//
// TODO: adapt as implementation progresses.
//
+// TODO: document lingo (pair, subscript, index)
+//
//===----------------------------------------------------------------------===//
#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/Analysis/ScalarEvolutionExpressions.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/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();
}
bObj, AA->getTypeStoreSize(bObj->getType()));
}
+static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) {
+ return SE->getConstant(Type::Int32Ty, 0L);
+}
+
//===----------------------------------------------------------------------===//
// Dependence Testing
//===----------------------------------------------------------------------===//
return false;
}
-void LoopDependenceAnalysis::analysePair(DependencePair *P) const {
- DEBUG(errs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n");
+bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const {
+ for (const Loop *L = this->L; L != 0; L = L->getParentLoop())
+ if (!S->isLoopInvariant(L))
+ return false;
+ return true;
+}
+
+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);
+}
- // Our default answer: we don't know anything, i.e. we failed to analyse this
- // pair to get a more specific answer (dependent, independent).
- P->Result = Unknown;
+LoopDependenceAnalysis::DependenceResult
+LoopDependenceAnalysis::analyseZIV(const SCEV *A,
+ const SCEV *B,
+ Subscript *S) const {
+ assert(isZIVPair(A, B));
+ const SCEV *diff = SE->getMinusSCEV(A, B);
+ return diff->isZero() ? Dependent : Independent;
+}
+
+LoopDependenceAnalysis::DependenceResult
+LoopDependenceAnalysis::analyseSubscript(const SCEV *A,
+ const SCEV *B,
+ Subscript *S) const {
+ DEBUG(errs() << " Testing subscript: " << *A << ", " << *B << "\n");
+
+ if (A == B) {
+ DEBUG(errs() << " -> [D] same SCEV\n");
+ return Dependent;
+ }
+
+ if (!isAffine(A) || !isAffine(B)) {
+ DEBUG(errs() << " -> [?] not affine\n");
+ return Unknown;
+ }
+
+ if (isZIVPair(A, B))
+ return analyseZIV(A, B, S);
+
+ // TODO: Implement SIV/MIV testers.
+
+ DEBUG(errs() << " -> [?] cannot analyse subscript\n");
+ return Unknown;
+}
+
+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;
+ return Unknown;
}
Value *aPtr = GetPointerOperand(P->A);
case AliasAnalysis::MayAlias:
// We can not analyse objects if we do not know about their aliasing.
DEBUG(errs() << "---> [?] may alias\n");
- return;
+ return Unknown;
case AliasAnalysis::NoAlias:
// If the objects noalias, they are distinct, accesses are independent.
DEBUG(errs() << "---> [I] no alias\n");
- P->Result = Independent;
- return;
+ return Independent;
case AliasAnalysis::MustAlias:
break; // The underlying objects alias, test accesses for dependence.
}
- DEBUG(errs() << "---> [?] cannot analyse\n");
- return;
+ 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.
- analysePair(p);
+ ++NumAnalysed;
+ switch (p->Result = analysePair(p)) {
+ case Dependent: ++NumDependent; break;
+ case Independent: ++NumIndependent; break;
+ case Unknown: ++NumUnknown; break;
+ }
}
return p->Result != Independent;
}
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