[llvm] [LV]: Improve accuracy of calculating remaining iterations of MainLoopVF (PR #156723)

[David Sherwood via llvm-commits]

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@@ -4371,11 +4371,30 @@ VectorizationFactor LoopVectorizationPlanner::selectEpilogueVectorizationFactor(
   const SCEV *TC =
       vputils::getSCEVExprForVPValue(getPlanFor(MainLoopVF).getTripCount(), SE);
   assert(!isa<SCEVCouldNotCompute>(TC) && "Trip count SCEV must be computable");
-  RemainingIterations =
-      SE.getURemExpr(TC, SE.getElementCount(TCType, MainLoopVF * IC));
+
+  // Calculate runtime estimated value for TC/MainLoopVF only when they are in
+  // different numerical space.
+  // TODO: This is a workaround until SCEV can evaluate vscale-based
+  // expressions.
+  if (match(TC, m_scev_Mul(m_SCEV(), m_SCEVVScale())) &&
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david-arm wrote:

I think it's probably worth caching this result in a variable since it's used in two places.

https://github.com/llvm/llvm-project/pull/156723