diff options
Diffstat (limited to 'llvm/lib/Transforms/Scalar')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/NaryReassociate.cpp | 24 |
1 files changed, 23 insertions, 1 deletions
diff --git a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp index 5b370e04088..6ac5ff85e32 100644 --- a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp +++ b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp @@ -234,6 +234,7 @@ bool NaryReassociate::doOneIteration(Function &F) { BasicBlock *BB = Node->getBlock(); for (auto I = BB->begin(); I != BB->end(); ++I) { if (SE->isSCEVable(I->getType()) && isPotentiallyNaryReassociable(I)) { + const SCEV *OldSCEV = SE->getSCEV(I); if (Instruction *NewI = tryReassociate(I)) { Changed = true; SE->forgetValue(I); @@ -243,7 +244,28 @@ bool NaryReassociate::doOneIteration(Function &F) { } // Add the rewritten instruction to SeenExprs; the original instruction // is deleted. - SeenExprs[SE->getSCEV(I)].push_back(I); + const SCEV *NewSCEV = SE->getSCEV(I); + SeenExprs[NewSCEV].push_back(I); + // Ideally, NewSCEV should equal OldSCEV because tryReassociate(I) + // is equivalent to I. However, ScalarEvolution::getSCEV may + // weaken nsw causing NewSCEV not to equal OldSCEV. For example, suppose + // we reassociate + // I = &a[sext(i +nsw j)] // assuming sizeof(a[0]) = 4 + // to + // NewI = &a[sext(i)] + sext(j). + // + // ScalarEvolution computes + // getSCEV(I) = a + 4 * sext(i + j) + // getSCEV(newI) = a + 4 * sext(i) + 4 * sext(j) + // which are different SCEVs. + // + // To alleviate this issue of ScalarEvolution not always capturing + // equivalence, we add I to SeenExprs[OldSCEV] as well so that we can + // map both SCEV before and after tryReassociate(I) to I. + // + // This improvement is exercised in @reassociate_gep_nsw in nary-gep.ll. + if (NewSCEV != OldSCEV) + SeenExprs[OldSCEV].push_back(I); } } } |
