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llvm-mirror/lib/Transforms/Scalar/EarlyCSE.cpp
Geoff Berry 9de6b5b6de [EarlyCSE] Handle calls with no MemorySSA info. 
Summary:
When checking for memory dependencies between calls using MemorySSA,
handle cases where the calls have no MemoryAccess associated with them
because the AA analysis being used has determined that the call does not
read/write memory.

Fixes PR33756

Reviewers: dberlin, davide

Subscribers: mcrosier, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D35317

llvm-svn: 308051
2017-07-14 20:13:21 +00:00

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//===- EarlyCSE.cpp - Simple and fast CSE pass ----------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs a simple dominator tree walk that eliminates trivially
// redundant instructions.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/EarlyCSE.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/RecyclingAllocator.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include <deque>
using namespace llvm;
using namespace llvm::PatternMatch;
#define DEBUG_TYPE "early-cse"
STATISTIC(NumSimplify, "Number of instructions simplified or DCE'd");
STATISTIC(NumCSE, "Number of instructions CSE'd");
STATISTIC(NumCSECVP, "Number of compare instructions CVP'd");
STATISTIC(NumCSELoad, "Number of load instructions CSE'd");
STATISTIC(NumCSECall, "Number of call instructions CSE'd");
STATISTIC(NumDSE, "Number of trivial dead stores removed");
//===----------------------------------------------------------------------===//
// SimpleValue
//===----------------------------------------------------------------------===//
namespace {
/// \brief Struct representing the available values in the scoped hash table.
struct SimpleValue {
Instruction *Inst;
SimpleValue(Instruction *I) : Inst(I) {
assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
}
bool isSentinel() const {
return Inst == DenseMapInfo<Instruction *>::getEmptyKey() ||
Inst == DenseMapInfo<Instruction *>::getTombstoneKey();
}
static bool canHandle(Instruction *Inst) {
// This can only handle non-void readnone functions.
if (CallInst *CI = dyn_cast<CallInst>(Inst))
return CI->doesNotAccessMemory() && !CI->getType()->isVoidTy();
return isa<CastInst>(Inst) || isa<BinaryOperator>(Inst) ||
isa<GetElementPtrInst>(Inst) || isa<CmpInst>(Inst) ||
isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) ||
isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst) ||
isa<ExtractValueInst>(Inst) || isa<InsertValueInst>(Inst);
}
};
}
namespace llvm {
template <> struct DenseMapInfo<SimpleValue> {
static inline SimpleValue getEmptyKey() {
return DenseMapInfo<Instruction *>::getEmptyKey();
}
static inline SimpleValue getTombstoneKey() {
return DenseMapInfo<Instruction *>::getTombstoneKey();
}
static unsigned getHashValue(SimpleValue Val);
static bool isEqual(SimpleValue LHS, SimpleValue RHS);
};
}
unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) {
Instruction *Inst = Val.Inst;
// Hash in all of the operands as pointers.
if (BinaryOperator *BinOp = dyn_cast<BinaryOperator>(Inst)) {
Value *LHS = BinOp->getOperand(0);
Value *RHS = BinOp->getOperand(1);
if (BinOp->isCommutative() && BinOp->getOperand(0) > BinOp->getOperand(1))
std::swap(LHS, RHS);
return hash_combine(BinOp->getOpcode(), LHS, RHS);
}
if (CmpInst *CI = dyn_cast<CmpInst>(Inst)) {
Value *LHS = CI->getOperand(0);
Value *RHS = CI->getOperand(1);
CmpInst::Predicate Pred = CI->getPredicate();
if (Inst->getOperand(0) > Inst->getOperand(1)) {
std::swap(LHS, RHS);
Pred = CI->getSwappedPredicate();
}
return hash_combine(Inst->getOpcode(), Pred, LHS, RHS);
}
if (CastInst *CI = dyn_cast<CastInst>(Inst))
return hash_combine(CI->getOpcode(), CI->getType(), CI->getOperand(0));
if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst))
return hash_combine(EVI->getOpcode(), EVI->getOperand(0),
hash_combine_range(EVI->idx_begin(), EVI->idx_end()));
if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst))
return hash_combine(IVI->getOpcode(), IVI->getOperand(0),
IVI->getOperand(1),
hash_combine_range(IVI->idx_begin(), IVI->idx_end()));
assert((isa<CallInst>(Inst) || isa<BinaryOperator>(Inst) ||
isa<GetElementPtrInst>(Inst) || isa<SelectInst>(Inst) ||
isa<ExtractElementInst>(Inst) || isa<InsertElementInst>(Inst) ||
isa<ShuffleVectorInst>(Inst)) &&
"Invalid/unknown instruction");
// Mix in the opcode.
