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

[Ryotaro Kasuga 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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kasuga-fj wrote:

Thanks for the comments. 

> I think the deduction is more general, only needs that the nominator is NW and the denominator is loop-invariant.

This seems correct, but I realized that there are no checks to ensure that Denominator is non-zero in the first place. Anyway, it looks like I should fix the other parts first, so I’ll start with those.

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