1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a 11 // simple alias analysis implemented in terms of ScalarEvolution queries. 12 // 13 // This differs from traditional loop dependence analysis in that it tests 14 // for dependencies within a single iteration of a loop, rather than 15 // dependencies between different iterations. 16 // 17 // ScalarEvolution has a more complete understanding of pointer arithmetic 18 // than BasicAliasAnalysis' collection of ad-hoc analyses. 19 // 20 //===----------------------------------------------------------------------===// 21 22 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" 23 using namespace llvm; 24 25 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA, 26 const MemoryLocation &LocB) { 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 == 0 || LocB.Size == 0) 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); 47 APInt BSizeInt(BitWidth, LocB.Size); 48 49 // Compute the difference between the two pointers. 50 const SCEV *BA = SE.getMinusSCEV(BS, AS); 51 52 // Test whether the difference is known to be great enough that memory of 53 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 54 // are non-zero, which is special-cased above. 55 if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) && 56 (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax())) 57 return NoAlias; 58 59 // Folding the subtraction while preserving range information can be tricky 60 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS 61 // and try again to see if things fold better that way. 62 63 // Compute the difference between the two pointers. 64 const SCEV *AB = SE.getMinusSCEV(AS, BS); 65 66 // Test whether the difference is known to be great enough that memory of 67 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 68 // are non-zero, which is special-cased above. 69 if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) && 70 (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax())) 71 return NoAlias; 72 } 73 74 // If ScalarEvolution can find an underlying object, form a new query. 75 // The correctness of this depends on ScalarEvolution not recognizing 76 // inttoptr and ptrtoint operators. 77 Value *AO = GetBaseValue(AS); 78 Value *BO = GetBaseValue(BS); 79 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr)) 80 if (alias(MemoryLocation(AO ? AO : LocA.Ptr, 81 AO ? +MemoryLocation::UnknownSize : LocA.Size, 82 AO ? AAMDNodes() : LocA.AATags), 83 MemoryLocation(BO ? BO : LocB.Ptr, 84 BO ? +MemoryLocation::UnknownSize : LocB.Size, 85 BO ? AAMDNodes() : LocB.AATags)) == NoAlias) 86 return NoAlias; 87 88 // Forward the query to the next analysis. 89 return AAResultBase::alias(LocA, LocB); 90 } 91 92 /// Given an expression, try to find a base value. 93 /// 94 /// Returns null if none was found. 95 Value *SCEVAAResult::GetBaseValue(const SCEV *S) { 96 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 97 // In an addrec, assume that the base will be in the start, rather 98 // than the step. 99 return GetBaseValue(AR->getStart()); 100 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) { 101 // If there's a pointer operand, it'll be sorted at the end of the list. 102 const SCEV *Last = A->getOperand(A->getNumOperands() - 1); 103 if (Last->getType()->isPointerTy()) 104 return GetBaseValue(Last); 105 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) { 106 // This is a leaf node. 107 return U->getValue(); 108 } 109 // No Identified object found. 110 return nullptr; 111 } 112 113 char SCEVAA::PassID; 114 115 SCEVAAResult SCEVAA::run(Function &F, AnalysisManager<Function> &AM) { 116 return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F)); 117 } 118 119 char SCEVAAWrapperPass::ID = 0; 120 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa", 121 "ScalarEvolution-based Alias Analysis", false, true) 122 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass) 123 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa", 124 "ScalarEvolution-based Alias Analysis", false, true) 125 126 FunctionPass *llvm::createSCEVAAWrapperPass() { 127 return new SCEVAAWrapperPass(); 128 } 129 130 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) { 131 initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry()); 132 } 133 134 bool SCEVAAWrapperPass::runOnFunction(Function &F) { 135 Result.reset( 136 new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE())); 137 return false; 138 } 139 140 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const { 141 AU.setPreservesAll(); 142 AU.addRequired<ScalarEvolutionWrapperPass>(); 143 } 144
