| //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===// |
| // |
| // 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 file defines the ScalarEvolutionAliasAnalysis pass, which implements a |
| // simple alias analysis implemented in terms of ScalarEvolution queries. |
| // |
| // This differs from traditional loop dependence analysis in that it tests |
| // for dependencies within a single iteration of a loop, rather than |
| // dependencies between different iterations. |
| // |
| // ScalarEvolution has a more complete understanding of pointer arithmetic |
| // than BasicAliasAnalysis' collection of ad-hoc analyses. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" |
| #include "llvm/Analysis/ScalarEvolution.h" |
| #include "llvm/InitializePasses.h" |
| using namespace llvm; |
| |
| static bool canComputePointerDiff(ScalarEvolution &SE, |
| const SCEV *A, const SCEV *B) { |
| if (SE.getEffectiveSCEVType(A->getType()) != |
| SE.getEffectiveSCEVType(B->getType())) |
| return false; |
| |
| return SE.instructionCouldExistWitthOperands(A, B); |
| } |
| |
| AliasResult SCEVAAResult::alias(const MemoryLocation &LocA, |
| const MemoryLocation &LocB, AAQueryInfo &AAQI) { |
| // If either of the memory references is empty, it doesn't matter what the |
| // pointer values are. This allows the code below to ignore this special |
| // case. |
| if (LocA.Size.isZero() || LocB.Size.isZero()) |
| return AliasResult::NoAlias; |
| |
| // This is SCEVAAResult. Get the SCEVs! |
| const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr)); |
| const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr)); |
| |
| // If they evaluate to the same expression, it's a MustAlias. |
| if (AS == BS) |
| return AliasResult::MustAlias; |
| |
| // If something is known about the difference between the two addresses, |
| // see if it's enough to prove a NoAlias. |
| if (canComputePointerDiff(SE, AS, BS)) { |
| unsigned BitWidth = SE.getTypeSizeInBits(AS->getType()); |
| APInt ASizeInt(BitWidth, LocA.Size.hasValue() |
| ? LocA.Size.getValue() |
| : MemoryLocation::UnknownSize); |
| APInt BSizeInt(BitWidth, LocB.Size.hasValue() |
| ? LocB.Size.getValue() |
| : MemoryLocation::UnknownSize); |
| |
| // Compute the difference between the two pointers. |
| const SCEV *BA = SE.getMinusSCEV(BS, AS); |
| |
| // Test whether the difference is known to be great enough that memory of |
| // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt |
| // are non-zero, which is special-cased above. |
| if (!isa<SCEVCouldNotCompute>(BA) && |
| ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) && |
| (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax())) |
| return AliasResult::NoAlias; |
| |
| // Folding the subtraction while preserving range information can be tricky |
| // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS |
| // and try again to see if things fold better that way. |
| |
| // Compute the difference between the two pointers. |
| const SCEV *AB = SE.getMinusSCEV(AS, BS); |
| |
| // Test whether the difference is known to be great enough that memory of |
| // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt |
| // are non-zero, which is special-cased above. |
| if (!isa<SCEVCouldNotCompute>(AB) && |
| BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) && |
| (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax())) |
| return AliasResult::NoAlias; |
| } |
| |
| // If ScalarEvolution can find an underlying object, form a new query. |
| // The correctness of this depends on ScalarEvolution not recognizing |
| // inttoptr and ptrtoint operators. |
| Value *AO = GetBaseValue(AS); |
| Value *BO = GetBaseValue(BS); |
| if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr)) |
| if (alias(MemoryLocation(AO ? AO : LocA.Ptr, |
| AO ? LocationSize::beforeOrAfterPointer() |
| : LocA.Size, |
| AO ? AAMDNodes() : LocA.AATags), |
| MemoryLocation(BO ? BO : LocB.Ptr, |
| BO ? LocationSize::beforeOrAfterPointer() |
| : LocB.Size, |
| BO ? AAMDNodes() : LocB.AATags), |
| AAQI) == AliasResult::NoAlias) |
| return AliasResult::NoAlias; |
| |
| // Forward the query to the next analysis. |
| return AAResultBase::alias(LocA, LocB, AAQI); |
| } |
| |
| /// Given an expression, try to find a base value. |
| /// |
| /// Returns null if none was found. |
| Value *SCEVAAResult::GetBaseValue(const SCEV *S) { |
| if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { |
| // In an addrec, assume that the base will be in the start, rather |
| // than the step. |
| return GetBaseValue(AR->getStart()); |
| } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) { |
| // If there's a pointer operand, it'll be sorted at the end of the list. |
| const SCEV *Last = A->getOperand(A->getNumOperands() - 1); |
| if (Last->getType()->isPointerTy()) |
| return GetBaseValue(Last); |
| } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) { |
| // This is a leaf node. |
| return U->getValue(); |
| } |
| // No Identified object found. |
| return nullptr; |
| } |
| |
| bool SCEVAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA, |
| FunctionAnalysisManager::Invalidator &Inv) { |
| // We don't care if this analysis itself is preserved, it has no state. But |
| // we need to check that the analyses it depends on have been. |
| return Inv.invalidate<ScalarEvolutionAnalysis>(Fn, PA); |
| } |
| |
| AnalysisKey SCEVAA::Key; |
| |
| SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) { |
| return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F)); |
| } |
| |
| char SCEVAAWrapperPass::ID = 0; |
| INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa", |
| "ScalarEvolution-based Alias Analysis", false, true) |
| INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass) |
| INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa", |
| "ScalarEvolution-based Alias Analysis", false, true) |
| |
| FunctionPass *llvm::createSCEVAAWrapperPass() { |
| return new SCEVAAWrapperPass(); |
| } |
| |
| SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) { |
| initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool SCEVAAWrapperPass::runOnFunction(Function &F) { |
| Result.reset( |
| new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE())); |
| return false; |
| } |
| |
| void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.setPreservesAll(); |
| AU.addRequired<ScalarEvolutionWrapperPass>(); |
| } |