181 lines · cpp
1//===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8//9// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a10// simple alias analysis implemented in terms of ScalarEvolution queries.11//12// This differs from traditional loop dependence analysis in that it tests13// for dependencies within a single iteration of a loop, rather than14// dependencies between different iterations.15//16// ScalarEvolution has a more complete understanding of pointer arithmetic17// than BasicAliasAnalysis' collection of ad-hoc analyses.18//19//===----------------------------------------------------------------------===//20 21#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"22#include "llvm/Analysis/ScalarEvolution.h"23#include "llvm/Analysis/ScalarEvolutionExpressions.h"24#include "llvm/InitializePasses.h"25using namespace llvm;26 27static bool canComputePointerDiff(ScalarEvolution &SE,28 const SCEV *A, const SCEV *B) {29 if (SE.getEffectiveSCEVType(A->getType()) !=30 SE.getEffectiveSCEVType(B->getType()))31 return false;32 33 return SE.instructionCouldExistWithOperands(A, B);34}35 36AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,37 const MemoryLocation &LocB, AAQueryInfo &AAQI,38 const Instruction *) {39 // If either of the memory references is empty, it doesn't matter what the40 // pointer values are. This allows the code below to ignore this special41 // case.42 if (LocA.Size.isZero() || LocB.Size.isZero())43 return AliasResult::NoAlias;44 45 // This is SCEVAAResult. Get the SCEVs!46 const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));47 const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));48 49 // If they evaluate to the same expression, it's a MustAlias.50 if (AS == BS)51 return AliasResult::MustAlias;52 53 // If something is known about the difference between the two addresses,54 // see if it's enough to prove a NoAlias.55 if (canComputePointerDiff(SE, AS, BS)) {56 unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());57 APInt ASizeInt(BitWidth, LocA.Size.hasValue()58 ? static_cast<uint64_t>(LocA.Size.getValue())59 : MemoryLocation::UnknownSize);60 APInt BSizeInt(BitWidth, LocB.Size.hasValue()61 ? static_cast<uint64_t>(LocB.Size.getValue())62 : MemoryLocation::UnknownSize);63 64 // Firstly, try to convert the two pointers into ptrtoint expressions to65 // handle two pointers with different pointer bases.66 // Either both pointers are used with ptrtoint or neither, so we can't end67 // up with a ptr + int mix.68 const SCEV *AInt =69 SE.getPtrToIntExpr(AS, SE.getEffectiveSCEVType(AS->getType()));70 const SCEV *BInt =71 SE.getPtrToIntExpr(BS, SE.getEffectiveSCEVType(BS->getType()));72 if (!isa<SCEVCouldNotCompute>(AInt) && !isa<SCEVCouldNotCompute>(BInt)) {73 AS = AInt;74 BS = BInt;75 }76 77 // Compute the difference between the two pointers.78 const SCEV *BA = SE.getMinusSCEV(BS, AS);79 80 // Test whether the difference is known to be great enough that memory of81 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt82 // are non-zero, which is special-cased above.83 if (!isa<SCEVCouldNotCompute>(BA) &&84 ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&85 (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))86 return AliasResult::NoAlias;87 88 // Folding the subtraction while preserving range information can be tricky89 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS90 // and try again to see if things fold better that way.91 92 // Compute the difference between the two pointers.93 const SCEV *AB = SE.getMinusSCEV(AS, BS);94 95 // Test whether the difference is known to be great enough that memory of96 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt97 // are non-zero, which is special-cased above.98 if (!isa<SCEVCouldNotCompute>(AB) &&99 BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&100 (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))101 return AliasResult::NoAlias;102 }103 104 // If ScalarEvolution can find an underlying object, form a new query.105 // The correctness of this depends on ScalarEvolution not recognizing106 // inttoptr and ptrtoint operators.107 Value *AO = GetBaseValue(AS);108 Value *BO = GetBaseValue(BS);109 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))110 if (alias(MemoryLocation(AO ? AO : LocA.Ptr,111 AO ? LocationSize::beforeOrAfterPointer()112 : LocA.Size,113 AO ? AAMDNodes() : LocA.AATags),114 MemoryLocation(BO ? BO : LocB.Ptr,115 BO ? LocationSize::beforeOrAfterPointer()116 : LocB.Size,117 BO ? AAMDNodes() : LocB.AATags),118 AAQI, nullptr) == AliasResult::NoAlias)119 return AliasResult::NoAlias;120 121 return AliasResult::MayAlias;122}123 124/// Given an expression, try to find a base value.125///126/// Returns null if none was found.127Value *SCEVAAResult::GetBaseValue(const SCEV *S) {128 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {129 // In an addrec, assume that the base will be in the start, rather130 // than the step.131 return GetBaseValue(AR->getStart());132 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {133 // If there's a pointer operand, it'll be sorted at the end of the list.134 const SCEV *Last = A->getOperand(A->getNumOperands() - 1);135 if (Last->getType()->isPointerTy())136 return GetBaseValue(Last);137 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {138 // This is a leaf node.139 return U->getValue();140 }141 // No Identified object found.142 return nullptr;143}144 145bool SCEVAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA,146 FunctionAnalysisManager::Invalidator &Inv) {147 // We don't care if this analysis itself is preserved, it has no state. But148 // we need to check that the analyses it depends on have been.149 return Inv.invalidate<ScalarEvolutionAnalysis>(Fn, PA);150}151 152AnalysisKey SCEVAA::Key;153 154SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {155 return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));156}157 158char SCEVAAWrapperPass::ID = 0;159INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",160 "ScalarEvolution-based Alias Analysis", false, true)161INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)162INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",163 "ScalarEvolution-based Alias Analysis", false, true)164 165FunctionPass *llvm::createSCEVAAWrapperPass() {166 return new SCEVAAWrapperPass();167}168 169SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {}170 171bool SCEVAAWrapperPass::runOnFunction(Function &F) {172 Result.reset(173 new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));174 return false;175}176 177void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {178 AU.setPreservesAll();179 AU.addRequired<ScalarEvolutionWrapperPass>();180}181