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130 lines
4.7 KiB
C++
130 lines
4.7 KiB
C++
//===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
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// simple alias analysis implemented in terms of ScalarEvolution queries.
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//
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// ScalarEvolution has a more complete understanding of pointer arithmetic
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// than BasicAliasAnalysis' collection of ad-hoc analyses.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Analysis/Passes.h"
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#include "llvm/Pass.h"
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using namespace llvm;
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namespace {
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/// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
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/// implementation that uses ScalarEvolution to answer queries.
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class ScalarEvolutionAliasAnalysis : public FunctionPass,
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public AliasAnalysis {
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ScalarEvolution *SE;
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public:
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static char ID; // Class identification, replacement for typeinfo
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ScalarEvolutionAliasAnalysis() : FunctionPass(&ID), SE(0) {}
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private:
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virtual void getAnalysisUsage(AnalysisUsage &AU) const;
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virtual bool runOnFunction(Function &F);
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virtual AliasResult alias(const Value *V1, unsigned V1Size,
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const Value *V2, unsigned V2Size);
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Value *GetBaseValue(const SCEV *S);
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};
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} // End of anonymous namespace
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// Register this pass...
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char ScalarEvolutionAliasAnalysis::ID = 0;
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static RegisterPass<ScalarEvolutionAliasAnalysis>
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X("scev-aa", "ScalarEvolution-based Alias Analysis", false, true);
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// Declare that we implement the AliasAnalysis interface
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static RegisterAnalysisGroup<AliasAnalysis> Y(X);
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FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
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return new ScalarEvolutionAliasAnalysis();
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}
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void
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ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequiredTransitive<ScalarEvolution>();
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AU.setPreservesAll();
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AliasAnalysis::getAnalysisUsage(AU);
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}
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bool
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ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
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InitializeAliasAnalysis(this);
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SE = &getAnalysis<ScalarEvolution>();
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return false;
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}
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/// GetBaseValue - Given an expression, try to find a
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/// base value. Return null is none was found.
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Value *
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ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
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if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
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// In an addrec, assume that the base will be in the start, rather
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// than the step.
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return GetBaseValue(AR->getStart());
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} else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
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// If there's a pointer operand, it'll be sorted at the end of the list.
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const SCEV *Last = A->getOperand(A->getNumOperands()-1);
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if (isa<PointerType>(Last->getType()))
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return GetBaseValue(Last);
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} else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
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// This is a leaf node.
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return U->getValue();
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}
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// No Identified object found.
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return 0;
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}
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AliasAnalysis::AliasResult
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ScalarEvolutionAliasAnalysis::alias(const Value *A, unsigned ASize,
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const Value *B, unsigned BSize) {
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// This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
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const SCEV *AS = SE->getSCEV(const_cast<Value *>(A));
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const SCEV *BS = SE->getSCEV(const_cast<Value *>(B));
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// If they evaluate to the same expression, it's a MustAlias.
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if (AS == BS) return MustAlias;
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// If something is known about the difference between the two addresses,
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// see if it's enough to prove a NoAlias.
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if (SE->getEffectiveSCEVType(AS->getType()) ==
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SE->getEffectiveSCEVType(BS->getType())) {
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unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
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APInt AI(BitWidth, ASize);
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const SCEV *BA = SE->getMinusSCEV(BS, AS);
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if (AI.ule(SE->getUnsignedRange(BA).getUnsignedMin())) {
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APInt BI(BitWidth, BSize);
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const SCEV *AB = SE->getMinusSCEV(AS, BS);
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if (BI.ule(SE->getUnsignedRange(AB).getUnsignedMin()))
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return NoAlias;
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}
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}
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// If ScalarEvolution can find an underlying object, form a new query.
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// The correctness of this depends on ScalarEvolution not recognizing
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// inttoptr and ptrtoint operators.
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Value *AO = GetBaseValue(AS);
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Value *BO = GetBaseValue(BS);
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if ((AO && AO != A) || (BO && BO != B))
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if (alias(AO ? AO : A, AO ? ~0u : ASize,
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BO ? BO : B, BO ? ~0u : BSize) == NoAlias)
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return NoAlias;
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// Forward the query to the next analysis.
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return AliasAnalysis::alias(A, ASize, B, BSize);
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}
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