return hash_combine(
Inst->getOpcode(),
hash_combine_range(Inst->value_op_begin(), Inst->value_op_end()));
}
bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) {
Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
if (LHS.isSentinel() || RHS.isSentinel())
return LHSI == RHSI;
if (LHSI->getOpcode() != RHSI->getOpcode())
return false;
if (LHSI->isIdenticalToWhenDefined(RHSI))
return true;
// If we're not strictly identical, we still might be a commutable instruction
if (BinaryOperator *LHSBinOp = dyn_cast<BinaryOperator>(LHSI)) {
if (!LHSBinOp->isCommutative())
return false;
assert(isa<BinaryOperator>(RHSI) &&
"same opcode, but different instruction type?");
BinaryOperator *RHSBinOp = cast<BinaryOperator>(RHSI);
// Commuted equality
return LHSBinOp->getOperand(0) == RHSBinOp->getOperand(1) &&
LHSBinOp->getOperand(1) == RHSBinOp->getOperand(0);
}
if (CmpInst *LHSCmp = dyn_cast<CmpInst>(LHSI)) {
assert(isa<CmpInst>(RHSI) &&
"same opcode, but different instruction type?");
CmpInst *RHSCmp = cast<CmpInst>(RHSI);
// Commuted equality
return LHSCmp->getOperand(0) == RHSCmp->getOperand(1) &&
LHSCmp->getOperand(1) == RHSCmp->getOperand(0) &&
LHSCmp->getSwappedPredicate() == RHSCmp->getPredicate();
}
return false;
}
//===----------------------------------------------------------------------===//
// CallValue
//===----------------------------------------------------------------------===//
namespace {
/// \brief Struct representing the available call values in the scoped hash
/// table.
struct CallValue {
Instruction *Inst;
CallValue(Instruction *I) : Inst(I) {
assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
}
bool isSentinel() const {
return Inst == DenseMapInfo<Instruction *>::getEmptyKey() ||
Inst == DenseMapInfo<Instruction *>::getTombstoneKey();
}
static bool canHandle(Instruction *Inst) {
// Don't value number anything that returns void.
if (Inst->getType()->isVoidTy())
return false;
CallInst *CI = dyn_cast<CallInst>(Inst);
if (!CI || !CI->onlyReadsMemory())
return false;
return true;
}
};
}
namespace llvm {
template <> struct DenseMapInfo<CallValue> {
static inline CallValue getEmptyKey() {
return DenseMapInfo<Instruction *>::getEmptyKey();
}
static inline CallValue getTombstoneKey() {
return DenseMapInfo<Instruction *>::getTombstoneKey();
}
static unsigned getHashValue(CallValue Val);
static bool isEqual(CallValue LHS, CallValue RHS);
};
}
unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) {
Instruction *Inst = Val.Inst;
// Hash all of the operands as pointers and mix in the opcode.
return hash_combine(
Inst->getOpcode(),
hash_combine_range(Inst->value_op_begin(), Inst->value_op_end()));
}
bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) {
Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
if (LHS.isSentinel() || RHS.isSentinel())
return LHSI == RHSI;
return LHSI->isIdenticalTo(RHSI);
}
//===----------------------------------------------------------------------===//
// EarlyCSE implementation
//===----------------------------------------------------------------------===//
namespace {
/// \brief A simple and fast domtree-based CSE pass.
///
/// This pass does a simple depth-first walk over the dominator tree,
/// eliminating trivially redundant instructions and using instsimplify to
/// canonicalize things as it goes. It is intended to be fast and catch obvious
/// cases so that instcombine and other passes are more effective. It is
/// expected that a later pass of GVN will catch the interesting/hard cases.
class EarlyCSE {
public:
const TargetLibraryInfo &TLI;
const TargetTransformInfo &TTI;
DominatorTree &DT;
AssumptionCache &AC;
const SimplifyQuery SQ;
MemorySSA *MSSA;
std::unique_ptr<MemorySSAUpdater> MSSAUpdater;
typedef RecyclingAllocator<
BumpPtrAllocator, ScopedHashTableVal<SimpleValue, Value *>> AllocatorTy;
typedef ScopedHashTable<SimpleValue, Value *, DenseMapInfo<SimpleValue>,
AllocatorTy> ScopedHTType;
/// \brief A scoped hash table of the current values of all of our simple
/// scalar expressions.
///
/// As we walk down the domtree, we look to see if instructions are in this:
/// if so, we replace them with what we find, otherwise we insert them so
/// that dominated values can succeed in their lookup.
ScopedHTType AvailableValues;
/// A scoped hash table of the current values of previously encounted memory
/// locations.
