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[MSSA] Avoid unnecessary use walks when calling getClobberingMemoryAc…
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…cess

Summary:
This allows us to mark when uses have been optimized.
This lets us avoid rewalking (IE when people call getClobberingAccess on everything), and also
enables us to later relax the requirement of use optimization during updates with less cost.

Reviewers: george.burgess.iv

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25172

llvm-svn: 284771
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@dberlin
dberlin committed Oct 20, 2016
1 parent 0a2cd96 commit cd2deac
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Showing 3 changed files with 88 additions and 26 deletions.
29 changes: 23 additions & 6 deletions llvm/include/llvm/Transforms/Utils/MemorySSA.h
View file
Expand Up @@ -110,6 +110,11 @@ class Instruction;
class MemoryAccess;
class LLVMContext;
class raw_ostream;
enum {
// Used to signify what the default invalid ID is for MemoryAccess's
// getID()
INVALID_MEMORYACCESS_ID = 0
};

template <class T> class memoryaccess_def_iterator_base;
using memoryaccess_def_iterator = memoryaccess_def_iterator_base<MemoryAccess>;
Expand Down Expand Up @@ -157,7 +162,8 @@ class MemoryAccess : public User, public ilist_node<MemoryAccess> {
friend class MemoryDef;
friend class MemoryPhi;

/// \brief Used internally to give IDs to MemoryAccesses for printing
/// \brief Used for debugging and tracking things about MemoryAccesses.
/// Guaranteed unique among MemoryAccesses, no guarantees otherwise.
virtual unsigned getID() const = 0;

MemoryAccess(LLVMContext &C, unsigned Vty, BasicBlock *BB,
Expand Down Expand Up @@ -235,21 +241,36 @@ class MemoryUse final : public MemoryUseOrDef {
void *operator new(size_t s) { return User::operator new(s, 1); }

MemoryUse(LLVMContext &C, MemoryAccess *DMA, Instruction *MI, BasicBlock *BB)
: MemoryUseOrDef(C, DMA, MemoryUseVal, MI, BB) {}
: MemoryUseOrDef(C, DMA, MemoryUseVal, MI, BB), OptimizedID(0) {}

static inline bool classof(const MemoryUse *) { return true; }
static inline bool classof(const Value *MA) {
return MA->getValueID() == MemoryUseVal;
}

void print(raw_ostream &OS) const override;
void setDefiningAccess(MemoryAccess *DMA, bool Optimized = false) {
if (Optimized)
OptimizedID = DMA->getID();
MemoryUseOrDef::setDefiningAccess(DMA);
}
bool isOptimized() const {
return getDefiningAccess() && OptimizedID == getDefiningAccess()->getID();
}
/// \brief Reset the ID of what this MemoryUse was optimized to, causing it to
/// be rewalked by the walker if necessary.
/// This really should only be called by tests.
void resetOptimized() { OptimizedID = INVALID_MEMORYACCESS_ID; }

protected:
friend class MemorySSA;

unsigned getID() const override {
llvm_unreachable("MemoryUses do not have IDs");
}

private:
unsigned int OptimizedID;
};

template <>
Expand Down Expand Up @@ -288,8 +309,6 @@ class MemoryDef final : public MemoryUseOrDef {
protected:
friend class MemorySSA;

// For debugging only. This gets used to give memory accesses pretty numbers
// when printing them out
unsigned getID() const override { return ID; }

private:
Expand Down Expand Up @@ -446,8 +465,6 @@ class MemoryPhi final : public MemoryAccess {
User::allocHungoffUses(N, /* IsPhi */ true);
}

/// For debugging only. This gets used to give memory accesses pretty numbers
/// when printing them out
unsigned getID() const final { return ID; }

private:
Expand Down
43 changes: 37 additions & 6 deletions llvm/lib/Transforms/Utils/MemorySSA.cpp
View file
Expand Up @@ -1229,7 +1229,7 @@ void MemorySSA::markUnreachableAsLiveOnEntry(BasicBlock *BB) {

MemorySSA::MemorySSA(Function &Func, AliasAnalysis *AA, DominatorTree *DT)
: AA(AA), DT(DT), F(Func), LiveOnEntryDef(nullptr), Walker(nullptr),
NextID(0) {
NextID(INVALID_MEMORYACCESS_ID) {
buildMemorySSA();
}

