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llvm-project/llvm/lib/Transforms/Scalar/NaryReassociate.cpp

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//===- NaryReassociate.cpp - Reassociate n-ary expressions ----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass reassociates n-ary add expressions and eliminates the redundancy
// exposed by the reassociation.
//
// A motivating example:
//
// void foo(int a, int b) {
// bar(a + b);
// bar((a + 2) + b);
// }
//
// An ideal compiler should reassociate (a + 2) + b to (a + b) + 2 and simplify
// the above code to
//
// int t = a + b;
// bar(t);
// bar(t + 2);
//
// However, the Reassociate pass is unable to do that because it processes each
// instruction individually and believes (a + 2) + b is the best form according
// to its rank system.
//
// To address this limitation, NaryReassociate reassociates an expression in a
// form that reuses existing instructions. As a result, NaryReassociate can
// reassociate (a + 2) + b in the example to (a + b) + 2 because it detects that
// (a + b) is computed before.
//
// NaryReassociate works as follows. For every instruction in the form of (a +
// b) + c, it checks whether a + c or b + c is already computed by a dominating
// instruction. If so, it then reassociates (a + b) + c into (a + c) + b or (b +
// c) + a and removes the redundancy accordingly. To efficiently look up whether
// an expression is computed before, we store each instruction seen and its SCEV
// into an SCEV-to-instruction map.
//
// Although the algorithm pattern-matches only ternary additions, it
// automatically handles many >3-ary expressions by walking through the function
// in the depth-first order. For example, given
//
// (a + c) + d
// ((a + b) + c) + d
//
// NaryReassociate first rewrites (a + b) + c to (a + c) + b, and then rewrites
// ((a + c) + b) + d into ((a + c) + d) + b.
//
// Finally, the above dominator-based algorithm may need to be run multiple
// iterations before emitting optimal code. One source of this need is that we
// only split an operand when it is used only once. The above algorithm can
// eliminate an instruction and decrease the usage count of its operands. As a
// result, an instruction that previously had multiple uses may become a
// single-use instruction and thus eligible for split consideration. For
// example,
//
// ac = a + c
// ab = a + b
// abc = ab + c
// ab2 = ab + b
// ab2c = ab2 + c
//
// In the first iteration, we cannot reassociate abc to ac+b because ab is used
// twice. However, we can reassociate ab2c to abc+b in the first iteration. As a
// result, ab2 becomes dead and ab will be used only once in the second
// iteration.
//
// Limitations and TODO items:
//
// 1) We only considers n-ary adds and muls for now. This should be extended
// and generalized.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "nary-reassociate"
namespace {
class NaryReassociate : public FunctionPass {
public:
static char ID;
NaryReassociate(): FunctionPass(ID) {
initializeNaryReassociatePass(*PassRegistry::getPassRegistry());
}
bool doInitialization(Module &M) override {
DL = &M.getDataLayout();
return false;
}
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<ScalarEvolutionWrapperPass>();
AU.addPreserved<TargetLibraryInfoWrapperPass>();
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<ScalarEvolutionWrapperPass>();
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);
// 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 binary operators for better CSE.
Instruction *tryReassociateBinaryOp(BinaryOperator *I);
// A helper function for tryReassociateBinaryOp. LHS and RHS are explicitly
// passed.
Instruction *tryReassociateBinaryOp(Value *LHS, Value *RHS,
BinaryOperator *I);
// Rewrites I to (LHS op RHS) if LHS is computed already.
