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llvm-project/llvm/lib/Analysis/LoopDependenceAnalysis.cpp

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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.
//
// TODO: document lingo (pair, subscript, index)
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "lda"
#include "llvm/ADT/DenseSet.h"
#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/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Instructions.h"
#include "llvm/Operator.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();
}
INITIALIZE_PASS_BEGIN(LoopDependenceAnalysis, "lda",
"Loop Dependence Analysis", false, true)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(LoopDependenceAnalysis, "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) {
// Returns true if the load or store can be analyzed. Atomic and volatile
// operations have properties which this analysis does not understand.
if (LoadInst *LI = dyn_cast<LoadInst>(I))
return LI->isUnordered();
else if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isUnordered();
return false;
}
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!");
}
static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA,
const Value *A,
const Value *B) {
const Value *aObj = GetUnderlyingObject(A);
const Value *bObj = GetUnderlyingObject(B);
return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()),
bObj, AA->getTypeStoreSize(bObj->getType()));
}
static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L);
}
//===----------------------------------------------------------------------===//
// 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 = new (PairAllocator) DependencePair(id, A, B);
Pairs.InsertNode(P, insertPos);
return false;
}
void LoopDependenceAnalysis::getLoops(const SCEV *S,
DenseSet<const Loop*>* Loops) const {
// Refactor this into an SCEVVisitor, if efficiency becomes a concern.
for (const Loop *L = this->L; L != 0; L = L->getParentLoop())
if (!SE->isLoopInvariant(S, L))
Loops->insert(L);
}
bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
DenseSet<const Loop*> loops;
getLoops(S, &loops);
return loops.empty();
}
bool LoopDependenceAnalysis::isAffine(const SCEV *S) const {
const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S);
return isLoopInvariant(S) || (rec && rec->isAffine());
}
bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const {
return isLoopInvariant(A) && isLoopInvariant(B);
}
bool LoopDependenceAnalysis::isSIVPair(const SCEV *A, const SCEV *B) const {
DenseSet<const Loop*> loops;
getLoops(A, &loops);
getLoops(B, &loops);
return loops.size() == 1;
}
LoopDependenceAnalysis::DependenceResult
LoopDependenceAnalysis::analyseZIV(const SCEV *A,
const SCEV *B,
Subscript *S) const {
assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!");
return A == B ? Dependent : Independent;
}
LoopDependenceAnalysis::DependenceResult
LoopDependenceAnalysis::analyseSIV(const SCEV *A,
const SCEV *B,
Subscript *S) const {
return Unknown; // TODO: Implement.
}
LoopDependenceAnalysis::DependenceResult
LoopDependenceAnalysis::analyseMIV(const SCEV *A,
const SCEV *B,
Subscript *S) const {
return Unknown; // TODO: Implement.
}
LoopDependenceAnalysis::DependenceResult
LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
const SCEV *B,
Subscript *S) const {
DEBUG(dbgs() << " Testing subscript: " << *A << ", " << *B << "\n");
if (A == B) {
DEBUG(dbgs() << " -> [D] same SCEV\n");
return Dependent;
}
if (!isAffine(A) || !isAffine(B)) {
DEBUG(dbgs() << " -> [?] not affine\n");
return Unknown;
}
if (isZIVPair(A, B))
return analyseZIV(A, B, S);
if (isSIVPair(A, B))
return analyseSIV(A, B, S);
return analyseMIV(A, B, S);
}
LoopDependenceAnalysis::DependenceResult
LoopDependenceAnalysis::analysePair(DependencePair *P) const {
DEBUG(dbgs() << "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(dbgs() << "--> [?] no load/store\n");
return Unknown;
}
Value *aPtr = GetPointerOperand(P->A);
Value *bPtr = GetPointerOperand(P->B);
switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) {
case AliasAnalysis::MayAlias:
case AliasAnalysis::PartialAlias:
// We can not analyse objects if we do not know about their aliasing.
DEBUG(dbgs() << "---> [?] may alias\n");
return Unknown;
case AliasAnalysis::NoAlias:
// If the objects noalias, they are distinct, accesses are independent.
DEBUG(dbgs() << "---> [I] no alias\n");
return Independent;
case AliasAnalysis::MustAlias:
break; // The underlying objects alias, test accesses for dependence.
}
const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr);
const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr);
if (!aGEP || !bGEP)
return Unknown;
// FIXME: Is filtering coupled subscripts necessary?
// Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding
// trailing zeroes to the smaller GEP, if needed.
typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy;
GEPOpdPairsTy opds;
for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(),
aEnd = aGEP->idx_end(),
bIdx = bGEP->idx_begin(),
bEnd = bGEP->idx_end();
aIdx != aEnd && bIdx != bEnd;
aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) {
const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE);
const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE);
opds.push_back(std::make_pair(aSCEV, bSCEV));
}
if (!opds.empty() && opds[0].first != opds[0].second) {
// We cannot (yet) handle arbitrary GEP pointer offsets. By limiting
//
// TODO: this could be relaxed by adding the size of the underlying object
// to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we
// know that x is a [100 x i8]*, we could modify the first subscript to be
// (i, 200-i) instead of (i, -i).
return Unknown;
}
// Now analyse the collected operand pairs (skipping the GEP ptr offsets).
for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end();
i != end; ++i) {
Subscript subscript;
DependenceResult result = analyseSubscript(i->first, i->second, &subscript);
if (result != Dependent) {
// We either proved independence or failed to analyse this subscript.
// Further subscripts will not improve the situation, so abort early.
return result;
}
P->Subscripts.push_back(subscript);
}
// We successfully analysed all subscripts but failed to prove independence.
return Dependent;
}
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);
}
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