ScalarEvolutionAliasAnalysis.cpp revision 212904
1198090Srdivacky//===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===// 2198090Srdivacky// 3198090Srdivacky// The LLVM Compiler Infrastructure 4198090Srdivacky// 5198090Srdivacky// This file is distributed under the University of Illinois Open Source 6198090Srdivacky// License. See LICENSE.TXT for details. 7198090Srdivacky// 8198090Srdivacky//===----------------------------------------------------------------------===// 9198090Srdivacky// 10198090Srdivacky// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a 11198090Srdivacky// simple alias analysis implemented in terms of ScalarEvolution queries. 12198090Srdivacky// 13204642Srdivacky// This differs from traditional loop dependence analysis in that it tests 14204642Srdivacky// for dependencies within a single iteration of a loop, rather than 15210299Sed// dependencies between different iterations. 16204642Srdivacky// 17198090Srdivacky// ScalarEvolution has a more complete understanding of pointer arithmetic 18198090Srdivacky// than BasicAliasAnalysis' collection of ad-hoc analyses. 19198090Srdivacky// 20198090Srdivacky//===----------------------------------------------------------------------===// 21198090Srdivacky 22198090Srdivacky#include "llvm/Analysis/AliasAnalysis.h" 23198090Srdivacky#include "llvm/Analysis/ScalarEvolutionExpressions.h" 24198090Srdivacky#include "llvm/Analysis/Passes.h" 25198090Srdivacky#include "llvm/Pass.h" 26198090Srdivackyusing namespace llvm; 27198090Srdivacky 28198090Srdivackynamespace { 29198090Srdivacky /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis 30198090Srdivacky /// implementation that uses ScalarEvolution to answer queries. 31198892Srdivacky class ScalarEvolutionAliasAnalysis : public FunctionPass, 32198892Srdivacky public AliasAnalysis { 33198090Srdivacky ScalarEvolution *SE; 34198090Srdivacky 35198090Srdivacky public: 36198090Srdivacky static char ID; // Class identification, replacement for typeinfo 37212904Sdim ScalarEvolutionAliasAnalysis() : FunctionPass(ID), SE(0) {} 38198090Srdivacky 39202878Srdivacky /// getAdjustedAnalysisPointer - This method is used when a pass implements 40202878Srdivacky /// an analysis interface through multiple inheritance. If needed, it 41202878Srdivacky /// should override this to adjust the this pointer as needed for the 42202878Srdivacky /// specified pass info. 43212904Sdim virtual void *getAdjustedAnalysisPointer(AnalysisID PI) { 44212904Sdim if (PI == &AliasAnalysis::ID) 45202878Srdivacky return (AliasAnalysis*)this; 46202878Srdivacky return this; 47202878Srdivacky } 48204642Srdivacky 49198090Srdivacky private: 50198090Srdivacky virtual void getAnalysisUsage(AnalysisUsage &AU) const; 51198090Srdivacky virtual bool runOnFunction(Function &F); 52198090Srdivacky virtual AliasResult alias(const Value *V1, unsigned V1Size, 53198090Srdivacky const Value *V2, unsigned V2Size); 54198090Srdivacky 55198892Srdivacky Value *GetBaseValue(const SCEV *S); 56198090Srdivacky }; 57198090Srdivacky} // End of anonymous namespace 58198090Srdivacky 59198090Srdivacky// Register this pass... 60198090Srdivackychar ScalarEvolutionAliasAnalysis::ID = 0; 61212904SdimINITIALIZE_AG_PASS(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa", 62212904Sdim "ScalarEvolution-based Alias Analysis", false, true, false); 63198090Srdivacky 64198090SrdivackyFunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() { 65198090Srdivacky return new ScalarEvolutionAliasAnalysis(); 66198090Srdivacky} 67198090Srdivacky 68198090Srdivackyvoid 69198090SrdivackyScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { 70198090Srdivacky AU.addRequiredTransitive<ScalarEvolution>(); 71198090Srdivacky AU.setPreservesAll(); 72198090Srdivacky AliasAnalysis::getAnalysisUsage(AU); 73198090Srdivacky} 74198090Srdivacky 75198090Srdivackybool 76198090SrdivackyScalarEvolutionAliasAnalysis::runOnFunction(Function &F) { 77198090Srdivacky InitializeAliasAnalysis(this); 78198090Srdivacky SE = &getAnalysis<ScalarEvolution>(); 79198090Srdivacky return false; 80198090Srdivacky} 81198090Srdivacky 82198892Srdivacky/// GetBaseValue - Given an expression, try to find a 83198892Srdivacky/// base value. Return null is none was found. 