Expand Down Expand Up @@ -1332,7 +1332,7 @@ void MemorySSA::OptimizeUses::optimizeUsesInBlock(
}

if (isUseTriviallyOptimizableToLiveOnEntry(*AA, MU->getMemoryInst())) {
MU->setDefiningAccess(MSSA->getLiveOnEntryDef());
MU->setDefiningAccess(MSSA->getLiveOnEntryDef(), true);
continue;
}

Expand Down Expand Up @@ -1428,13 +1428,13 @@ void MemorySSA::OptimizeUses::optimizeUsesInBlock(
// At the end of this loop, UpperBound is either a clobber, or lower bound
// PHI walking may cause it to be < LowerBound, and in fact, < LastKill.
if (FoundClobberResult || UpperBound < LocInfo.LastKill) {
MU->setDefiningAccess(VersionStack[UpperBound]);
MU->setDefiningAccess(VersionStack[UpperBound], true);
// We were last killed now by where we got to
LocInfo.LastKill = UpperBound;
} else {
// Otherwise, we checked all the new ones, and now we know we can get to
// LastKill.
MU->setDefiningAccess(VersionStack[LocInfo.LastKill]);
MU->setDefiningAccess(VersionStack[LocInfo.LastKill], true);
}
LocInfo.LowerBound = VersionStack.size() - 1;
LocInfo.LowerBoundBlock = BB;
Expand Down Expand Up @@ -1754,8 +1754,27 @@ void MemorySSA::removeMemoryAccess(MemoryAccess *MA) {
}

// Re-point the uses at our defining access
if (!MA->use_empty())
MA->replaceAllUsesWith(NewDefTarget);
if (!MA->use_empty()) {
// Reset optimized on users of this store, and reset the uses.
// A few notes:
// 1. This is a slightly modified version of RAUW to avoid walking the
// uses twice here.
// 2. If we wanted to be complete, we would have to reset the optimized
// flags on users of phi nodes if doing the below makes a phi node have all
// the same arguments. Instead, we prefer users to removeMemoryAccess those
// phi nodes, because doing it here would be N^3.
if (MA->hasValueHandle())
ValueHandleBase::ValueIsRAUWd(MA, NewDefTarget);
// Note: We assume MemorySSA is not used in metadata since it's not really
// part of the IR.

while (!MA->use_empty()) {
Use &U = *MA->use_begin();
if (MemoryUse *MU = dyn_cast<MemoryUse>(U.getUser()))
MU->resetOptimized();
U.set(NewDefTarget);
}
}

// The call below to erase will destroy MA, so we can't change the order we
// are doing things here
Expand Down Expand Up @@ -2115,6 +2134,7 @@ void MemorySSA::CachingWalker::invalidateInfo(MemoryAccess *MA) {
if (MemoryUse *MU = dyn_cast<MemoryUse>(MA)) {
UpwardsMemoryQuery Q(MU->getMemoryInst(), MU);
Cache.remove(MU, Q.StartingLoc, Q.IsCall);
MU->resetOptimized();
} else {
// If it is not a use, the best we can do right now is destroy the cache.
Cache.clear();
Expand Down Expand Up @@ -2188,6 +2208,13 @@ MemorySSA::CachingWalker::getClobberingMemoryAccess(MemoryAccess *MA) {
if (!StartingAccess)
return MA;

// If this is an already optimized use or def, return the optimized result.
// Note: Currently, we do not store the optimized def result because we'd need
// a separate field, since we can't use it as the defining access.
if (MemoryUse *MU = dyn_cast<MemoryUse>(StartingAccess))
if (MU->isOptimized())
return MU->getDefiningAccess();

const Instruction *I = StartingAccess->getMemoryInst();
UpwardsMemoryQuery Q(I, StartingAccess);
// We can't sanely do anything with a fences, they conservatively
Expand All @@ -2202,6 +2229,8 @@ MemorySSA::CachingWalker::getClobberingMemoryAccess(MemoryAccess *MA) {
if (isUseTriviallyOptimizableToLiveOnEntry(*MSSA->AA, I)) {
MemoryAccess *LiveOnEntry = MSSA->getLiveOnEntryDef();
Cache.insert(StartingAccess, LiveOnEntry, Q.StartingLoc, Q.IsCall);
if (MemoryUse *MU = dyn_cast<MemoryUse>(StartingAccess))
MU->setDefiningAccess(LiveOnEntry, true);
return LiveOnEntry;
}

