https://llvm.org/bugs/show_bug.cgi?id=27691
Bug ID: 27691
Summary: SCEV is too conservative around proving no-overflow
for loops without computable backedge counts
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: san...@playingwithpointers.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Currently the following logic is guarded under
!isa<SCEVCouldNotCompute>(MaxBECount), when I think it applies even if
MaxBECount is not computable.
// If the backedge is guarded by a comparison with the pre-inc value
// the addrec is safe. Also, if the entry is guarded by a comparison
// with the start value and the backedge is guarded by a comparison
// with the post-inc value, the addrec is safe.
ICmpInst::Predicate Pred;
const SCEV *OverflowLimit =
getSignedOverflowLimitForStep(Step, &Pred, this);
if (OverflowLimit &&
(isLoopBackedgeGuardedByCond(L, Pred, AR, OverflowLimit) ||
(isLoopEntryGuardedByCond(L, Pred, Start, OverflowLimit) &&
isLoopBackedgeGuardedByCond(L, Pred, AR->getPostIncExpr(*this),
OverflowLimit)))) {
// Cache knowledge of AR NSW, then propagate NSW to the wide AddRec.
const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNSW);
return getAddRecExpr(
getExtendAddRecStart<SCEVSignExtendExpr>(AR, Ty, this),
getSignExtendExpr(Step, Ty), L, AR->getNoWrapFlags());
}
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