forked from OSchip/llvm-project
138 lines
5.7 KiB
C++
138 lines
5.7 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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// This differs from traditional loop dependence analysis in that it tests
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// for dependencies within a single iteration of a loop, rather than
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// dependencies between different iterations.
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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/ScalarEvolutionAliasAnalysis.h"
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using namespace llvm;
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// Register this pass...
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char ScalarEvolutionAliasAnalysis::ID = 0;
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INITIALIZE_AG_PASS_BEGIN(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
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"ScalarEvolution-based Alias Analysis", false, true,
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false)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
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INITIALIZE_AG_PASS_END(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
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"ScalarEvolution-based Alias Analysis", false, true,
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false)
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FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
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return new ScalarEvolutionAliasAnalysis();
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}
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void ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequiredTransitive<ScalarEvolutionWrapperPass>();
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AU.setPreservesAll();
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AliasAnalysis::getAnalysisUsage(AU);
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}
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bool ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
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InitializeAliasAnalysis(this, &F.getParent()->getDataLayout());
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SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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return false;
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}
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/// Given an expression, try to find a base value.
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///
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/// Returns null if none was found.
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Value *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 (Last->getType()->isPointerTy())
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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 nullptr;
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}
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AliasResult ScalarEvolutionAliasAnalysis::alias(const MemoryLocation &LocA,
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const MemoryLocation &LocB) {
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// If either of the memory references is empty, it doesn't matter what the
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// pointer values are. This allows the code below to ignore this special
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// case.
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if (LocA.Size == 0 || LocB.Size == 0)
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return NoAlias;
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// This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
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const SCEV *AS = SE->getSCEV(const_cast<Value *>(LocA.Ptr));
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const SCEV *BS = SE->getSCEV(const_cast<Value *>(LocB.Ptr));
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// If they evaluate to the same expression, it's a MustAlias.
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if (AS == BS)
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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 ASizeInt(BitWidth, LocA.Size);
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APInt BSizeInt(BitWidth, LocB.Size);
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// Compute the difference between the two pointers.
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const SCEV *BA = SE->getMinusSCEV(BS, AS);
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// Test whether the difference is known to be great enough that memory of
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// the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
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// are non-zero, which is special-cased above.
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if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
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(-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
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return NoAlias;
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// Folding the subtraction while preserving range information can be tricky
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// (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
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// and try again to see if things fold better that way.
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// Compute the difference between the two pointers.
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const SCEV *AB = SE->getMinusSCEV(AS, BS);
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// Test whether the difference is known to be great enough that memory of
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// the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
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// are non-zero, which is special-cased above.
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if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
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(-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
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return NoAlias;
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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 != LocA.Ptr) || (BO && BO != LocB.Ptr))
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if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
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AO ? +MemoryLocation::UnknownSize : LocA.Size,
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AO ? AAMDNodes() : LocA.AATags),
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MemoryLocation(BO ? BO : LocB.Ptr,
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BO ? +MemoryLocation::UnknownSize : LocB.Size,
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BO ? AAMDNodes() : LocB.AATags)) == 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(LocA, LocB);
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}
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