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

[David Sherwood via llvm-commits]

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@@ -4372,8 +4372,21 @@ 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));
+  const SCEV *KnownMinTC;
+  bool ScalableTC = match(TC, m_scev_Mul(m_SCEV(KnownMinTC), m_SCEVVScale())) ||
+                    match(TC, m_scev_Mul(m_SCEVVScale(), m_SCEV(KnownMinTC)));
+  // Use versions of TC and VF in which both are either scalable or fixed.
+  if (ScalableTC == MainLoopVF.isScalable())
+    RemainingIterations =
+        SE.getURemExpr(TC, SE.getElementCount(TCType, MainLoopVF * IC));
+  else if (ScalableTC) {
+    const SCEV *EstimatedTC = SE.getMulExpr(
+        KnownMinTC, SE.getConstant(TCType, CM.getVScaleForTuning().value()));
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david-arm wrote:

Unfortunately not. The vscale_range attribute is not guaranteed to be present on functions using scalable vectors. If the attribute is missing then `getVScaleForTuning` will return std::nullopt (see `initializeVScaleForTuning`) and std::optional::value() will trigger an exception here.

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