///
/// This allows us to get efficient access to dominating loads or stores when
/// we have a fully redundant load. In addition to the most recent load, we
/// keep track of a generation count of the read, which is compared against
/// the current generation count. The current generation count is incremented
/// after every possibly writing memory operation, which ensures that we only
/// CSE loads with other loads that have no intervening store. Ordering
/// events (such as fences or atomic instructions) increment the generation
/// count as well; essentially, we model these as writes to all possible
/// locations. Note that atomic and/or volatile loads and stores can be
/// present the table; it is the responsibility of the consumer to inspect
/// the atomicity/volatility if needed.
struct LoadValue {
Instruction *DefInst;
unsigned Generation;
int MatchingId;
bool IsAtomic;
bool IsInvariant;
LoadValue()
: DefInst(nullptr), Generation(0), MatchingId(-1), IsAtomic(false),
IsInvariant(false) {}
LoadValue(Instruction *Inst, unsigned Generation, unsigned MatchingId,
bool IsAtomic, bool IsInvariant)
: DefInst(Inst), Generation(Generation), MatchingId(MatchingId),
IsAtomic(IsAtomic), IsInvariant(IsInvariant) {}
};
typedef RecyclingAllocator<BumpPtrAllocator,
ScopedHashTableVal<Value *, LoadValue>>
LoadMapAllocator;
typedef ScopedHashTable<Value *, LoadValue, DenseMapInfo<Value *>,
LoadMapAllocator> LoadHTType;
LoadHTType AvailableLoads;
/// \brief A scoped hash table of the current values of read-only call
/// values.
///
/// It uses the same generation count as loads.
typedef ScopedHashTable<CallValue, std::pair<Instruction *, unsigned>>
CallHTType;
CallHTType AvailableCalls;
/// \brief This is the current generation of the memory value.
unsigned CurrentGeneration;
/// \brief Set up the EarlyCSE runner for a particular function.
EarlyCSE(const DataLayout &DL, const TargetLibraryInfo &TLI,
const TargetTransformInfo &TTI, DominatorTree &DT,
AssumptionCache &AC, MemorySSA *MSSA)
: TLI(TLI), TTI(TTI), DT(DT), AC(AC), SQ(DL, &TLI, &DT, &AC), MSSA(MSSA),
MSSAUpdater(make_unique<MemorySSAUpdater>(MSSA)), CurrentGeneration(0) {
}
bool run();
private:
// Almost a POD, but needs to call the constructors for the scoped hash
// tables so that a new scope gets pushed on. These are RAII so that the
// scope gets popped when the NodeScope is destroyed.
class NodeScope {
public:
NodeScope(ScopedHTType &AvailableValues, LoadHTType &AvailableLoads,
CallHTType &AvailableCalls)
: Scope(AvailableValues), LoadScope(AvailableLoads),
CallScope(AvailableCalls) {}
private:
NodeScope(const NodeScope &) = delete;
void operator=(const NodeScope &) = delete;
ScopedHTType::ScopeTy Scope;
LoadHTType::ScopeTy LoadScope;
CallHTType::ScopeTy CallScope;
};
// Contains all the needed information to create a stack for doing a depth
// first traversal of the tree. This includes scopes for values, loads, and
// calls as well as the generation. There is a child iterator so that the
// children do not need to be store separately.
class StackNode {
public:
StackNode(ScopedHTType &AvailableValues, LoadHTType &AvailableLoads,
CallHTType &AvailableCalls, unsigned cg, DomTreeNode *n,
DomTreeNode::iterator child, DomTreeNode::iterator end)
: CurrentGeneration(cg), ChildGeneration(cg), Node(n), ChildIter(child),
EndIter(end), Scopes(AvailableValues, AvailableLoads, AvailableCalls),
Processed(false) {}
// Accessors.
unsigned currentGeneration() { return CurrentGeneration; }
unsigned childGeneration() { return ChildGeneration; }
void childGeneration(unsigned generation) { ChildGeneration = generation; }
DomTreeNode *node() { return Node; }
DomTreeNode::iterator childIter() { return ChildIter; }
DomTreeNode *nextChild() {
DomTreeNode *child = *ChildIter;
++ChildIter;
return child;
}
DomTreeNode::iterator end() { return EndIter; }
bool isProcessed() { return Processed; }
void process() { Processed = true; }
private:
StackNode(const StackNode &) = delete;
void operator=(const StackNode &) = delete;
// Members.
unsigned CurrentGeneration;
unsigned ChildGeneration;
DomTreeNode *Node;
DomTreeNode::iterator ChildIter;
DomTreeNode::iterator EndIter;
NodeScope Scopes;
bool Processed;
};
/// \brief Wrapper class to handle memory instructions, including loads,
/// stores and intrinsic loads and stores defined by the target.
