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//===- LoopDependenceAnalysis.cpp - LDA Implementation ----------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This is the (beginning) of an implementation of a loop dependence analysis
// framework, which is used to detect dependences in memory accesses in loops.
//
// Please note that this is work in progress and the interface is subject to
// change.
//
// TODO: adapt as implementation progresses.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "lda"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/LoopDependenceAnalysis.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Instructions.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetData.h"
using namespace llvm;
STATISTIC(NumAnswered, "Number of dependence queries answered");
STATISTIC(NumAnalysed, "Number of distinct dependence pairs analysed");
STATISTIC(NumDependent, "Number of pairs with dependent accesses");
STATISTIC(NumIndependent, "Number of pairs with independent accesses");
STATISTIC(NumUnknown, "Number of pairs with unknown accesses");
LoopPass *llvm::createLoopDependenceAnalysisPass() {
return new LoopDependenceAnalysis();
}
static RegisterPass<LoopDependenceAnalysis>
R("lda", "Loop Dependence Analysis", false, true);
char LoopDependenceAnalysis::ID = 0;
//===----------------------------------------------------------------------===//
// Utility Functions
//===----------------------------------------------------------------------===//
static inline bool IsMemRefInstr(const Value *V) {
const Instruction *I = dyn_cast<const Instruction>(V);
return I && (I->mayReadFromMemory() || I->mayWriteToMemory());
}
static void GetMemRefInstrs(const Loop *L,
SmallVectorImpl<Instruction*> &Memrefs) {
for (Loop::block_iterator b = L->block_begin(), be = L->block_end();
b != be; ++b)
for (BasicBlock::iterator i = (*b)->begin(), ie = (*b)->end();
i != ie; ++i)
if (IsMemRefInstr(i))
Memrefs.push_back(i);
}
static bool IsLoadOrStoreInst(Value *I) {
return isa<LoadInst>(I) || isa<StoreInst>(I);
}
static Value *GetPointerOperand(Value *I) {
if (LoadInst *i = dyn_cast<LoadInst>(I))
return i->getPointerOperand();
if (StoreInst *i = dyn_cast<StoreInst>(I))
return i->getPointerOperand();
llvm_unreachable("Value is no load or store instruction!");
// Never reached.
return 0;
}
static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
const Value *A,
const Value *B) {
const Value *aObj = A->getUnderlyingObject();
const Value *bObj = B->getUnderlyingObject();
return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()),
bObj, AA->getTypeStoreSize(bObj->getType()));
}
//===----------------------------------------------------------------------===//
// Dependence Testing
//===----------------------------------------------------------------------===//
bool LoopDependenceAnalysis::isDependencePair(const Value *A,
const Value *B) const {
return IsMemRefInstr(A) &&
IsMemRefInstr(B) &&
(cast<const Instruction>(A)->mayWriteToMemory() ||
cast<const Instruction>(B)->mayWriteToMemory());
}
bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A,
Value *B,
DependencePair *&P) {
void *insertPos = 0;
FoldingSetNodeID id;
id.AddPointer(A);
id.AddPointer(B);
P = Pairs.FindNodeOrInsertPos(id, insertPos);
if (P) return true;
P = PairAllocator.Allocate<DependencePair>();
new (P) DependencePair(id, A, B);
Pairs.InsertNode(P, insertPos);
return false;
}
LoopDependenceAnalysis::DependenceResult
LoopDependenceAnalysis::analysePair(DependencePair *P) const {
DEBUG(errs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
// We only analyse loads and stores but no possible memory accesses by e.g.
// free, call, or invoke instructions.
if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) {
DEBUG(errs() << "--> [?] no load/store\n");
return Unknown;
}
Value *aPtr = GetPointerOperand(P->A);
Value *bPtr = GetPointerOperand(P->B);
switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
case AliasAnalysis::MayAlias:
// We can not analyse objects if we do not know about their aliasing.
DEBUG(errs() << "---> [?] may alias\n");
return Unknown;
case AliasAnalysis::NoAlias:
// If the objects noalias, they are distinct, accesses are independent.
DEBUG(errs() << "---> [I] no alias\n");
return Independent;
case AliasAnalysis::MustAlias:
break; // The underlying objects alias, test accesses for dependence.
}
// We failed to analyse this pair to get a more specific answer.
DEBUG(errs() << "---> [?] cannot analyse\n");
return Unknown;
}
bool LoopDependenceAnalysis::depends(Value *A, Value *B) {
assert(isDependencePair(A, B) && "Values form no dependence pair!");
++NumAnswered;
DependencePair *p;
if (!findOrInsertDependencePair(A, B, p)) {
// The pair is not cached, so analyse it.
++NumAnalysed;
switch (p->Result = analysePair(p)) {
case Dependent: ++NumDependent; break;
case Independent: ++NumIndependent; break;
case Unknown: ++NumUnknown; break;
}
}
return p->Result != Independent;
}
//===----------------------------------------------------------------------===//
// LoopDependenceAnalysis Implementation
//===----------------------------------------------------------------------===//
bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
this->L = L;
AA = &getAnalysis<AliasAnalysis>();
SE = &getAnalysis<ScalarEvolution>();
return false;
}
void LoopDependenceAnalysis::releaseMemory() {
Pairs.clear();
PairAllocator.Reset();
}
void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequiredTransitive<AliasAnalysis>();
AU.addRequiredTransitive<ScalarEvolution>();
}
static void PrintLoopInfo(raw_ostream &OS,
LoopDependenceAnalysis *LDA, const Loop *L) {
if (!L->empty()) return; // ignore non-innermost loops
SmallVector<Instruction*, 8> memrefs;
GetMemRefInstrs(L, memrefs);
OS << "Loop at depth " << L->getLoopDepth() << ", header block: ";
WriteAsOperand(OS, L->getHeader(), false);
OS << "\n";
OS << " Load/store instructions: " << memrefs.size() << "\n";
for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
end = memrefs.end(); x != end; ++x)
OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n";
OS << " Pairwise dependence results:\n";
for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(),
end = memrefs.end(); x != end; ++x)
for (SmallVector<Instruction*, 8>::const_iterator y = x + 1;
y != end; ++y)
if (LDA->isDependencePair(*x, *y))
OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin())
<< ": " << (LDA->depends(*x, *y) ? "dependent" : "independent")
<< "\n";
}
void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const {
// TODO: doc why const_cast is safe
PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L);
}
void LoopDependenceAnalysis::print(std::ostream &OS, const Module *M) const {
raw_os_ostream os(OS);
print(os, M);
}
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