[llvm] [SCEV] Use power of two facts involving vscale when inferring wrap flags (PR #101380)

[Nikita Popov via llvm-commits]

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@@ -9149,23 +9149,21 @@ ScalarEvolution::ExitLimit ScalarEvolution::computeExitLimitFromICmp(
   // behaviour), and we can prove the test sequence produced must repeat
   // the same values on self-wrap of the IV, then we can infer that IV
   // doesn't self wrap because if it did, we'd have an infinite (undefined)
-  // loop.
+  // loop.  Note that a stride of 0 is trivially no-self-wrap by definition.
   if (ControllingFiniteLoop && isLoopInvariant(RHS, L)) {
     // TODO: We can peel off any functions which are invertible *in L*.  Loop
     // invariant terms are effectively constants for our purposes here.
     auto *InnerLHS = LHS;
     if (auto *ZExt = dyn_cast<SCEVZeroExtendExpr>(LHS))
       InnerLHS = ZExt->getOperand();
-    if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(InnerLHS)) {
-      auto *StrideC = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*this));
-      if (!AR->hasNoSelfWrap() && AR->getLoop() == L && AR->isAffine() &&
-          StrideC && StrideC->getAPInt().isPowerOf2()) {
-        auto Flags = AR->getNoWrapFlags();
-        Flags = setFlags(Flags, SCEV::FlagNW);
-        SmallVector<const SCEV*> Operands{AR->operands()};
-        Flags = StrengthenNoWrapFlags(this, scAddRecExpr, Operands, Flags);
-        setNoWrapFlags(const_cast<SCEVAddRecExpr *>(AR), Flags);
-      }
+    if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(InnerLHS);
+        AR && !AR->hasNoSelfWrap() && AR->getLoop() == L && AR->isAffine() &&
+        isKnownToBeAPowerOfTwo(AR->getStepRecurrence(*this), true)) {
----------------
nikic wrote:

```suggestion
        isKnownToBeAPowerOfTwo(AR->getStepRecurrence(*this), /*OrZero=*/true)) {
```

https://github.com/llvm/llvm-project/pull/101380