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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/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/Debug.h"
#include "llvm/Target/TargetData.h"
using namespace llvm;
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();
assert(0 && "Value is no load or store instruction!");
// Never reached.
return 0;
}
//===----------------------------------------------------------------------===//
// Dependence Testing
//===----------------------------------------------------------------------===//
bool LoopDependenceAnalysis::isDependencePair(const Value *x,
const Value *y) const {
return IsMemRefInstr(x) &&
IsMemRefInstr(y) &&
(cast<const Instruction>(x)->mayWriteToMemory() ||
cast<const Instruction>(y)->mayWriteToMemory());
}
bool LoopDependenceAnalysis::depends(Value *src, Value *dst) {
assert(isDependencePair(src, dst) && "Values form no dependence pair!");
DOUT << "== LDA test ==\n" << *src << *dst;
// We only analyse loads and stores; for possible memory accesses by e.g.
// free, call, or invoke instructions we conservatively assume dependence.
if (!IsLoadOrStoreInst(src) || !IsLoadOrStoreInst(dst))
return true;
Value *srcPtr = GetPointerOperand(src);
Value *dstPtr = GetPointerOperand(dst);
const Value *srcObj = srcPtr->getUnderlyingObject();
const Value *dstObj = dstPtr->getUnderlyingObject();
AliasAnalysis::AliasResult alias = AA->alias(
srcObj, AA->getTargetData().getTypeStoreSize(srcObj->getType()),
dstObj, AA->getTargetData().getTypeStoreSize(dstObj->getType()));
// If we don't know whether or not the two objects alias, assume dependence.
if (alias == AliasAnalysis::MayAlias)
return true;
// If the objects noalias, they are distinct, accesses are independent.
if (alias == AliasAnalysis::NoAlias)
return false;
// TODO: the underlying objects MustAlias, test for dependence
// We couldn't establish a more precise result, so we have to conservatively
// assume full dependence.
return true;
}
//===----------------------------------------------------------------------===//
// LoopDependenceAnalysis Implementation
//===----------------------------------------------------------------------===//
bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) {
this->L = L;
AA = &getAnalysis<AliasAnalysis>();
SE = &getAnalysis<ScalarEvolution>();
return false;
}
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;
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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