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+//===- NaryReassociate.cpp - Reassociate n-ary expressions ----------------===//
+//
+// 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 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/Transforms/Scalar/NaryReassociate.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GetElementPtrTypeIterator.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/InstrTypes.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/IR/Type.h"
+#include "llvm/IR/Value.h"
+#include "llvm/IR/ValueHandle.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Transforms/Scalar.h"
+#include <cassert>
+#include <cstdint>
+
+using namespace llvm;
+using namespace PatternMatch;
+
+#define DEBUG_TYPE "nary-reassociate"
+
+namespace {
+
+class NaryReassociateLegacyPass : public FunctionPass {
+public:
+ static char ID;
+
+ NaryReassociateLegacyPass() : FunctionPass(ID) {
+ initializeNaryReassociateLegacyPassPass(*PassRegistry::getPassRegistry());
+ }
+
+ bool doInitialization(Module &M) override {
+ 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:
+ NaryReassociatePass Impl;
+};
+
+} // end anonymous namespace
+
+char NaryReassociateLegacyPass::ID = 0;
+
+INITIALIZE_PASS_BEGIN(NaryReassociateLegacyPass, "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(NaryReassociateLegacyPass, "nary-reassociate",
+ "Nary reassociation", false, false)
+
+FunctionPass *llvm::createNaryReassociatePass() {
+ return new NaryReassociateLegacyPass();
+}
+
+bool NaryReassociateLegacyPass::runOnFunction(Function &F) {
+ if (skipFunction(F))
+ return false;
+
+ auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
+ auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+ auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
+ auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
+
+ return Impl.runImpl(F, AC, DT, SE, TLI, TTI);
+}
+
+PreservedAnalyses NaryReassociatePass::run(Function &F,
+ FunctionAnalysisManager &AM) {
+ auto *AC = &AM.getResult<AssumptionAnalysis>(F);
+ auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
+ auto *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
+ auto *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
+ auto *TTI = &AM.getResult<TargetIRAnalysis>(F);
+
+ if (!runImpl(F, AC, DT, SE, TLI, TTI))
+ return PreservedAnalyses::all();
+
+ PreservedAnalyses PA;
+ PA.preserveSet<CFGAnalyses>();
+ PA.preserve<ScalarEvolutionAnalysis>();
+ return PA;
+}
+
+bool NaryReassociatePass::runImpl(Function &F, AssumptionCache *AC_,
+ DominatorTree *DT_, ScalarEvolution *SE_,
+ TargetLibraryInfo *TLI_,
+ TargetTransformInfo *TTI_) {
+ AC = AC_;
+ DT = DT_;
+ SE = SE_;
+ TLI = TLI_;
+ TTI = TTI_;
+ DL = &F.getParent()->getDataLayout();
+
+ 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 NaryReassociatePass::doOneIteration(Function &F) {
+ bool Changed = false;
+ SeenExprs.clear();
+ // Process the basic blocks in a depth first traversal of the dominator
+ // tree. This order ensures that all bases of a candidate are in Candidates
+ // when we process it.
+ for (const auto Node : depth_first(DT)) {
+ 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);
+ WeakVH NewIExist = NewI;
+ // If SeenExprs/NewIExist contains I's WeakTrackingVH/WeakVH, that
+ // entry will be replaced with nullptr if deleted.
+ RecursivelyDeleteTriviallyDeadInstructions(&*I, TLI);
+ if (!NewIExist) {
+ // Rare occation where the new instruction (NewI) have been removed,
+ // probably due to parts of the input code was dead from the
+ // beginning, reset the iterator and start over from the beginning
+ I = BB->begin();
+ continue;
+ }
+ I = NewI->getIterator();
+ }
+ // Add the rewritten instruction to SeenExprs; the original instruction
+ // is deleted.
+ const SCEV *NewSCEV = SE->getSCEV(&*I);
+ SeenExprs[NewSCEV].push_back(WeakTrackingVH(&*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(WeakTrackingVH(&*I));
+ }
+ }
+ }
+ return Changed;
+}
+
+Instruction *NaryReassociatePass::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");
+ }
+}
+
+static bool isGEPFoldable(GetElementPtrInst *GEP,
+ const TargetTransformInfo *TTI) {
+ SmallVector<const Value*, 4> Indices;
+ for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
+ Indices.push_back(*I);
+ return TTI->getGEPCost(GEP->getSourceElementType(), GEP->getPointerOperand(),
+ Indices) == TargetTransformInfo::TCC_Free;
+}
+
+Instruction *NaryReassociatePass::tryReassociateGEP(GetElementPtrInst *GEP) {
+ // Not worth reassociating GEP if it is foldable.
+ if (isGEPFoldable(GEP, TTI))
+ return nullptr;
+
+ gep_type_iterator GTI = gep_type_begin(*GEP);
+ for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
+ if (GTI.isSequential()) {
+ if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I - 1,
+ GTI.getIndexedType())) {
+ return NewGEP;
+ }
+ }
+ }
+ return nullptr;
+}
+
+bool NaryReassociatePass::requiresSignExtension(Value *Index,
+ GetElementPtrInst *GEP) {
+ unsigned PointerSizeInBits =
+ DL->getPointerSizeInBits(GEP->getType()->getPointerAddressSpace());
+ return cast<IntegerType>(Index->getType())->getBitWidth() < PointerSizeInBits;
+}
+
+GetElementPtrInst *
+NaryReassociatePass::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 *
+NaryReassociatePass::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(cast<GEPOperator>(GEP),
+ IndexExprs);
+
+ Value *Candidate = findClosestMatchingDominator(CandidateExpr, GEP);
+ if (Candidate == nullptr)
+ return nullptr;
+
+ IRBuilder<> Builder(GEP);
+ // Candidate does not necessarily have the same pointer type as GEP. Use
+ // bitcast or pointer cast to make sure they have the same type, so that the
+ // later RAUW doesn't complain.
+ Candidate = Builder.CreateBitOrPointerCast(Candidate, GEP->getType());
+ assert(Candidate->getType() == GEP->getType());
+
+ // NewGEP = (char *)Candidate + RHS * sizeof(IndexedType)
+ uint64_t IndexedSize = DL->getTypeAllocSize(IndexedType);
+ Type *ElementType = GEP->getResultElementType();
+ 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])));
+ Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
+ 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(GEP->getResultElementType(), Candidate, RHS));
+ NewGEP->setIsInBounds(GEP->isInBounds());
+ NewGEP->takeName(GEP);
+ return NewGEP;
+}
+
+Instruction *NaryReassociatePass::tryReassociateBinaryOp(BinaryOperator *I) {
+ Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
+ // There is no need to reassociate 0.
+ if (SE->getSCEV(I)->isZero())
+ return nullptr;
+ if (auto *NewI = tryReassociateBinaryOp(LHS, RHS, I))
+ return NewI;
+ if (auto *NewI = tryReassociateBinaryOp(RHS, LHS, I))
+ return NewI;
+ return nullptr;
+}
+
+Instruction *NaryReassociatePass::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 *NaryReassociatePass::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 NaryReassociatePass::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 *NaryReassociatePass::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 *
+NaryReassociatePass::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 WeakTrackingVHs, 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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