class ParseMemoryInst {
public:
ParseMemoryInst(Instruction *Inst, const TargetTransformInfo &TTI)
: IsTargetMemInst(false), Inst(Inst) {
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst))
if (TTI.getTgtMemIntrinsic(II, Info))
IsTargetMemInst = true;
}
bool isLoad() const {
if (IsTargetMemInst) return Info.ReadMem;
return isa<LoadInst>(Inst);
}
bool isStore() const {
if (IsTargetMemInst) return Info.WriteMem;
return isa<StoreInst>(Inst);
}
bool isAtomic() const {
if (IsTargetMemInst)
return Info.Ordering != AtomicOrdering::NotAtomic;
return Inst->isAtomic();
}
bool isUnordered() const {
if (IsTargetMemInst)
return Info.isUnordered();
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
return LI->isUnordered();
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
return SI->isUnordered();
}
// Conservative answer
return !Inst->isAtomic();
}
bool isVolatile() const {
if (IsTargetMemInst)
return Info.IsVolatile;
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
return LI->isVolatile();
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
return SI->isVolatile();
}
// Conservative answer
return true;
}
bool isInvariantLoad() const {
if (auto *LI = dyn_cast<LoadInst>(Inst))
return LI->getMetadata(LLVMContext::MD_invariant_load) != nullptr;
return false;
}
bool isMatchingMemLoc(const ParseMemoryInst &Inst) const {
return (getPointerOperand() == Inst.getPointerOperand() &&
getMatchingId() == Inst.getMatchingId());
}
bool isValid() const { return getPointerOperand() != nullptr; }
// For regular (non-intrinsic) loads/stores, this is set to -1. For
// intrinsic loads/stores, the id is retrieved from the corresponding
// field in the MemIntrinsicInfo structure. That field contains
// non-negative values only.
int getMatchingId() const {
if (IsTargetMemInst) return Info.MatchingId;
return -1;
}
Value *getPointerOperand() const {
if (IsTargetMemInst) return Info.PtrVal;
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
return LI->getPointerOperand();
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
return SI->getPointerOperand();
}
return nullptr;
}
bool mayReadFromMemory() const {
if (IsTargetMemInst) return Info.ReadMem;
return Inst->mayReadFromMemory();
}
bool mayWriteToMemory() const {
if (IsTargetMemInst) return Info.WriteMem;
return Inst->mayWriteToMemory();
}
private:
bool IsTargetMemInst;
MemIntrinsicInfo Info;
Instruction *Inst;
};
bool processNode(DomTreeNode *Node);
Value *getOrCreateResult(Value *Inst, Type *ExpectedType) const {
if (auto *LI = dyn_cast<LoadInst>(Inst))
return LI;
if (auto *SI = dyn_cast<StoreInst>(Inst))
return SI->getValueOperand();
assert(isa<IntrinsicInst>(Inst) && "Instruction not supported");
return TTI.getOrCreateResultFromMemIntrinsic(cast<IntrinsicInst>(Inst),
ExpectedType);
}
bool isSameMemGeneration(unsigned EarlierGeneration, unsigned LaterGeneration,
Instruction *EarlierInst, Instruction *LaterInst);
void removeMSSA(Instruction *Inst) {
if (!MSSA)
return;
// Removing a store here can leave MemorySSA in an unoptimized state by
// creating MemoryPhis that have identical arguments and by creating
// MemoryUses whose defining access is not an actual clobber. We handle the
// phi case eagerly here. The non-optimized MemoryUse case is lazily
// updated by MemorySSA getClobberingMemoryAccess.
if (MemoryAccess *MA = MSSA->getMemoryAccess(Inst)) {
// Optimize MemoryPhi nodes that may become redundant by having all the
// same input values once MA is removed.
SmallSetVector<MemoryPhi *, 4> PhisToCheck;
SmallVector<MemoryAccess *, 8> WorkQueue;
WorkQueue.push_back(MA);
// Process MemoryPhi nodes in FIFO order using a ever-growing vector since
// we shouldn't be processing that many phis and this will avoid an
// allocation in almost all cases.
for (unsigned I = 0; I < WorkQueue.size(); ++I) {
MemoryAccess *WI = WorkQueue[I];
for (auto *U : WI->users())
if (MemoryPhi *MP = dyn_cast<MemoryPhi>(U))
PhisToCheck.insert(MP);
MSSAUpdater->removeMemoryAccess(WI);
for (MemoryPhi *MP : PhisToCheck) {
MemoryAccess *FirstIn = MP->getIncomingValue(0);
if (all_of(MP->incoming_values(),
[=](Use &In) { return In == FirstIn; }))
WorkQueue.push_back(MP);
}
PhisToCheck.clear();
}
}
}
};
}
/// Determine if the memory referenced by LaterInst is from the same heap
/// version as EarlierInst.
/// This is currently called in two scenarios:
///
/// load p
/// ...
/// load p
///
/// and
///
/// x = load p
/// ...
