352 lines
12 KiB
C++
352 lines
12 KiB
C++
//===- LoopDependenceAnalysis.cpp - LDA Implementation ----------*- C++ -*-===//
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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 is the (beginning) of an implementation of a loop dependence analysis
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// framework, which is used to detect dependences in memory accesses in loops.
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//
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// Please note that this is work in progress and the interface is subject to
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// change.
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//
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// TODO: adapt as implementation progresses.
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//
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// TODO: document lingo (pair, subscript, index)
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "lda"
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#include "llvm/ADT/DenseSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/LoopDependenceAnalysis.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Instructions.h"
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#include "llvm/Operator.h"
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#include "llvm/Support/Allocator.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Target/TargetData.h"
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using namespace llvm;
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STATISTIC(NumAnswered, "Number of dependence queries answered");
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STATISTIC(NumAnalysed, "Number of distinct dependence pairs analysed");
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STATISTIC(NumDependent, "Number of pairs with dependent accesses");
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STATISTIC(NumIndependent, "Number of pairs with independent accesses");
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STATISTIC(NumUnknown, "Number of pairs with unknown accesses");
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LoopPass *llvm::createLoopDependenceAnalysisPass() {
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return new LoopDependenceAnalysis();
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}
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INITIALIZE_PASS(LoopDependenceAnalysis, "lda",
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"Loop Dependence Analysis", false, true);
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char LoopDependenceAnalysis::ID = 0;
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//===----------------------------------------------------------------------===//
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// Utility Functions
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//===----------------------------------------------------------------------===//
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static inline bool IsMemRefInstr(const Value *V) {
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const Instruction *I = dyn_cast<const Instruction>(V);
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return I && (I->mayReadFromMemory() || I->mayWriteToMemory());
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}
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static void GetMemRefInstrs(const Loop *L,
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SmallVectorImpl<Instruction*> &Memrefs) {
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for (Loop::block_iterator b = L->block_begin(), be = L->block_end();
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b != be; ++b)
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for (BasicBlock::iterator i = (*b)->begin(), ie = (*b)->end();
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i != ie; ++i)
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if (IsMemRefInstr(i))
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Memrefs.push_back(i);
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}
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static bool IsLoadOrStoreInst(Value *I) {
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return isa<LoadInst>(I) || isa<StoreInst>(I);
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}
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static Value *GetPointerOperand(Value *I) {
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if (LoadInst *i = dyn_cast<LoadInst>(I))
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return i->getPointerOperand();
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if (StoreInst *i = dyn_cast<StoreInst>(I))
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return i->getPointerOperand();
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llvm_unreachable("Value is no load or store instruction!");
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// Never reached.
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return 0;
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}
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static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
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const Value *A,
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const Value *B) {
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const Value *aObj = A->getUnderlyingObject();
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const Value *bObj = B->getUnderlyingObject();
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return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()),
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bObj, AA->getTypeStoreSize(bObj->getType()));
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}
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static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
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return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L);
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}
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//===----------------------------------------------------------------------===//
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// Dependence Testing
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//===----------------------------------------------------------------------===//
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bool LoopDependenceAnalysis::isDependencePair(const Value *A,
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const Value *B) const {
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return IsMemRefInstr(A) &&
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IsMemRefInstr(B) &&
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(cast<const Instruction>(A)->mayWriteToMemory() ||
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cast<const Instruction>(B)->mayWriteToMemory());
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}
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bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A,
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Value *B,
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DependencePair *&P) {
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void *insertPos = 0;
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FoldingSetNodeID id;
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id.AddPointer(A);
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id.AddPointer(B);
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P = Pairs.FindNodeOrInsertPos(id, insertPos);
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if (P) return true;
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P = new (PairAllocator) DependencePair(id, A, B);
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Pairs.InsertNode(P, insertPos);
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return false;
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}
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void LoopDependenceAnalysis::getLoops(const SCEV *S,
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DenseSet<const Loop*>* Loops) const {
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// Refactor this into an SCEVVisitor, if efficiency becomes a concern.
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for (const Loop *L = this->L; L != 0; L = L->getParentLoop())
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if (!S->isLoopInvariant(L))
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Loops->insert(L);
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}
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bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
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DenseSet<const Loop*> loops;
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getLoops(S, &loops);
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return loops.empty();
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}
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bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
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const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
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return isLoopInvariant(S) || (rec && rec->isAffine());
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}
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bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
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return isLoopInvariant(A) && isLoopInvariant(B);
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}
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bool LoopDependenceAnalysis::isSIVPair(const SCEV *A, const SCEV *B) const {
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DenseSet<const Loop*> loops;
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getLoops(A, &loops);
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getLoops(B, &loops);
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return loops.size() == 1;
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}
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LoopDependenceAnalysis::DependenceResult
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LoopDependenceAnalysis::analyseZIV(const SCEV *A,
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const SCEV *B,
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Subscript *S) const {
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assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!");
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return A == B ? Dependent : Independent;
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}
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LoopDependenceAnalysis::DependenceResult
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LoopDependenceAnalysis::analyseSIV(const SCEV *A,
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const SCEV *B,
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Subscript *S) const {
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return Unknown; // TODO: Implement.
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}
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LoopDependenceAnalysis::DependenceResult
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LoopDependenceAnalysis::analyseMIV(const SCEV *A,
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const SCEV *B,
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Subscript *S) const {
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return Unknown; // TODO: Implement.
