapping[&*BI] = IV;
if (!New->mayHaveSideEffects()) {
New->deleteValue();
New = nullptr;
}
} else {
ValueMapping[&*BI] = New;
}
if (New) {
// Otherwise, insert the new instruction into the block.
New->setName(BI->getName());
PredBB->getInstList().insert(OldPredBranch->getIterator(), New);
// Update Dominance from simplified New instruction operands.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (BasicBlock *SuccBB = dyn_cast<BasicBlock>(New->getOperand(i)))
Updates.push_back({DominatorTree::Insert, PredBB, SuccBB});
}
}
// Check to see if the targets of the branch had PHI nodes. If so, we need to
// add entries to the PHI nodes for branch from PredBB now.
BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
addPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
ValueMapping);
addPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
ValueMapping);
updateSSA(BB, PredBB, ValueMapping);
// PredBB no longer jumps to BB, remove entries in the PHI node for the edge
// that we nuked.
BB->removePredecessor(PredBB, true);
// Remove the unconditional branch at the end of the PredBB block.
OldPredBranch->eraseFromParent();
if (HasProfileData)
BPI->copyEdgeProbabilities(BB, PredBB);
DTU->applyUpdatesPermissive(Updates);
++NumDupes;
return true;
}
// Pred is a predecessor of BB with an unconditional branch to BB. SI is
// a Select instruction in Pred. BB has other predecessors and SI is used in
// a PHI node in BB. SI has no other use.
// A new basic block, NewBB, is created and SI is converted to compare and
// conditional branch. SI is erased from parent.
void JumpThreadingPass::unfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB,
SelectInst *SI, PHINode *SIUse,
unsigned Idx) {
// Expand the select.
//
// Pred --
// | v
// | NewBB
// | |
// |-----
// v
// BB
BranchInst *PredTerm = cast<BranchInst>(Pred->getTerminator());
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
BB->getParent(), BB);
// Move the unconditional branch to NewBB.
PredTerm->removeFromParent();
NewBB->getInstList().insert(NewBB->end(), PredTerm);
// Create a conditional branch and update PHI nodes.
BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
SIUse->setIncomingValue(Idx, SI->getFalseValue());
SIUse->addIncoming(SI->getTrueValue(), NewBB);
// The select is now dead.
SI->eraseFromParent();
DTU->applyUpdatesPermissive({{DominatorTree::Insert, NewBB, BB},
{DominatorTree::Insert, Pred, NewBB}});
// Update any other PHI nodes in BB.
for (BasicBlock::iterator BI = BB->begin();
PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
if (Phi != SIUse)
Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
}
bool JumpThreadingPass::tryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB) {
PHINode *CondPHI = dyn_cast<PHINode>(SI->getCondition());
if (!CondPHI || CondPHI->getParent() != BB)
return false;
for (unsigned I = 0, E = CondPHI->getNumIncomingValues(); I != E; ++I) {
BasicBlock *Pred = CondPHI->getIncomingBlock(I);
SelectInst *PredSI = dyn_cast<SelectInst>(CondPHI->getIncomingValue(I));
// The second and third condition can be potentially relaxed. Currently
// the conditions help to simplify the code and allow us to reuse existing
// code, developed for tryToUnfoldSelect(CmpInst *, BasicBlock *)
if (!PredSI || PredSI->getParent() != Pred || !PredSI->hasOneUse())
continue;
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredTerm || !PredTerm->isUnconditional())
continue;
unfoldSelectInstr(Pred, BB, PredSI, CondPHI, I);
return true;
}
return false;
}
/// tryToUnfoldSelect - Look for blocks of the form
/// bb1:
/// %a = select
/// br bb2
///
/// bb2:
/// %p = phi [%a, %bb1] ...
/// %c = icmp %p
/// br i1 %c
///
/// And expand the select into a branch structure if one of its arms allows %c
/// to be folded. This later enables threading from bb1 over bb2.
bool JumpThreadingPass::tryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
if (!CondBr || !CondBr->isConditional() || !CondLHS ||
CondLHS->getParent() != BB)
return false;
for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
BasicBlock *Pred = CondLHS->getIncomingBlock(I);
SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
// Look if one of the incoming values is a select in the corresponding
// predecessor.
if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
continue;
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredTerm || !PredTerm->isUnconditional())
continue;
// Now check if one of the select values would allow us to constant fold the
// terminator in BB. We don't do the transform if both sides fold, those
// cases will be threaded in any case.
