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/InitializePasses.h" 23 using namespace llvm; 24 25 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA, 26 const MemoryLocation &LocB, AAQueryInfo &AAQI) { 27 // If either of the memory references is empty, it doesn't matter what the 28 // pointer values are. This allows the code below to ignore this special 29 // case. 30 if (LocA.Size.isZero() || LocB.Size.isZero()) 31 return NoAlias; 32 33 // This is SCEVAAResult. Get the SCEVs! 34 const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr)); 35 const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr)); 36 37 // If they evaluate to the same expression, it's a MustAlias. 38 if (AS == BS) 39 return MustAlias; 40 41 // If something is known about the difference between the two addresses, 42 // see if it's enough to prove a NoAlias. 43 if (SE.getEffectiveSCEVType(AS->getType()) == 44 SE.getEffectiveSCEVType(BS->getType())) { 45 unsigned BitWidth = SE.getTypeSizeInBits(AS->getType()); 46 APInt ASizeInt(BitWidth, LocA.Size.hasValue() 47 ? LocA.Size.getValue() 48 : MemoryLocation::UnknownSize); 49 APInt BSizeInt(BitWidth, LocB.Size.hasValue() 50 ? LocB.Size.getValue() 51 : MemoryLocation::UnknownSize); 52 53 // Compute the difference between the two pointers. 54 const SCEV *BA = SE.getMinusSCEV(BS, AS); 55 56 // Test whether the difference is known to be great enough that memory of 57 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 58 // are non-zero, which is special-cased above. 59 if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) && 60 (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax())) 61 return NoAlias; 62 63 // Folding the subtraction while preserving range information can be tricky 64 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS 65 // and try again to see if things fold better that way. 66 67 // Compute the difference between the two pointers. 68 const SCEV *AB = SE.getMinusSCEV(AS, BS); 69 70 // Test whether the difference is known to be great enough that memory of 71 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 72 // are non-zero, which is special-cased above. 73 if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) && 74 (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax())) 75 return NoAlias; 76 } 77 78 // If ScalarEvolution can find an underlying object, form a new query. 79 // The correctness of this depends on ScalarEvolution not recognizing 80 // inttoptr and ptrtoint operators. 81 Value *AO = GetBaseValue(AS); 82 Value *BO = GetBaseValue(BS); 83 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr)) 84 if (alias(MemoryLocation(AO ? AO : LocA.Ptr, 85 AO ? LocationSize::unknown() : LocA.Size, 86 AO ? AAMDNodes() : LocA.AATags), 87 MemoryLocation(BO ? BO : LocB.Ptr, 88 BO ? LocationSize::unknown() : LocB.Size, 89 BO ? AAMDNodes() : LocB.AATags), 90 AAQI) == NoAlias) 91 return NoAlias; 92 93 // Forward the query to the next analysis. 94 return AAResultBase::alias(LocA, LocB, AAQI); 95 } 96 97 /// Given an expression, try to find a base value. 98 /// 99 /// Returns null if none was found. 100 Value *SCEVAAResult::GetBaseValue(const SCEV *S) { 101 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 102 // In an addrec, assume that the base will be in the start, rather 103 // than the step. 104 return GetBaseValue(AR->getStart()); 105 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) { 106 // If there's a pointer operand, it'll be sorted at the end of the list. 107 const SCEV *Last = A->getOperand(A->getNumOperands() - 1); 108 if (Last->getType()->isPointerTy()) 109 return GetBaseValue(Last); 110 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) { 111 // This is a leaf node. 112 return U->getValue(); 113 } 114 // No Identified object found. 115 return nullptr; 116 } 117 118 AnalysisKey SCEVAA::Key; 119 120 SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) { 121 return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F)); 122 } 123 124 char SCEVAAWrapperPass::ID = 0; 125 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa", 126 "ScalarEvolution-based Alias Analysis", false, true) 127 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass) 128 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa", 129 "ScalarEvolution-based Alias Analysis", false, true) 130 131 FunctionPass *llvm::createSCEVAAWrapperPass() { 132 return new SCEVAAWrapperPass(); 133 } 134 135 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) { 136 initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry()); 137 } 138 139 bool SCEVAAWrapperPass::runOnFunction(Function &F) { 140 Result.reset( 141 new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE())); 142 return false; 143 } 144 145 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const { 146 AU.setPreservesAll(); 147 AU.addRequired<ScalarEvolutionWrapperPass>(); 148 } 149