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-exception 6// 7//===----------------------------------------------------------------------===// 8// 9// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a 10// simple alias analysis implemented in terms of ScalarEvolution queries. 11// 12// This differs from traditional loop dependence analysis in that it tests 13// for dependencies within a single iteration of a loop, rather than 14// dependencies between different iterations. 15// 16// ScalarEvolution has a more complete understanding of pointer arithmetic 17// 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.instructionCouldExistWitthOperands(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 the 40 // pointer values are. This allows the code below to ignore this special 41 // 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 ? LocA.Size.getValue() 59 : MemoryLocation::UnknownSize); 60 APInt BSizeInt(BitWidth, LocB.Size.hasValue() 61 ? LocB.Size.getValue() 62 : MemoryLocation::UnknownSize); 63 64 // Compute the difference between the two pointers. 65 const SCEV *BA = SE.getMinusSCEV(BS, AS); 66 67 // Test whether the difference is known to be great enough that memory of 68 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 69 // are non-zero, which is special-cased above. 70 if (!isa<SCEVCouldNotCompute>(BA) && 71 ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) && 72 (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax())) 73 return AliasResult::NoAlias; 74 75 // Folding the subtraction while preserving range information can be tricky 76 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS 77 // and try again to see if things fold better that way. 78 79 // Compute the difference between the two pointers. 80 const SCEV *AB = SE.getMinusSCEV(AS, BS); 81 82 // Test whether the difference is known to be great enough that memory of 83 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 84 // are non-zero, which is special-cased above. 85 if (!isa<SCEVCouldNotCompute>(AB) && 86 BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) && 87 (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax())) 88 return AliasResult::NoAlias; 89 } 90 91 // If ScalarEvolution can find an underlying object, form a new query. 92 // The correctness of this depends on ScalarEvolution not recognizing 93 // inttoptr and ptrtoint operators. 94 Value *AO = GetBaseValue(AS); 95 Value *BO = GetBaseValue(BS); 96 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr)) 97 if (alias(MemoryLocation(AO ? AO : LocA.Ptr, 98 AO ? LocationSize::beforeOrAfterPointer() 99 : LocA.Size, 100 AO ? AAMDNodes() : LocA.AATags), 101 MemoryLocation(BO ? BO : LocB.Ptr, 102 BO ? LocationSize::beforeOrAfterPointer() 103 : LocB.Size, 104 BO ? AAMDNodes() : LocB.AATags), 105 AAQI, nullptr) == AliasResult::NoAlias) 106 return AliasResult::NoAlias; 107 108 // Forward the query to the next analysis. 109 return AAResultBase::alias(LocA, LocB, AAQI, nullptr); 110} 111 112/// Given an expression, try to find a base value. 113/// 114/// Returns null if none was found. 115Value *SCEVAAResult::GetBaseValue(const SCEV *S) { 116 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 117 // In an addrec, assume that the base will be in the start, rather 118 // than the step. 119 return GetBaseValue(AR->getStart()); 120 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) { 121 // If there's a pointer operand, it'll be sorted at the end of the list. 122 const SCEV *Last = A->getOperand(A->getNumOperands() - 1); 123 if (Last->getType()->isPointerTy()) 124 return GetBaseValue(Last); 125 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) { 126 // This is a leaf node. 127 return U->getValue(); 128 } 129 // No Identified object found. 130 return nullptr; 131} 132 133bool SCEVAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA, 134 FunctionAnalysisManager::Invalidator &Inv) { 135 // We don't care if this analysis itself is preserved, it has no state. But 136 // we need to check that the analyses it depends on have been. 137 return Inv.invalidate<ScalarEvolutionAnalysis>(Fn, PA); 138} 139 140AnalysisKey SCEVAA::Key; 141 142SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) { 143 return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F)); 144} 145 146char SCEVAAWrapperPass::ID = 0; 147INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa", 148 "ScalarEvolution-based Alias Analysis", false, true) 149INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass) 150INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa", 151 "ScalarEvolution-based Alias Analysis", false, true) 152 153FunctionPass *llvm::createSCEVAAWrapperPass() { 154 return new SCEVAAWrapperPass(); 155} 156 157SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) { 158 initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry()); 159} 160 161bool SCEVAAWrapperPass::runOnFunction(Function &F) { 162 Result.reset( 163 new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE())); 164 return false; 165} 166 167void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const { 168 AU.setPreservesAll(); 169 AU.addRequired<ScalarEvolutionWrapperPass>(); 170} 171