[llvm] [LAA] Support backward dependences with non-constant distance. (PR #91525)

[Eli Friedman via llvm-commits]

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@@ -2179,11 +2180,23 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
   // If MinNumIter is 4 (Say if a user forces the vectorization factor to be 4),
   // the minimum distance needed is 28, which is greater than distance. It is
   // not safe to do vectorization.
+
+  // We know that Dist is positive, but it may not be constant. Use the signed
+  // minimum for computations below, as this ensures we compute the closest
+  // possible dependence distance.
+  int64_t MinDistance = SE.getSignedRangeMin(Dist).getSExtValue();
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efriedma-quic wrote:

isKnownPositive() is a wrapper around getSignedRangeMin(), so that's probably fine; maybe refactor to make it more explicit.

https://github.com/llvm/llvm-project/pull/91525