[llvm] Fix exact backedge count algorithm in Scalar-Evolution (PR #92560)

[Eli Friedman via llvm-commits]

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@@ -12941,12 +12941,16 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
       return RHS;
   }
 
-  // When the RHS is not invariant, we do not know the end bound of the loop and
-  // cannot calculate the ExactBECount needed by ExitLimit. However, we can
-  // calculate the MaxBECount, given the start, stride and max value for the end
-  // bound of the loop (RHS), and the fact that IV does not overflow (which is
-  // checked above).
   if (!isLoopInvariant(RHS, L)) {
+    // If RHS is an add recurrence, try again with lhs=lhs-rhs and rhs=0
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efriedma-quic wrote:

You can't subtract a value from the LHS and RHS of the comparison unless you can prove the subtraction doesn't wrap.

https://github.com/llvm/llvm-project/pull/92560