LazyValueInfo::Tristate LHSFolds =
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
CondRHS, Pred, BB, CondCmp);
LazyValueInfo::Tristate RHSFolds =
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
CondRHS, Pred, BB, CondCmp);
if ((LHSFolds != LazyValueInfo::Unknown ||
RHSFolds != LazyValueInfo::Unknown) &&
LHSFolds != RHSFolds) {
unfoldSelectInstr(Pred, BB, SI, CondLHS, I);
return true;
}
}
return false;
}
/// tryToUnfoldSelectInCurrBB - Look for PHI/Select or PHI/CMP/Select in the
/// same BB in the form
/// bb:
/// %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
/// %s = select %p, trueval, falseval
///
/// or
///
/// bb:
/// %p = phi [0, %bb1], [1, %bb2], [0, %bb3], [1, %bb4], ...
/// %c = cmp %p, 0
/// %s = select %c, trueval, falseval
///
/// And expand the select into a branch structure. This later enables
/// jump-threading over bb in this pass.
///
/// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
/// select if the associated PHI has at least one constant. If the unfolded
/// select is not jump-threaded, it will be folded again in the later
/// optimizations.
bool JumpThreadingPass::tryToUnfoldSelectInCurrBB(BasicBlock *BB) {
// This transform would reduce the quality of msan diagnostics.
// Disable this transform under MemorySanitizer.
if (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory))
return false;
// If threading this would thread across a loop header, don't thread the edge.
// See the comments above findLoopHeaders for justifications and caveats.
if (LoopHeaders.count(BB))
return false;
for (BasicBlock::iterator BI = BB->begin();
PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
// Look for a Phi having at least one constant incoming value.
if (llvm::all_of(PN->incoming_values(),
[](Value *V) { return !isa<ConstantInt>(V); }))
continue;
auto isUnfoldCandidate = [BB](SelectInst *SI, Value *V) {
using namespace PatternMatch;
// Check if SI is in BB and use V as condition.
if (SI->getParent() != BB)
return false;
Value *Cond = SI->getCondition();
bool IsAndOr = match(SI, m_CombineOr(m_LogicalAnd(), m_LogicalOr()));
return Cond && Cond == V && Cond->getType()->isIntegerTy(1) && !IsAndOr;
};
SelectInst *SI = nullptr;
for (Use &U : PN->uses()) {
if (ICmpInst *Cmp = dyn_cast<ICmpInst>(U.getUser())) {
// Look for a ICmp in BB that compares PN with a constant and is the
// condition of a Select.
if (Cmp->getParent() == BB && Cmp->hasOneUse() &&
isa<ConstantInt>(Cmp->getOperand(1 - U.getOperandNo())))
if (SelectInst *SelectI = dyn_cast<SelectInst>(Cmp->user_back()))
if (isUnfoldCandidate(SelectI, Cmp->use_begin()->get())) {
SI = SelectI;
break;
}
} else if (SelectInst *SelectI = dyn_cast<SelectInst>(U.getUser())) {
// Look for a Select in BB that uses PN as condition.
if (isUnfoldCandidate(SelectI, U.get())) {
SI = SelectI;
break;
}
}
}
if (!SI)
continue;
// Expand the select.
Value *Cond = SI->getCondition();
if (InsertFreezeWhenUnfoldingSelect &&
!isGuaranteedNotToBeUndefOrPoison(Cond, nullptr, SI,
&DTU->getDomTree()))
Cond = new FreezeInst(Cond, "cond.fr", SI);
Instruction *Term = SplitBlockAndInsertIfThen(Cond, SI, false);
BasicBlock *SplitBB = SI->getParent();
BasicBlock *NewBB = Term->getParent();
PHINode *NewPN = PHINode::Create(SI->getType(), 2, "", SI);
NewPN->addIncoming(SI->getTrueValue(), Term->getParent());
NewPN->addIncoming(SI->getFalseValue(), BB);
SI->replaceAllUsesWith(NewPN);
SI->eraseFromParent();
// NewBB and SplitBB are newly created blocks which require insertion.
std::vector<DominatorTree::UpdateType> Updates;
Updates.reserve((2 * SplitBB->getTerminator()->getNumSuccessors()) + 3);
Updates.push_back({DominatorTree::Insert, BB, SplitBB});
Updates.push_back({DominatorTree::Insert, BB, NewBB});
Updates.push_back({DominatorTree::Insert, NewBB, SplitBB});
// BB's successors were moved to SplitBB, update DTU accordingly.
for (auto *Succ : successors(SplitBB)) {
Updates.push_back({DominatorTree::Delete, BB, Succ});
Updates.push_back({DominatorTree::Insert, SplitBB, Succ});
}
DTU->applyUpdatesPermissive(Updates);
return true;
}
return false;
}
/// Try to propagate a guard from the current BB into one of its predecessors
/// in case if another branch of execution implies that the condition of this
/// guard is always true. Currently we only process the simplest case that
/// looks like:
///
/// Start:
/// %cond = ...
/// br i1 %cond, label %T1, label %F1
/// T1:
/// br label %Merge
/// F1:
/// br label %Merge
/// Merge:
/// %condGuard = ...