/// store x, p
///
/// in both cases we want to verify that there are no possible writes to the
/// memory referenced by p between the earlier and later instruction.
bool EarlyCSE::isSameMemGeneration(unsigned EarlierGeneration,
unsigned LaterGeneration,
Instruction *EarlierInst,
Instruction *LaterInst) {
// Check the simple memory generation tracking first.
if (EarlierGeneration == LaterGeneration)
return true;
if (!MSSA)
return false;
// If MemorySSA has determined that one of EarlierInst or LaterInst does not
// read/write memory, then we can safely return true here.
// FIXME: We could be more aggressive when checking doesNotAccessMemory(),
// onlyReadsMemory(), mayReadFromMemory(), and mayWriteToMemory() in this pass
// by also checking the MemorySSA MemoryAccess on the instruction. Initial
// experiments suggest this isn't worthwhile, at least for C/C++ code compiled
// with the default optimization pipeline.
auto *EarlierMA = MSSA->getMemoryAccess(EarlierInst);
if (!EarlierMA)
return true;
auto *LaterMA = MSSA->getMemoryAccess(LaterInst);
if (!LaterMA)
return true;
// Since we know LaterDef dominates LaterInst and EarlierInst dominates
// LaterInst, if LaterDef dominates EarlierInst then it can't occur between
// EarlierInst and LaterInst and neither can any other write that potentially
// clobbers LaterInst.
MemoryAccess *LaterDef =
MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
return MSSA->dominates(LaterDef, EarlierMA);
}
bool EarlyCSE::processNode(DomTreeNode *Node) {
bool Changed = false;
BasicBlock *BB = Node->getBlock();
// If this block has a single predecessor, then the predecessor is the parent
// of the domtree node and all of the live out memory values are still current
// in this block. If this block has multiple predecessors, then they could
// have invalidated the live-out memory values of our parent value. For now,
// just be conservative and invalidate memory if this block has multiple
// predecessors.
if (!BB->getSinglePredecessor())
++CurrentGeneration;
// If this node has a single predecessor which ends in a conditional branch,
// we can infer the value of the branch condition given that we took this
// path. We need the single predecessor to ensure there's not another path
// which reaches this block where the condition might hold a different
// value. Since we're adding this to the scoped hash table (like any other
// def), it will have been popped if we encounter a future merge block.
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
auto *BI = dyn_cast<BranchInst>(Pred->getTerminator());
if (BI && BI->isConditional()) {
auto *CondInst = dyn_cast<Instruction>(BI->getCondition());
if (CondInst && SimpleValue::canHandle(CondInst)) {
assert(BI->getSuccessor(0) == BB || BI->getSuccessor(1) == BB);
auto *TorF = (BI->getSuccessor(0) == BB)
? ConstantInt::getTrue(BB->getContext())
: ConstantInt::getFalse(BB->getContext());
AvailableValues.insert(CondInst, TorF);
DEBUG(dbgs() << "EarlyCSE CVP: Add conditional value for '"
<< CondInst->getName() << "' as " << *TorF << " in "
<< BB->getName() << "\n");
// Replace all dominated uses with the known value.
if (unsigned Count = replaceDominatedUsesWith(
CondInst, TorF, DT, BasicBlockEdge(Pred, BB))) {
Changed = true;
NumCSECVP += Count;
}
}
}
}
/// LastStore - Keep track of the last non-volatile store that we saw... for
/// as long as there in no instruction that reads memory. If we see a store
/// to the same location, we delete the dead store. This zaps trivial dead
/// stores which can occur in bitfield code among other things.
Instruction *LastStore = nullptr;
// See if any instructions in the block can be eliminated. If so, do it. If
// not, add them to AvailableValues.
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
Instruction *Inst = &*I++;
// Dead instructions should just be removed.
if (isInstructionTriviallyDead(Inst, &TLI)) {
DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n');
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumSimplify;
continue;
}
// Skip assume intrinsics, they don't really have side effects (although
// they're marked as such to ensure preservation of control dependencies),
// and this pass will not bother with its removal. However, we should mark
// its condition as true for all dominated blocks.
if (match(Inst, m_Intrinsic<Intrinsic::assume>())) {
auto *CondI =
dyn_cast<Instruction>(cast<CallInst>(Inst)->getArgOperand(0));
if (CondI && SimpleValue::canHandle(CondI)) {
DEBUG(dbgs() << "EarlyCSE considering assumption: " << *Inst << '\n');
AvailableValues.insert(CondI, ConstantInt::getTrue(BB->getContext()));
} else
DEBUG(dbgs() << "EarlyCSE skipping assumption: " << *Inst << '\n');
continue;
}
// Skip invariant.start intrinsics since they only read memory, and we can
// forward values across it. Also, we dont need to consume the last store
// since the semantics of invariant.start allow us to perform DSE of the
// last store, if there was a store following invariant.start. Consider:
//
// store 30, i8* p
// invariant.start(p)
// store 40, i8* p
// We can DSE the store to 30, since the store 40 to invariant location p
// causes undefined behaviour.
