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//===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
// simple alias analysis implemented in terms of ScalarEvolution queries.
//
// This differs from traditional loop dependence analysis in that it tests
// for dependencies within a single iteration of a loop, rather than
// dependencies between different iterations.
//
// ScalarEvolution has a more complete understanding of pointer arithmetic
// than BasicAliasAnalysis' collection of ad-hoc analyses.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/Passes.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Pass.h"
using namespace llvm;
namespace {
/// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
/// implementation that uses ScalarEvolution to answer queries.
class ScalarEvolutionAliasAnalysis : public FunctionPass,
public AliasAnalysis {
ScalarEvolution *SE;
public:
static char ID; // Class identification, replacement for typeinfo
ScalarEvolutionAliasAnalysis() : FunctionPass(ID), SE(nullptr) {
initializeScalarEvolutionAliasAnalysisPass(
*PassRegistry::getPassRegistry());
}
/// getAdjustedAnalysisPointer - This method is used when a pass implements
/// an analysis interface through multiple inheritance. If needed, it
/// should override this to adjust the this pointer as needed for the
/// specified pass info.
void *getAdjustedAnalysisPointer(AnalysisID PI) override {
if (PI == &AliasAnalysis::ID)
return (AliasAnalysis*)this;
return this;
}
private:
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnFunction(Function &F) override;
AliasResult alias(const Location &LocA, const Location &LocB) override;
Value *GetBaseValue(const SCEV *S);
};
} // End of anonymous namespace
// Register this pass...
char ScalarEvolutionAliasAnalysis::ID = 0;
INITIALIZE_AG_PASS_BEGIN(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
"ScalarEvolution-based Alias Analysis", false, true, false)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_AG_PASS_END(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
"ScalarEvolution-based Alias Analysis", false, true, false)
FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
return new ScalarEvolutionAliasAnalysis();
}
void
ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequiredTransitive<ScalarEvolution>();
AU.setPreservesAll();
AliasAnalysis::getAnalysisUsage(AU);
}
bool
ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
InitializeAliasAnalysis(this);
SE = &getAnalysis<ScalarEvolution>();
return false;
}
/// GetBaseValue - Given an expression, try to find a
/// base value. Return null is none was found.
Value *
ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
// In an addrec, assume that the base will be in the start, rather
// than the step.
return GetBaseValue(AR->getStart());
} else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
// If there's a pointer operand, it'll be sorted at the end of the list.
const SCEV *Last = A->getOperand(A->getNumOperands()-1);
if (Last->getType()->isPointerTy())
return GetBaseValue(Last);
} else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
// This is a leaf node.
return U->getValue();
}
// No Identified object found.
return nullptr;
}
AliasAnalysis::AliasResult
ScalarEvolutionAliasAnalysis::alias(const Location &LocA,
const Location &LocB) {
// If either of the memory references is empty, it doesn't matter what the
// pointer values are. This allows the code below to ignore this special
// case.
if (LocA.Size == 0 || LocB.Size == 0)
return NoAlias;
// This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
const SCEV *AS = SE->getSCEV(const_cast<Value *>(LocA.Ptr));
const SCEV *BS = SE->getSCEV(const_cast<Value *>(LocB.Ptr));
// If they evaluate to the same expression, it's a MustAlias.
if (AS == BS) return MustAlias;
// If something is known about the difference between the two addresses,
// see if it's enough to prove a NoAlias.
if (SE->getEffectiveSCEVType(AS->getType()) ==
SE->getEffectiveSCEVType(BS->getType())) {
unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
APInt ASizeInt(BitWidth, LocA.Size);
APInt BSizeInt(BitWidth, LocB.Size);
// Compute the difference between the two pointers.
const SCEV *BA = SE->getMinusSCEV(BS, AS);
// Test whether the difference is known to be great enough that memory of
// the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
// are non-zero, which is special-cased above.
if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
(-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
return NoAlias;
// Folding the subtraction while preserving range information can be tricky
// (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
// and try again to see if things fold better that way.
// Compute the difference between the two pointers.
const SCEV *AB = SE->getMinusSCEV(AS, BS);
// Test whether the difference is known to be great enough that memory of
// the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
// are non-zero, which is special-cased above.
if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
(-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
return NoAlias;
}
// If ScalarEvolution can find an underlying object, form a new query.
// The correctness of this depends on ScalarEvolution not recognizing
// inttoptr and ptrtoint operators.
Value *AO = GetBaseValue(AS);
Value *BO = GetBaseValue(BS);
if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
if (alias(Location(AO ? AO : LocA.Ptr,
AO ? +UnknownSize : LocA.Size,
AO ? nullptr : LocA.TBAATag),
Location(BO ? BO : LocB.Ptr,
BO ? +UnknownSize : LocB.Size,
BO ? nullptr : LocB.TBAATag)) == NoAlias)
return NoAlias;
// Forward the query to the next analysis.
return AliasAnalysis::alias(LocA, LocB);
}
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