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}
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LoopDependenceAnalysis::DependenceResult
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LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
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const SCEV *B,
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Subscript *S) const {
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DEBUG(dbgs() << " Testing subscript: " << *A << ", " << *B << "\n");
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if (A == B) {
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DEBUG(dbgs() << " -> [D] same SCEV\n");
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return Dependent;
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}
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if (!isAffine(A) || !isAffine(B)) {
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DEBUG(dbgs() << " -> [?] not affine\n");
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return Unknown;
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}
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if (isZIVPair(A, B))
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return analyseZIV(A, B, S);
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if (isSIVPair(A, B))
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return analyseSIV(A, B, S);
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return analyseMIV(A, B, S);
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}
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LoopDependenceAnalysis::DependenceResult
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LoopDependenceAnalysis::analysePair(DependencePair *P) const {
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DEBUG(dbgs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
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// We only analyse loads and stores but no possible memory accesses by e.g.
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// free, call, or invoke instructions.
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if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
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DEBUG(dbgs() << "--> [?] no load/store\n");
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return Unknown;
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}
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Value *aPtr = GetPointerOperand(P->A);
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Value *bPtr = GetPointerOperand(P->B);
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switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
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case AliasAnalysis::MayAlias:
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// We can not analyse objects if we do not know about their aliasing.
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DEBUG(dbgs() << "---> [?] may alias\n");
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return Unknown;
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case AliasAnalysis::NoAlias:
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// If the objects noalias, they are distinct, accesses are independent.
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DEBUG(dbgs() << "---> [I] no alias\n");
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return Independent;
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case AliasAnalysis::MustAlias:
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break; // The underlying objects alias, test accesses for dependence.
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}
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const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr);
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const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr);
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if (!aGEP || !bGEP)
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return Unknown;
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// FIXME: Is filtering coupled subscripts necessary?
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// Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding
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// trailing zeroes to the smaller GEP, if needed.
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typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy;
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GEPOpdPairsTy opds;
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for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(),
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aEnd = aGEP->idx_end(),
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bIdx = bGEP->idx_begin(),
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bEnd = bGEP->idx_end();
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aIdx != aEnd && bIdx != bEnd;
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aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) {
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const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE);
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const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE);
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opds.push_back(std::make_pair(aSCEV, bSCEV));
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}
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if (!opds.empty() && opds[0].first != opds[0].second) {
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// We cannot (yet) handle arbitrary GEP pointer offsets. By limiting
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//
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// TODO: this could be relaxed by adding the size of the underlying object
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// to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we
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// know that x is a [100 x i8]*, we could modify the first subscript to be
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// (i, 200-i) instead of (i, -i).
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return Unknown;
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}
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// Now analyse the collected operand pairs (skipping the GEP ptr offsets).
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for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end();
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i != end; ++i) {
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Subscript subscript;
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DependenceResult result = analyseSubscript(i->first, i->second, &subscript);
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if (result != Dependent) {
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// We either proved independence or failed to analyse this subscript.
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// Further subscripts will not improve the situation, so abort early.
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return result;
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}
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P->Subscripts.push_back(subscript);
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}
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// We successfully analysed all subscripts but failed to prove independence.
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return Dependent;
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}
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bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
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assert(isDependencePair(A, B) && "Values form no dependence pair!");
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++NumAnswered;
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DependencePair *p;
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if (!findOrInsertDependencePair(A, B, p)) {
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// The pair is not cached, so analyse it.
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++NumAnalysed;
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switch (p->Result = analysePair(p)) {
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case Dependent: ++NumDependent; break;
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case Independent: ++NumIndependent; break;
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case Unknown: ++NumUnknown; break;
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}
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}
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return p->Result != Independent;
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}
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//===----------------------------------------------------------------------===//
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// LoopDependenceAnalysis Implementation
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//===----------------------------------------------------------------------===//
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bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
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this->L = L;
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AA = &getAnalysis<AliasAnalysis>();
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SE = &getAnalysis<ScalarEvolution>();
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return false;
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}
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void LoopDependenceAnalysis::releaseMemory() {
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Pairs.clear();
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PairAllocator.Reset();
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}
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void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesAll();
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AU.addRequiredTransitive<AliasAnalysis>();
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AU.addRequiredTransitive<ScalarEvolution>();
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}
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static void PrintLoopInfo(raw_ostream &OS,
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LoopDependenceAnalysis *LDA, const Loop *L) {
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if (!L->empty()) return; // ignore non-innermost loops
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SmallVector<Instruction*, 8> memrefs;
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GetMemRefInstrs(L, memrefs);
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OS << "Loop at depth " << L->getLoopDepth() << ", header block: ";
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WriteAsOperand(OS, L->getHeader(), false);
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OS << "\n";
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OS << " Load/store instructions: " << memrefs.size() << "\n";
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for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
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end = memrefs.end(); x != end; ++x)
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OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n";
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OS << " Pairwise dependence results:\n";
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for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
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end = memrefs.end(); x != end; ++x)
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for (SmallVector<Instruction*, 8>::const_iterator y = x + 1;
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y != end; ++y)
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if (LDA->isDependencePair(*x, *y))
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OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin())
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<< ": " << (LDA->depends(*x, *y) ? "dependent" : "independent")
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<< "\n";
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}
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void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const {
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// TODO: doc why const_cast is safe
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PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L);
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}
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