diff options
Diffstat (limited to 'llvm/lib/Analysis/ScalarEvolution.cpp')
| -rw-r--r-- | llvm/lib/Analysis/ScalarEvolution.cpp | 19 |
1 files changed, 18 insertions, 1 deletions
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp index 9b9e7dc5eea..8f2e4f2ba28 100644 --- a/llvm/lib/Analysis/ScalarEvolution.cpp +++ b/llvm/lib/Analysis/ScalarEvolution.cpp @@ -1364,7 +1364,24 @@ static const SCEV *getPreStartForSignExtend(const SCEVAddRecExpr *AR, const SCEVAddRecExpr *PreAR = dyn_cast<SCEVAddRecExpr>( SE->getAddRecExpr(PreStart, Step, L, SCEV::FlagAnyWrap)); - if (PreAR && PreAR->getNoWrapFlags(SCEV::FlagNSW)) + // WARNING: FIXME: the optimization below assumes that a sign-overflowing nsw + // operation is undefined behavior. This is strictly more aggressive than the + // interpretation of nsw in other parts of LLVM (for instance, they may + // unconditionally hoist nsw arithmetic through control flow). This logic + // needs to be revisited once we have a consistent semantics for poison + // values. + // + // "{S,+,X} is <nsw>" and "{S,+,X} is evaluated at least once" implies "S+X + // does not sign-overflow" (we'd have undefined behavior if it did). If + // `L->getExitingBlock() == L->getLoopLatch()` then `PreAR` (= {S,+,X}<nsw>) + // is evaluated every-time `AR` (= {S+X,+,X}) is evaluated, and hence within + // `AR` we are safe to assume that "S+X" will not sign-overflow. + // + + BasicBlock *ExitingBlock = L->getExitingBlock(); + BasicBlock *LatchBlock = L->getLoopLatch(); + if (PreAR && PreAR->getNoWrapFlags(SCEV::FlagNSW) && + ExitingBlock != nullptr && ExitingBlock == LatchBlock) return PreStart; // 2. Direct overflow check on the step operation's expression. |