Instruction *tryReassociatedBinaryOp(const SCEV *LHS, Value *RHS,
BinaryOperator *I);
// Tries to match Op1 and Op2 by using V.
bool matchTernaryOp(BinaryOperator *I, Value *V, Value *&Op1, Value *&Op2);
// Gets SCEV for (LHS op RHS).
const SCEV *getBinarySCEV(BinaryOperator *I, const SCEV *LHS,
const SCEV *RHS);
// 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);
AssumptionCache *AC;
const DataLayout *DL;
DominatorTree *DT;
ScalarEvolution *SE;
TargetLibraryInfo *TLI;
TargetTransformInfo *TTI;
// 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
// example,
//
// if (p1)
// foo(a + b);
// if (p2)
// bar(a + b);
DenseMap<const SCEV *, SmallVector<WeakVH, 2>> SeenExprs;
};
} // anonymous namespace
char NaryReassociate::ID = 0;
INITIALIZE_PASS_BEGIN(NaryReassociate, "nary-reassociate", "Nary reassociation",
false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(NaryReassociate, "nary-reassociate", "Nary reassociation",
false, false)
FunctionPass *llvm::createNaryReassociatePass() {
return new NaryReassociate();
}
bool NaryReassociate::runOnFunction(Function &F) {
if (skipOptnoneFunction(F))
return false;
AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
bool Changed = false, ChangedInThisIteration;
do {
ChangedInThisIteration = doOneIteration(F);
Changed |= ChangedInThisIteration;
} while (ChangedInThisIteration);
return Changed;
}
// Whitelist the instruction types NaryReassociate handles for now.
static bool isPotentiallyNaryReassociable(Instruction *I) {
switch (I->getOpcode()) {
case Instruction::Add:
case Instruction::GetElementPtr:
case Instruction::Mul:
return true;
default:
return false;
}
}
bool NaryReassociate::doOneIteration(Function &F) {
bool Changed = false;
SeenExprs.clear();
// 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) {
if (SE->isSCEVable(I->getType()) && isPotentiallyNaryReassociable(&*I)) {
const SCEV *OldSCEV = SE->getSCEV(&*I);
if (Instruction *NewI = tryReassociate(&*I)) {
Changed = true;
SE->forgetValue(&*I);
I->replaceAllUsesWith(NewI);
// If SeenExprs constains I's WeakVH, that entry will be replaced with
// nullptr.
RecursivelyDeleteTriviallyDeadInstructions(&*I, TLI);
I = NewI->getIterator();
}
// Add the rewritten instruction to SeenExprs; the original instruction
// is deleted.
const SCEV *NewSCEV = SE->getSCEV(&*I);
SeenExprs[NewSCEV].push_back(WeakVH(&*I));
// Ideally, NewSCEV should equal OldSCEV because tryReassociate(I)
// is equivalent to I. However, ScalarEvolution::getSCEV may
// weaken nsw causing NewSCEV not to equal OldSCEV. For example, suppose
// we reassociate
// I = &a[sext(i +nsw j)] // assuming sizeof(a[0]) = 4
// to
// NewI = &a[sext(i)] + sext(j).
//
// ScalarEvolution computes
// getSCEV(I) = a + 4 * sext(i + j)
// getSCEV(newI) = a + 4 * sext(i) + 4 * sext(j)
// which are different SCEVs.
//
// To alleviate this issue of ScalarEvolution not always capturing
// equivalence, we add I to SeenExprs[OldSCEV] as well so that we can
// map both SCEV before and after tryReassociate(I) to I.
//
// This improvement is exercised in @reassociate_gep_nsw in nary-gep.ll.
if (NewSCEV != OldSCEV)
SeenExprs[OldSCEV].push_back(WeakVH(&*I));
}
}
}
return Changed;
}
Instruction *NaryReassociate::tryReassociate(Instruction *I) {
switch (I->getOpcode()) {
case Instruction::Add:
case Instruction::Mul:
return tryReassociateBinaryOp(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);
}
}
unsigned AddrSpace = GEP->getPointerAddressSpace();
return TTI->isLegalAddressingMode(GEP->getType()->getElementType(), BaseGV,
BaseOffset, HasBaseReg, Scale, AddrSpace);
}
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);
} else if (ZExtInst *ZExt = dyn_cast<ZExtInst>(IndexToSplit)) {
// zext can be treated as sext if the source is non-negative.
if (isKnownNonNegative(ZExt->getOperand(0), *DL, 0, AC, GEP, DT))
IndexToSplit = ZExt->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) &&
computeOverflowForSignedAdd(AO, *DL, AC, GEP, DT) !=
OverflowResult::NeverOverflows)
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);
if (isKnownNonNegative(LHS, *DL, 0, AC, GEP, DT) &&
DL->getTypeSizeInBits(LHS->getType()) <
DL->getTypeSizeInBits(GEP->getOperand(I)->getType())) {
// Zero-extend LHS if it is non-negative. InstCombine canonicalizes sext to
// zext if the source operand is proved non-negative. We should do that
// consistently so that CandidateExpr more likely appears before. See
// @reassociate_gep_assume for an example of this canonicalization.
