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In order to enable the preservation of noalias function parameter information after inlining, and the representation of block-level __restrict__ pointer information (etc.), additional kinds of aliasing metadata will be introduced. This metadata needs to be carried around in AliasAnalysis::Location objects (and MMOs at the SDAG level), and so we need to generalize the current scheme (which is hard-coded to just one TBAA MDNode*). This commit introduces only the necessary refactoring to allow for the introduction of other aliasing metadata types, but does not actually introduce any (that will come in a follow-up commit). What it does introduce is a new AAMDNodes structure to hold all of the aliasing metadata nodes associated with a particular memory-accessing instruction, and uses that structure instead of the raw MDNode* in AliasAnalysis::Location, etc. No functionality change intended. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213859 91177308-0d34-0410-b5e6-96231b3b80d8
174 lines
6.8 KiB
C++
174 lines
6.8 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/Passes.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.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(nullptr) {
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initializeScalarEvolutionAliasAnalysisPass(
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*PassRegistry::getPassRegistry());
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}
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/// getAdjustedAnalysisPointer - This method is used when a pass implements
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/// an analysis interface through multiple inheritance. If needed, it
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/// should override this to adjust the this pointer as needed for the
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/// specified pass info.
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void *getAdjustedAnalysisPointer(AnalysisID PI) override {
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if (PI == &AliasAnalysis::ID)
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return (AliasAnalysis*)this;
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return this;
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}
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private:
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void getAnalysisUsage(AnalysisUsage &AU) const override;
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bool runOnFunction(Function &F) override;
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AliasResult alias(const Location &LocA, const Location &LocB) override;
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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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INITIALIZE_AG_PASS_BEGIN(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
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"ScalarEvolution-based Alias Analysis", false, true, false)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
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INITIALIZE_AG_PASS_END(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
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"ScalarEvolution-based Alias Analysis", false, true, false)
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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 (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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AliasAnalysis::AliasResult
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ScalarEvolutionAliasAnalysis::alias(const Location &LocA,
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const Location &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) 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(Location(AO ? AO : LocA.Ptr,
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AO ? +UnknownSize : LocA.Size,
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AO ? AAMDNodes() : LocA.AATags),
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Location(BO ? BO : LocB.Ptr,
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BO ? +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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