if (match(Inst, m_Intrinsic<Intrinsic::invariant_start>()))
continue;
if (match(Inst, m_Intrinsic<Intrinsic::experimental_guard>())) {
if (auto *CondI =
dyn_cast<Instruction>(cast<CallInst>(Inst)->getArgOperand(0))) {
if (SimpleValue::canHandle(CondI)) {
// Do we already know the actual value of this condition?
if (auto *KnownCond = AvailableValues.lookup(CondI)) {
// Is the condition known to be true?
if (isa<ConstantInt>(KnownCond) &&
cast<ConstantInt>(KnownCond)->isOne()) {
DEBUG(dbgs() << "EarlyCSE removing guard: " << *Inst << '\n');
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
continue;
} else
// Use the known value if it wasn't true.
cast<CallInst>(Inst)->setArgOperand(0, KnownCond);
}
// The condition we're on guarding here is true for all dominated
// locations.
AvailableValues.insert(CondI, ConstantInt::getTrue(BB->getContext()));
}
}
// Guard intrinsics read all memory, but don't write any memory.
// Accordingly, don't update the generation but consume the last store (to
// avoid an incorrect DSE).
LastStore = nullptr;
continue;
}
// If the instruction can be simplified (e.g. X+0 = X) then replace it with
// its simpler value.
if (Value *V = SimplifyInstruction(Inst, SQ)) {
DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << " to: " << *V << '\n');
bool Killed = false;
if (!Inst->use_empty()) {
Inst->replaceAllUsesWith(V);
Changed = true;
}
if (isInstructionTriviallyDead(Inst, &TLI)) {
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
Killed = true;
}
if (Changed)
++NumSimplify;
if (Killed)
continue;
}
// If this is a simple instruction that we can value number, process it.
if (SimpleValue::canHandle(Inst)) {
// See if the instruction has an available value. If so, use it.
if (Value *V = AvailableValues.lookup(Inst)) {
DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << " to: " << *V << '\n');
if (auto *I = dyn_cast<Instruction>(V))
I->andIRFlags(Inst);
Inst->replaceAllUsesWith(V);
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumCSE;
continue;
}
// Otherwise, just remember that this value is available.
AvailableValues.insert(Inst, Inst);
continue;
}
ParseMemoryInst MemInst(Inst, TTI);
// If this is a non-volatile load, process it.
if (MemInst.isValid() && MemInst.isLoad()) {
// (conservatively) we can't peak past the ordering implied by this
// operation, but we can add this load to our set of available values
if (MemInst.isVolatile() || !MemInst.isUnordered()) {
LastStore = nullptr;
++CurrentGeneration;
}
// If we have an available version of this load, and if it is the right
// generation or the load is known to be from an invariant location,
// replace this instruction.
//
// If either the dominating load or the current load are invariant, then
// we can assume the current load loads the same value as the dominating
// load.
LoadValue InVal = AvailableLoads.lookup(MemInst.getPointerOperand());
if (InVal.DefInst != nullptr &&
InVal.MatchingId == MemInst.getMatchingId() &&
// We don't yet handle removing loads with ordering of any kind.
!MemInst.isVolatile() && MemInst.isUnordered() &&
// We can't replace an atomic load with one which isn't also atomic.
InVal.IsAtomic >= MemInst.isAtomic() &&
(InVal.IsInvariant || MemInst.isInvariantLoad() ||
isSameMemGeneration(InVal.Generation, CurrentGeneration,
InVal.DefInst, Inst))) {
Value *Op = getOrCreateResult(InVal.DefInst, Inst->getType());
if (Op != nullptr) {
DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst
<< " to: " << *InVal.DefInst << '\n');
if (!Inst->use_empty())
Inst->replaceAllUsesWith(Op);
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumCSELoad;
continue;
}
}
// Otherwise, remember that we have this instruction.
AvailableLoads.insert(
MemInst.getPointerOperand(),
LoadValue(Inst, CurrentGeneration, MemInst.getMatchingId(),
MemInst.isAtomic(), MemInst.isInvariantLoad()));
LastStore = nullptr;
continue;
}
// If this instruction may read from memory or throw (and potentially read
// from memory in the exception handler), forget LastStore. Load/store
// intrinsics will indicate both a read and a write to memory. The target
// may override this (e.g. so that a store intrinsic does not read from
// memory, and thus will be treated the same as a regular store for
// commoning purposes).
if ((Inst->mayReadFromMemory() || Inst->mayThrow()) &&
!(MemInst.isValid() && !MemInst.mayReadFromMemory()))
LastStore = nullptr;
// If this is a read-only call, process it.
if (CallValue::canHandle(Inst)) {
// If we have an available version of this call, and if it is the right
// generation, replace this instruction.
std::pair<Instruction *, unsigned> InVal = AvailableCalls.lookup(Inst);
if (InVal.first != nullptr &&
isSameMemGeneration(InVal.second, CurrentGeneration, InVal.first,
Inst)) {
DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst
<< " to: " << *InVal.first << '\n');
if (!Inst->use_empty())
Inst->replaceAllUsesWith(InVal.first);
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumCSECall;
continue;
}
// Otherwise, remember that we have this instruction.
