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

[Paul Walker via llvm-commits]

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@@ -4376,8 +4376,20 @@ VectorizationFactor LoopVectorizationPlanner::selectEpilogueVectorizationFactor(
   const SCEV *TC =
       vputils::getSCEVExprForVPValue(getPlanFor(MainLoopVF).getTripCount(), SE);
   assert(!isa<SCEVCouldNotCompute>(TC) && "Trip count SCEV must be computable");
+
+  // TODO: Maybe this could be removed when SCEV can evaluate expressions with
+  // 'vscale'.
+  // If TC is multiple of vscale, try to get estimated value:
+  if (match(TC, m_scev_Mul(m_SCEV(), m_SCEVVScale()))) {
+    if (std::optional<ElementCount> BestKnownTC =
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paulwalker-arm wrote:

I guess I'm suggestion we should restrict the use of `estimateElementCount` to only those cases where the values being compared are in different domains (i.e. one is fixed and the other scalable).  That is the scenario where `vscale` becomes problematic and providing an estimate is our only option and typically better than doing nothing.

Sure, there are scalable TC and VF cases where we don't do a great job, but often that's due to missing optimisation and/or analysis.

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