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[NaryReassoc] reassociate GEP for CSE
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Summary:
x = &a[i];
y = &a[i + j];

=>

y = x + j;

along with some refactoring work such as extracting method
findClosestMatchingDominator.

Depends on D9786 which provides the ScalarEvolution::getGEPExpr interface.

Test Plan: nary-gep.ll

Reviewers: meheff, broune

Reviewed By: broune

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D9802

llvm-svn: 237971
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Jingyue Wu committed May 21, 2015
1 parent abc3984 commit 4fc97f6
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266 changes: 245 additions & 21 deletions llvm/lib/Transforms/Scalar/NaryReassociate.cpp
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Expand Up @@ -85,6 +85,7 @@

#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
Expand All @@ -104,6 +105,10 @@ class NaryReassociate : public FunctionPass {
initializeNaryReassociatePass(*PassRegistry::getPassRegistry());
}

bool doInitialization(Module &M) override {
DL = &M.getDataLayout();
return false;
}
bool runOnFunction(Function &F) override;

void getAnalysisUsage(AnalysisUsage &AU) const override {
Expand All @@ -113,23 +118,57 @@ class NaryReassociate : public FunctionPass {
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<ScalarEvolution>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
AU.setPreservesCFG();
}

private:
// Runs only one iteration of the dominator-based algorithm. See the header
// comments for why we need multiple iterations.
bool doOneIteration(Function &F);
// Reasssociates I to a better form.
Instruction *tryReassociateAdd(Instruction *I);

// Reassociates I for better CSE.
Instruction *tryReassociate(Instruction *I);

// Reassociate GEP for better CSE.
Instruction *tryReassociateGEP(GetElementPtrInst *GEP);
// Try splitting GEP at the I-th index and see whether either part can be
// CSE'ed. This is a helper function for tryReassociateGEP.
//
// \p IndexedType The element type indexed by GEP's I-th index. This is
// equivalent to
// GEP->getIndexedType(GEP->getPointerOperand(), 0-th index,
// ..., i-th index).
GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
unsigned I, Type *IndexedType);
// Given GEP's I-th index = LHS + RHS, see whether &Base[..][LHS][..] or
// &Base[..][RHS][..] can be CSE'ed and rewrite GEP accordingly.
GetElementPtrInst *tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
unsigned I, Value *LHS,
Value *RHS, Type *IndexedType);

// Reassociate Add for better CSE.
Instruction *tryReassociateAdd(BinaryOperator *I);
// A helper function for tryReassociateAdd. LHS and RHS are explicitly passed.
Instruction *tryReassociateAdd(Value *LHS, Value *RHS, Instruction *I);
// Rewrites I to LHS + RHS if LHS is computed already.
Instruction *tryReassociatedAdd(const SCEV *LHS, Value *RHS, Instruction *I);

// Returns the closest dominator of \c Dominatee that computes
// \c CandidateExpr. Returns null if not found.
Instruction *findClosestMatchingDominator(const SCEV *CandidateExpr,
Instruction *Dominatee);
// GetElementPtrInst implicitly sign-extends an index if the index is shorter
// than the pointer size. This function returns whether Index is shorter than
// GEP's pointer size, i.e., whether Index needs to be sign-extended in order
// to be an index of GEP.
bool requiresSignExtension(Value *Index, GetElementPtrInst *GEP);

DominatorTree *DT;
ScalarEvolution *SE;
TargetLibraryInfo *TLI;
TargetTransformInfo *TTI;
const DataLayout *DL;
// A lookup table quickly telling which instructions compute the given SCEV.
// Note that there can be multiple instructions at different locations
// computing to the same SCEV, so we map a SCEV to an instruction list. For
Expand All @@ -149,6 +188,7 @@ INITIALIZE_PASS_BEGIN(NaryReassociate, "nary-reassociate", "Nary reassociation",
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(NaryReassociate, "nary-reassociate", "Nary reassociation",
false, false)

Expand All @@ -163,6 +203,7 @@ bool NaryReassociate::runOnFunction(Function &F) {
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
SE = &getAnalysis<ScalarEvolution>();
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);

bool Changed = false, ChangedInThisIteration;
do {
Expand All @@ -172,34 +213,205 @@ bool NaryReassociate::runOnFunction(Function &F) {
return Changed;
}