Expand All @@ -2218,6 +2247,8 @@ MemorySSA::CachingWalker::getClobberingMemoryAccess(MemoryAccess *MA) {
DEBUG(dbgs() << *DefiningAccess << "\n");
DEBUG(dbgs() << "Final Memory SSA clobber for " << *I << " is ");
DEBUG(dbgs() << *Result << "\n");
if (MemoryUse *MU = dyn_cast<MemoryUse>(StartingAccess))
MU->setDefiningAccess(Result, true);

return Result;
}
Expand Down
42 changes: 28 additions & 14 deletions llvm/unittests/Transforms/Utils/MemorySSA.cpp
View file
Expand Up @@ -225,9 +225,15 @@ TEST_F(MemorySSATest, RemoveMemoryAccess) {
// but we should now get live on entry for the clobbering definition of the
// load, since it will walk past the phi node since every argument is the
// same.
// XXX: This currently requires either removing the phi or resetting optimized
// on the load

EXPECT_FALSE(
MSSA.isLiveOnEntryDef(Walker->getClobberingMemoryAccess(LoadInst)));
// If we reset optimized, we get live on entry.
LoadAccess->resetOptimized();
EXPECT_TRUE(
MSSA.isLiveOnEntryDef(Walker->getClobberingMemoryAccess(LoadInst)));

// The phi should now be a two entry phi with two live on entry defs.
for (const auto &Op : DefiningAccess->operands()) {
MemoryAccess *Operand = cast<MemoryAccess>(&*Op);
Expand Down Expand Up @@ -450,24 +456,32 @@ TEST_F(MemorySSATest, WalkerInvariantLoadOpt) {
EXPECT_EQ(LoadClobber, MSSA.getLiveOnEntryDef());
}

// At one point, we were building MSSA with 0 AA passes. This ensures that we
// actually use BasicAA.
TEST_F(MemorySSATest, AAIsPresentAtBuildTime) {
// Test loads get reoptimized properly by the walker.
TEST_F(MemorySSATest, WalkerReopt) {
F = Function::Create(FunctionType::get(B.getVoidTy(), {}, false),
GlobalValue::ExternalLinkage, "F", &M);
B.SetInsertPoint(BasicBlock::Create(C, "", F));

Type *Int8 = Type::getInt8Ty(C);
Constant *One = ConstantInt::get(Int8, 1);
Value *AllocaA = B.CreateAlloca(Int8);
Instruction *StoreA = B.CreateStore(One, AllocaA);

Value *AllocaB = B.CreateAlloca(Int8);
B.CreateStore(One, AllocaB);
Instruction *LoadA = B.CreateLoad(AllocaA);
Value *AllocaA = B.CreateAlloca(Int8, ConstantInt::get(Int8, 1), "A");
Instruction *SIA = B.CreateStore(ConstantInt::get(Int8, 0), AllocaA);
Value *AllocaB = B.CreateAlloca(Int8, ConstantInt::get(Int8, 1), "B");
Instruction *SIB = B.CreateStore(ConstantInt::get(Int8, 0), AllocaB);
Instruction *LIA = B.CreateLoad(AllocaA);

setupAnalyses();
MemorySSA &MSSA = *Analyses->MSSA;
auto *MU = cast<MemoryUse>(MSSA.getMemoryAccess(LoadA));
EXPECT_EQ(MU->getDefiningAccess(), MSSA.getMemoryAccess(StoreA));
MemorySSAWalker *Walker = Analyses->Walker;

MemoryAccess *LoadClobber = Walker->getClobberingMemoryAccess(LIA);
MemoryUse *LoadAccess = cast<MemoryUse>(MSSA.getMemoryAccess(LIA));
EXPECT_EQ(LoadClobber, MSSA.getMemoryAccess(SIA));
EXPECT_TRUE(MSSA.isLiveOnEntryDef(Walker->getClobberingMemoryAccess(SIA)));
MSSA.removeMemoryAccess(LoadAccess);

// Create the load memory access pointing to an unoptimized place.
MemoryUse *NewLoadAccess = cast<MemoryUse>(MSSA.createMemoryAccessInBB(
LIA, MSSA.getMemoryAccess(SIB), LIA->getParent(), MemorySSA::End));
// This should it cause it to be optimized
EXPECT_EQ(Walker->getClobberingMemoryAccess(NewLoadAccess), LoadClobber);
EXPECT_EQ(NewLoadAccess->getDefiningAccess(), LoadClobber);
}

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