AvailableCalls.insert(
Inst, std::pair<Instruction *, unsigned>(Inst, CurrentGeneration));
continue;
}
// A release fence requires that all stores complete before it, but does
// not prevent the reordering of following loads 'before' the fence. As a
// result, we don't need to consider it as writing to memory and don't need
// to advance the generation. We do need to prevent DSE across the fence,
// but that's handled above.
if (FenceInst *FI = dyn_cast<FenceInst>(Inst))
if (FI->getOrdering() == AtomicOrdering::Release) {
assert(Inst->mayReadFromMemory() && "relied on to prevent DSE above");
continue;
}
// write back DSE - If we write back the same value we just loaded from
// the same location and haven't passed any intervening writes or ordering
// operations, we can remove the write. The primary benefit is in allowing
// the available load table to remain valid and value forward past where
// the store originally was.
if (MemInst.isValid() && MemInst.isStore()) {
LoadValue InVal = AvailableLoads.lookup(MemInst.getPointerOperand());
if (InVal.DefInst &&
InVal.DefInst == getOrCreateResult(Inst, InVal.DefInst->getType()) &&
InVal.MatchingId == MemInst.getMatchingId() &&
// We don't yet handle removing stores with ordering of any kind.
!MemInst.isVolatile() && MemInst.isUnordered() &&
isSameMemGeneration(InVal.Generation, CurrentGeneration,
InVal.DefInst, Inst)) {
// It is okay to have a LastStore to a different pointer here if MemorySSA
// tells us that the load and store are from the same memory generation.
// In that case, LastStore should keep its present value since we're
// removing the current store.
assert((!LastStore ||
ParseMemoryInst(LastStore, TTI).getPointerOperand() ==
MemInst.getPointerOperand() ||
MSSA) &&
"can't have an intervening store if not using MemorySSA!");
DEBUG(dbgs() << "EarlyCSE DSE (writeback): " << *Inst << '\n');
removeMSSA(Inst);
Inst->eraseFromParent();
Changed = true;
++NumDSE;
// We can avoid incrementing the generation count since we were able
// to eliminate this store.
continue;
}
}
// Okay, this isn't something we can CSE at all. Check to see if it is
// something that could modify memory. If so, our available memory values
// cannot be used so bump the generation count.
if (Inst->mayWriteToMemory()) {
++CurrentGeneration;
if (MemInst.isValid() && MemInst.isStore()) {
// We do a trivial form of DSE if there are two stores to the same
// location with no intervening loads. Delete the earlier store.
// At the moment, we don't remove ordered stores, but do remove
// unordered atomic stores. There's no special requirement (for
// unordered atomics) about removing atomic stores only in favor of
// other atomic stores since we we're going to execute the non-atomic
// one anyway and the atomic one might never have become visible.
if (LastStore) {
ParseMemoryInst LastStoreMemInst(LastStore, TTI);
assert(LastStoreMemInst.isUnordered() &&
!LastStoreMemInst.isVolatile() &&
"Violated invariant");
if (LastStoreMemInst.isMatchingMemLoc(MemInst)) {
DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore
<< " due to: " << *Inst << '\n');
removeMSSA(LastStore);
LastStore->eraseFromParent();
Changed = true;
++NumDSE;
LastStore = nullptr;
}
// fallthrough - we can exploit information about this store
}
// Okay, we just invalidated anything we knew about loaded values. Try
// to salvage *something* by remembering that the stored value is a live
// version of the pointer. It is safe to forward from volatile stores
// to non-volatile loads, so we don't have to check for volatility of
// the store.