// Whitelist the instruction types NaryReassociate handles for now.
static bool isPotentiallyNaryReassociable(Instruction *I) {
switch (I->getOpcode()) {
case Instruction::Add:
case Instruction::GetElementPtr:
return true;
default:
return false;
}
}

bool NaryReassociate::doOneIteration(Function &F) {
bool Changed = false;
SeenExprs.clear();
// Traverse the dominator tree in the depth-first order. This order makes sure
// all bases of a candidate are in Candidates when we process it.
// Process the basic blocks in pre-order of the dominator tree. This order
// ensures that all bases of a candidate are in Candidates when we process it.
for (auto Node = GraphTraits<DominatorTree *>::nodes_begin(DT);
Node != GraphTraits<DominatorTree *>::nodes_end(DT); ++Node) {
BasicBlock *BB = Node->getBlock();
for (auto I = BB->begin(); I != BB->end(); ++I) {
// Skip vector types which are not SCEVable.
if (I->getOpcode() == Instruction::Add && !I->getType()->isVectorTy()) {
if (Instruction *NewI = tryReassociateAdd(I)) {
if (SE->isSCEVable(I->getType()) && isPotentiallyNaryReassociable(I)) {
if (Instruction *NewI = tryReassociate(I)) {
Changed = true;
SE->forgetValue(I);
I->replaceAllUsesWith(NewI);
RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
I = NewI;
}
// We should add the rewritten instruction because tryReassociateAdd may
// have invalidated the original one.
// Add the rewritten instruction to SeenExprs; the original instruction
// is deleted.
SeenExprs[SE->getSCEV(I)].push_back(I);
}
}
}
return Changed;
}

Instruction *NaryReassociate::tryReassociateAdd(Instruction *I) {
Instruction *NaryReassociate::tryReassociate(Instruction *I) {
switch (I->getOpcode()) {
case Instruction::Add:
return tryReassociateAdd(cast<BinaryOperator>(I));
case Instruction::GetElementPtr:
return tryReassociateGEP(cast<GetElementPtrInst>(I));
default:
llvm_unreachable("should be filtered out by isPotentiallyNaryReassociable");
}
}

// FIXME: extract this method into TTI->getGEPCost.
static bool isGEPFoldable(GetElementPtrInst *GEP,
const TargetTransformInfo *TTI,
const DataLayout *DL) {
GlobalVariable *BaseGV = nullptr;
int64_t BaseOffset = 0;
bool HasBaseReg = false;
int64_t Scale = 0;

if (GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getPointerOperand()))
BaseGV = GV;
else
HasBaseReg = true;

gep_type_iterator GTI = gep_type_begin(GEP);
for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I, ++GTI) {
if (isa<SequentialType>(*GTI)) {
int64_t ElementSize = DL->getTypeAllocSize(GTI.getIndexedType());
if (ConstantInt *ConstIdx = dyn_cast<ConstantInt>(*I)) {
BaseOffset += ConstIdx->getSExtValue() * ElementSize;
} else {
// Needs scale register.
if (Scale != 0) {
// No addressing mode takes two scale registers.
return false;
}
Scale = ElementSize;
}
} else {
StructType *STy = cast<StructType>(*GTI);
uint64_t Field = cast<ConstantInt>(*I)->getZExtValue();
BaseOffset += DL->getStructLayout(STy)->getElementOffset(Field);
}
}
return TTI->isLegalAddressingMode(GEP->getType()->getElementType(), BaseGV,
BaseOffset, HasBaseReg, Scale);
}

Instruction *NaryReassociate::tryReassociateGEP(GetElementPtrInst *GEP) {
// Not worth reassociating GEP if it is foldable.
if (isGEPFoldable(GEP, TTI, DL))
return nullptr;

gep_type_iterator GTI = gep_type_begin(*GEP);
for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I) {
if (isa<SequentialType>(*GTI++)) {
if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I - 1, *GTI)) {
return NewGEP;
}
}
}
return nullptr;
}

bool NaryReassociate::requiresSignExtension(Value *Index,
GetElementPtrInst *GEP) {
unsigned PointerSizeInBits =
DL->getPointerSizeInBits(GEP->getType()->getPointerAddressSpace());
return cast<IntegerType>(Index->getType())->getBitWidth() < PointerSizeInBits;
}

GetElementPtrInst *
NaryReassociate::tryReassociateGEPAtIndex(GetElementPtrInst *GEP, unsigned I,
Type *IndexedType) {
Value *IndexToSplit = GEP->getOperand(I + 1);
if (SExtInst *SExt = dyn_cast<SExtInst>(IndexToSplit))
IndexToSplit = SExt->getOperand(0);

if (AddOperator *AO = dyn_cast<AddOperator>(IndexToSplit)) {
// If the I-th index needs sext and the underlying add is not equipped with
// nsw, we cannot split the add because
// sext(LHS + RHS) != sext(LHS) + sext(RHS).
if (requiresSignExtension(IndexToSplit, GEP) && !AO->hasNoSignedWrap())
return nullptr;
Value *LHS = AO->getOperand(0), *RHS = AO->getOperand(1);
// IndexToSplit = LHS + RHS.
if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I, LHS, RHS, IndexedType))
return NewGEP;
// Symmetrically, try IndexToSplit = RHS + LHS.
if (LHS != RHS) {
if (auto *NewGEP =
tryReassociateGEPAtIndex(GEP, I, RHS, LHS, IndexedType))
return NewGEP;
}
}
return nullptr;
}

