[llvm] [SCEVDivision] Prevent propagation of incorrect no-wrap flags (PR #154745)

[Nikita Popov via llvm-commits]

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@@ -141,10 +141,26 @@ void SCEVDivision::visitAddRecExpr(const SCEVAddRecExpr *Numerator) {
   if (Ty != StartQ->getType() || Ty != StartR->getType() ||
       Ty != StepQ->getType() || Ty != StepR->getType())
     return cannotDivide(Numerator);
+
+  // Infer no-wrap flags for Remainder.
+  // TODO: Catch more cases.
+  SCEV::NoWrapFlags NumFlags = Numerator->getNoWrapFlags();
+  SCEV::NoWrapFlags RemFlags = SCEV::NoWrapFlags::FlagAnyWrap;
+  const SCEV *StepNumAbs =
+      SE.getAbsExpr(Numerator->getStepRecurrence(SE), /*IsNSW=*/false);
+  const SCEV *StepRAbs = SE.getAbsExpr(StepR, /*IsNSW=*/false);
+  const Loop *L = Numerator->getLoop();
+
+  // If abs(StepR) <=u abs(StepNumAbs) and both are loop invariant, propagate
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nikic wrote:

Something we need to be careful about is that SCEV flags are global, not use-site specific. So even if we know that division by zero is not possible *in context*, we may not be able to assume it for the purpose of flag propagation. (I haven't checked whether this is a problem for the delinearization case or not.)

https://github.com/llvm/llvm-project/pull/154745