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