84198090SrdivackyValue * 85198892SrdivackyScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) { 86198090Srdivacky if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 87198090Srdivacky // In an addrec, assume that the base will be in the start, rather 88198090Srdivacky // than the step. 89198892Srdivacky return GetBaseValue(AR->getStart()); 90198090Srdivacky } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) { 91198090Srdivacky // If there's a pointer operand, it'll be sorted at the end of the list. 92198090Srdivacky const SCEV *Last = A->getOperand(A->getNumOperands()-1); 93204642Srdivacky if (Last->getType()->isPointerTy()) 94198892Srdivacky return GetBaseValue(Last); 95198090Srdivacky } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) { 96198892Srdivacky // This is a leaf node. 97198892Srdivacky return U->getValue(); 98198090Srdivacky } 99198090Srdivacky // No Identified object found. 100198090Srdivacky return 0; 101198090Srdivacky} 102198090Srdivacky 103198090SrdivackyAliasAnalysis::AliasResult 104198090SrdivackyScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize, 105198090Srdivacky const Value *B, unsigned BSize) { 106210299Sed // If either of the memory references is empty, it doesn't matter what the 107210299Sed // pointer values are. This allows the code below to ignore this special 108210299Sed // case. 109210299Sed if (ASize == 0 || BSize == 0) 110210299Sed return NoAlias; 111210299Sed 112198090Srdivacky // This is ScalarEvolutionAliasAnalysis. Get the SCEVs! 113198090Srdivacky const SCEV *AS = SE->getSCEV(const_cast<Value *>(A)); 114198090Srdivacky const SCEV *BS = SE->getSCEV(const_cast<Value *>(B)); 115198090Srdivacky 116198090Srdivacky // If they evaluate to the same expression, it's a MustAlias. 117198090Srdivacky if (AS == BS) return MustAlias; 118198090Srdivacky 119198090Srdivacky // If something is known about the difference between the two addresses, 120198090Srdivacky // see if it's enough to prove a NoAlias. 121198090Srdivacky if (SE->getEffectiveSCEVType(AS->getType()) == 122198090Srdivacky SE->getEffectiveSCEVType(BS->getType())) { 123198090Srdivacky unsigned BitWidth = SE->getTypeSizeInBits(AS->getType()); 124210299Sed APInt ASizeInt(BitWidth, ASize); 125210299Sed APInt BSizeInt(BitWidth, BSize); 126210299Sed 127210299Sed // Compute the difference between the two pointers. 128198090Srdivacky const SCEV *BA = SE->getMinusSCEV(BS, AS); 129210299Sed 130210299Sed // Test whether the difference is known to be great enough that memory of 131210299Sed // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 132210299Sed // are non-zero, which is special-cased above. 133210299Sed if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) && 134210299Sed (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax())) 135210299Sed return NoAlias; 136210299Sed 137210299Sed // Folding the subtraction while preserving range information can be tricky 138210299Sed // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS 139210299Sed // and try again to see if things fold better that way. 140210299Sed 141210299Sed // Compute the difference between the two pointers. 142210299Sed const SCEV *AB = SE->getMinusSCEV(AS, BS); 143210299Sed 144210299Sed // Test whether the difference is known to be great enough that memory of 145210299Sed // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt 146210299Sed // are non-zero, which is special-cased above. 147210299Sed if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) && 148210299Sed (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax())) 149210299Sed return NoAlias; 150198090Srdivacky } 151198090Srdivacky 152198090Srdivacky // If ScalarEvolution can find an underlying object, form a new query. 153198090Srdivacky // The correctness of this depends on ScalarEvolution not recognizing 154198090Srdivacky // inttoptr and ptrtoint operators. 155198892Srdivacky Value *AO = GetBaseValue(AS); 156198892Srdivacky Value *BO = GetBaseValue(BS); 157198090Srdivacky if ((AO && AO != A) || (BO && BO != B)) 158212904Sdim if (alias(AO ? AO : A, AO ? UnknownSize : ASize, 159212904Sdim BO ? BO : B, BO ? UnknownSize : BSize) == NoAlias) 160198090Srdivacky return NoAlias; 161198090Srdivacky 162198090Srdivacky // Forward the query to the next analysis. 163198090Srdivacky return AliasAnalysis::alias(A, ASize, B, BSize); 164198090Srdivacky} 165