GetElementPtrInst *
NaryReassociate::tryReassociateGEPAtIndex(GetElementPtrInst *GEP, unsigned I,
Value *LHS, Value *RHS,
Type *IndexedType) {
// Look for GEP's closest dominator that has the same SCEV as GEP except that
// the I-th index is replaced with LHS.
SmallVector<const SCEV *, 4> IndexExprs;
for (auto Index = GEP->idx_begin(); Index != GEP->idx_end(); ++Index)
IndexExprs.push_back(SE->getSCEV(*Index));
// Replace the I-th index with LHS.
IndexExprs[I] = SE->getSCEV(LHS);
const SCEV *CandidateExpr = SE->getGEPExpr(
GEP->getSourceElementType(), SE->getSCEV(GEP->getPointerOperand()),
IndexExprs, GEP->isInBounds());

auto *Candidate = findClosestMatchingDominator(CandidateExpr, GEP);
if (Candidate == nullptr)
return nullptr;

PointerType *TypeOfCandidate = dyn_cast<PointerType>(Candidate->getType());
// Pretty rare but theoretically possible when a numeric value happens to
// share CandidateExpr.
if (TypeOfCandidate == nullptr)
return nullptr;

// NewGEP = (char *)Candidate + RHS * sizeof(IndexedType)
uint64_t IndexedSize = DL->getTypeAllocSize(IndexedType);
Type *ElementType = TypeOfCandidate->getElementType();
uint64_t ElementSize = DL->getTypeAllocSize(ElementType);
// Another less rare case: because I is not necessarily the last index of the
// GEP, the size of the type at the I-th index (IndexedSize) is not
// necessarily divisible by ElementSize. For example,
//
// #pragma pack(1)
// struct S {
// int a[3];
// int64 b[8];
// };
// #pragma pack()
//
// sizeof(S) = 100 is indivisible by sizeof(int64) = 8.
//
// TODO: bail out on this case for now. We could emit uglygep.
if (IndexedSize % ElementSize != 0)
return nullptr;

// NewGEP = &Candidate[RHS * (sizeof(IndexedType) / sizeof(Candidate[0])));
IRBuilder<> Builder(GEP);
Type *IntPtrTy = DL->getIntPtrType(TypeOfCandidate);
if (RHS->getType() != IntPtrTy)
RHS = Builder.CreateSExtOrTrunc(RHS, IntPtrTy);
if (IndexedSize != ElementSize) {
RHS = Builder.CreateMul(
RHS, ConstantInt::get(IntPtrTy, IndexedSize / ElementSize));
}
GetElementPtrInst *NewGEP =
cast<GetElementPtrInst>(Builder.CreateGEP(Candidate, RHS));
NewGEP->setIsInBounds(GEP->isInBounds());
NewGEP->takeName(GEP);
return NewGEP;
}

Instruction *NaryReassociate::tryReassociateAdd(BinaryOperator *I) {
Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
if (auto *NewI = tryReassociateAdd(LHS, RHS, I))
return NewI;
Expand Down Expand Up @@ -236,22 +448,34 @@ Instruction *NaryReassociate::tryReassociatedAdd(const SCEV *LHSExpr,
if (Pos == SeenExprs.end())
return nullptr;

auto &LHSCandidates = Pos->second;
// Look for the closest dominator LHS of I that computes LHSExpr, and replace
// I with LHS + RHS.
//
// Because we traverse the dominator tree in the pre-order, a
auto *LHS = findClosestMatchingDominator(LHSExpr, I);
if (LHS == nullptr)
return nullptr;

Instruction *NewI = BinaryOperator::CreateAdd(LHS, RHS, "", I);
NewI->takeName(I);
return NewI;
}

Instruction *
NaryReassociate::findClosestMatchingDominator(const SCEV *CandidateExpr,
Instruction *Dominatee) {
auto Pos = SeenExprs.find(CandidateExpr);
if (Pos == SeenExprs.end())
return nullptr;

auto &Candidates = Pos->second;
// Because we process the basic blocks in pre-order of the dominator tree, a
// candidate that doesn't dominate the current instruction won't dominate any
// future instruction either. Therefore, we pop it out of the stack. This
// optimization makes the algorithm O(n).
while (!LHSCandidates.empty()) {
Instruction *LHS = LHSCandidates.back();
if (DT->dominates(LHS, I)) {
Instruction *NewI = BinaryOperator::CreateAdd(LHS, RHS, "", I);
NewI->takeName(I);
return NewI;
}
LHSCandidates.pop_back();
while (!Candidates.empty()) {
Instruction *Candidate = Candidates.back();
if (DT->dominates(Candidate, Dominatee))
return Candidate;
Candidates.pop_back();
}
return nullptr;
}

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