AvailableLoads.insert(
MemInst.getPointerOperand(),
LoadValue(Inst, CurrentGeneration, MemInst.getMatchingId(),
MemInst.isAtomic(), /*IsInvariant=*/false));
// Remember that this was the last unordered store we saw for DSE. We
// don't yet handle DSE on ordered or volatile stores since we don't
// have a good way to model the ordering requirement for following
// passes once the store is removed. We could insert a fence, but
// since fences are slightly stronger than stores in their ordering,
// it's not clear this is a profitable transform. Another option would
// be to merge the ordering with that of the post dominating store.
if (MemInst.isUnordered() && !MemInst.isVolatile())
LastStore = Inst;
else
LastStore = nullptr;
}
}
}
return Changed;
}
bool EarlyCSE::run() {
// Note, deque is being used here because there is significant performance
// gains over vector when the container becomes very large due to the
// specific access patterns. For more information see the mailing list
// discussion on this:
// http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20120116/135228.html
std::deque<StackNode *> nodesToProcess;
bool Changed = false;
// Process the root node.
nodesToProcess.push_back(new StackNode(
AvailableValues, AvailableLoads, AvailableCalls, CurrentGeneration,
DT.getRootNode(), DT.getRootNode()->begin(), DT.getRootNode()->end()));
// Save the current generation.
unsigned LiveOutGeneration = CurrentGeneration;
// Process the stack.
while (!nodesToProcess.empty()) {
// Grab the first item off the stack. Set the current generation, remove
// the node from the stack, and process it.
StackNode *NodeToProcess = nodesToProcess.back();
// Initialize class members.
CurrentGeneration = NodeToProcess->currentGeneration();
// Check if the node needs to be processed.
if (!NodeToProcess->isProcessed()) {
// Process the node.
Changed |= processNode(NodeToProcess->node());
NodeToProcess->childGeneration(CurrentGeneration);
NodeToProcess->process();
} else if (NodeToProcess->childIter() != NodeToProcess->end()) {
// Push the next child onto the stack.
DomTreeNode *child = NodeToProcess->nextChild();
nodesToProcess.push_back(
new StackNode(AvailableValues, AvailableLoads, AvailableCalls,
NodeToProcess->childGeneration(), child, child->begin(),
child->end()));
} else {
// It has been processed, and there are no more children to process,
// so delete it and pop it off the stack.
delete NodeToProcess;
nodesToProcess.pop_back();
}
} // while (!nodes...)
// Reset the current generation.
CurrentGeneration = LiveOutGeneration;
return Changed;
}
PreservedAnalyses EarlyCSEPass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
auto &TTI = AM.getResult<TargetIRAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &AC = AM.getResult<AssumptionAnalysis>(F);
auto *MSSA =
UseMemorySSA ? &AM.getResult<MemorySSAAnalysis>(F).getMSSA() : nullptr;
EarlyCSE CSE(F.getParent()->getDataLayout(), TLI, TTI, DT, AC, MSSA);
if (!CSE.run())
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
PA.preserve<GlobalsAA>();
if (UseMemorySSA)
PA.preserve<MemorySSAAnalysis>();
return PA;
}
namespace {
/// \brief A simple and fast domtree-based CSE pass.
///
/// This pass does a simple depth-first walk over the dominator tree,
/// eliminating trivially redundant instructions and using instsimplify to
/// canonicalize things as it goes. It is intended to be fast and catch obvious
/// cases so that instcombine and other passes are more effective. It is
/// expected that a later pass of GVN will catch the interesting/hard cases.
template<bool UseMemorySSA>
class EarlyCSELegacyCommonPass : public FunctionPass {
public:
static char ID;
EarlyCSELegacyCommonPass() : FunctionPass(ID) {
if (UseMemorySSA)
initializeEarlyCSEMemSSALegacyPassPass(*PassRegistry::getPassRegistry());
else
initializeEarlyCSELegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto *MSSA =
UseMemorySSA ? &getAnalysis<MemorySSAWrapperPass>().getMSSA() : nullptr;
EarlyCSE CSE(F.getParent()->getDataLayout(), TLI, TTI, DT, AC, MSSA);
return CSE.run();
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
if (UseMemorySSA) {
AU.addRequired<MemorySSAWrapperPass>();
AU.addPreserved<MemorySSAWrapperPass>();
}
AU.addPreserved<GlobalsAAWrapperPass>();
AU.setPreservesCFG();
}
};
}
using EarlyCSELegacyPass = EarlyCSELegacyCommonPass</*UseMemorySSA=*/false>;
template<>
char EarlyCSELegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(EarlyCSELegacyPass, "early-cse", "Early CSE", false,
false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(EarlyCSELegacyPass, "early-cse", "Early CSE", false, false)
using EarlyCSEMemSSALegacyPass =
EarlyCSELegacyCommonPass</*UseMemorySSA=*/true>;
template<>
char EarlyCSEMemSSALegacyPass::ID = 0;
FunctionPass *llvm::createEarlyCSEPass(bool UseMemorySSA) {
if (UseMemorySSA)
return new EarlyCSEMemSSALegacyPass();
else
return new EarlyCSELegacyPass();
}
INITIALIZE_PASS_BEGIN(EarlyCSEMemSSALegacyPass, "early-cse-memssa",
"Early CSE w/ MemorySSA", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_END(EarlyCSEMemSSALegacyPass, "early-cse-memssa",
"Early CSE w/ MemorySSA", false, false)
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