/// call void(i1, ...) @llvm.experimental.guard( i1 %condGuard )[ "deopt"() ]
///
/// And cond either implies condGuard or !condGuard. In this case all the
/// instructions before the guard can be duplicated in both branches, and the
/// guard is then threaded to one of them.
bool JumpThreadingPass::processGuards(BasicBlock *BB) {
using namespace PatternMatch;
// We only want to deal with two predecessors.
BasicBlock *Pred1, *Pred2;
auto PI = pred_begin(BB), PE = pred_end(BB);
if (PI == PE)
return false;
Pred1 = *PI++;
if (PI == PE)
return false;
Pred2 = *PI++;
if (PI != PE)
return false;
if (Pred1 == Pred2)
return false;
// Try to thread one of the guards of the block.
// TODO: Look up deeper than to immediate predecessor?
auto *Parent = Pred1->getSinglePredecessor();
if (!Parent || Parent != Pred2->getSinglePredecessor())
return false;
if (auto *BI = dyn_cast<BranchInst>(Parent->getTerminator()))
for (auto &I : *BB)
if (isGuard(&I) && threadGuard(BB, cast<IntrinsicInst>(&I), BI))
return true;
return false;
}
/// Try to propagate the guard from BB which is the lower block of a diamond
/// to one of its branches, in case if diamond's condition implies guard's
/// condition.
bool JumpThreadingPass::threadGuard(BasicBlock *BB, IntrinsicInst *Guard,
BranchInst *BI) {
assert(BI->getNumSuccessors() == 2 && "Wrong number of successors?");
assert(BI->isConditional() && "Unconditional branch has 2 successors?");
Value *GuardCond = Guard->getArgOperand(0);
Value *BranchCond = BI->getCondition();
BasicBlock *TrueDest = BI->getSuccessor(0);
BasicBlock *FalseDest = BI->getSuccessor(1);
auto &DL = BB->getModule()->getDataLayout();
bool TrueDestIsSafe = false;
bool FalseDestIsSafe = false;
// True dest is safe if BranchCond => GuardCond.
auto Impl = isImpliedCondition(BranchCond, GuardCond, DL);
if (Impl && *Impl)
TrueDestIsSafe = true;
else {
// False dest is safe if !BranchCond => GuardCond.
Impl = isImpliedCondition(BranchCond, GuardCond, DL, /* LHSIsTrue */ false);
if (Impl && *Impl)
FalseDestIsSafe = true;
}
if (!TrueDestIsSafe && !FalseDestIsSafe)
return false;
BasicBlock *PredUnguardedBlock = TrueDestIsSafe ? TrueDest : FalseDest;
BasicBlock *PredGuardedBlock = FalseDestIsSafe ? TrueDest : FalseDest;
ValueToValueMapTy UnguardedMapping, GuardedMapping;
Instruction *AfterGuard = Guard->getNextNode();
unsigned Cost = getJumpThreadDuplicationCost(BB, AfterGuard, BBDupThreshold);
if (Cost > BBDupThreshold)
return false;
// Duplicate all instructions before the guard and the guard itself to the
// branch where implication is not proved.
BasicBlock *GuardedBlock = DuplicateInstructionsInSplitBetween(
BB, PredGuardedBlock, AfterGuard, GuardedMapping, *DTU);
assert(GuardedBlock && "Could not create the guarded block?");
// Duplicate all instructions before the guard in the unguarded branch.
// Since we have successfully duplicated the guarded block and this block
// has fewer instructions, we expect it to succeed.
BasicBlock *UnguardedBlock = DuplicateInstructionsInSplitBetween(
BB, PredUnguardedBlock, Guard, UnguardedMapping, *DTU);
assert(UnguardedBlock && "Could not create the unguarded block?");
LLVM_DEBUG(dbgs() << "Moved guard " << *Guard << " to block "
<< GuardedBlock->getName() << "\n");
// Some instructions before the guard may still have uses. For them, we need
// to create Phi nodes merging their copies in both guarded and unguarded
// branches. Those instructions that have no uses can be just removed.
SmallVector<Instruction *, 4> ToRemove;
for (auto BI = BB->begin(); &*BI != AfterGuard; ++BI)
if (!isa<PHINode>(&*BI))
ToRemove.push_back(&*BI);
Instruction *InsertionPoint = &*BB->getFirstInsertionPt();
assert(InsertionPoint && "Empty block?");
// Substitute with Phis & remove.
for (auto *Inst : reverse(ToRemove)) {
if (!Inst->use_empty()) {
PHINode *NewPN = PHINode::Create(Inst->getType(), 2);
NewPN->addIncoming(UnguardedMapping[Inst], UnguardedBlock);
NewPN->addIncoming(GuardedMapping[Inst], GuardedBlock);
NewPN->insertBefore(InsertionPoint);
Inst->replaceAllUsesWith(NewPN);
}
Inst->eraseFromParent();
}
return true;
}
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