IndexExprs[I] =
SE->getZeroExtendExpr(IndexExprs[I], GEP->getOperand(I)->getType());
}
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::tryReassociateBinaryOp(BinaryOperator *I) {
Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
if (auto *NewI = tryReassociateBinaryOp(LHS, RHS, I))
return NewI;
if (auto *NewI = tryReassociateBinaryOp(RHS, LHS, I))
return NewI;
return nullptr;
}
Instruction *NaryReassociate::tryReassociateBinaryOp(Value *LHS, Value *RHS,
BinaryOperator *I) {
Value *A = nullptr, *B = nullptr;
// To be conservative, we reassociate I only when it is the only user of (A op
// B).
if (LHS->hasOneUse() && matchTernaryOp(I, LHS, A, B)) {
// I = (A op B) op RHS
// = (A op RHS) op B or (B op RHS) op A
const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
const SCEV *RHSExpr = SE->getSCEV(RHS);
if (BExpr != RHSExpr) {
if (auto *NewI =
tryReassociatedBinaryOp(getBinarySCEV(I, AExpr, RHSExpr), B, I))
return NewI;
}
if (AExpr != RHSExpr) {
if (auto *NewI =
tryReassociatedBinaryOp(getBinarySCEV(I, BExpr, RHSExpr), A, I))
return NewI;
}
}
return nullptr;
}
Instruction *NaryReassociate::tryReassociatedBinaryOp(const SCEV *LHSExpr,
Value *RHS,
BinaryOperator *I) {
// Look for the closest dominator LHS of I that computes LHSExpr, and replace
// I with LHS op RHS.
auto *LHS = findClosestMatchingDominator(LHSExpr, I);
if (LHS == nullptr)
return nullptr;
Instruction *NewI = nullptr;
switch (I->getOpcode()) {
case Instruction::Add:
NewI = BinaryOperator::CreateAdd(LHS, RHS, "", I);
break;
case Instruction::Mul:
NewI = BinaryOperator::CreateMul(LHS, RHS, "", I);
break;
default:
llvm_unreachable("Unexpected instruction.");
}
NewI->takeName(I);
return NewI;
}
bool NaryReassociate::matchTernaryOp(BinaryOperator *I, Value *V, Value *&Op1,
Value *&Op2) {
switch (I->getOpcode()) {
case Instruction::Add:
return match(V, m_Add(m_Value(Op1), m_Value(Op2)));
case Instruction::Mul:
return match(V, m_Mul(m_Value(Op1), m_Value(Op2)));
default:
llvm_unreachable("Unexpected instruction.");
}
return false;
}
const SCEV *NaryReassociate::getBinarySCEV(BinaryOperator *I, const SCEV *LHS,
const SCEV *RHS) {
switch (I->getOpcode()) {
case Instruction::Add:
return SE->getAddExpr(LHS, RHS);
case Instruction::Mul:
return SE->getMulExpr(LHS, RHS);
default:
llvm_unreachable("Unexpected instruction.");
}
return nullptr;
}
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 (!Candidates.empty()) {
// Candidates stores WeakVHs, so a candidate can be nullptr if it's removed
// during rewriting.
if (Value *Candidate = Candidates.back()) {
Instruction *CandidateInstruction = cast<Instruction>(Candidate);
if (DT->dominates(CandidateInstruction, Dominatee))
return CandidateInstruction;
}
Candidates.pop_back();
}
return nullptr;
}
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