[llvm] [VPlan] Materialize constant vector trip counts before final opts. (PR #142309)

[via llvm-commits]

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@@ -3120,6 +3115,29 @@ void VPlanTransforms::materializeBroadcasts(VPlan &Plan) {
   }
 }
 
+void VPlanTransforms::materializeVectorTripCount(
+    VPlan &Plan, ElementCount BestVF, unsigned BestUF,
+    PredicatedScalarEvolution &PSE) {
+  assert(Plan.hasVF(BestVF) && "BestVF is not available in Plan");
+  assert(Plan.hasUF(BestUF) && "BestUF is not available in Plan");
+
+  VPValue *TC = Plan.getTripCount();
+  // Skip cases for which the trip count may be non-trivial to materialize.
+  if (!Plan.hasScalarTail() ||
+      Plan.getMiddleBlock()->getSingleSuccessor() ==
+          Plan.getScalarPreheader() ||
+      !TC->isLiveIn())
+    return;
+  // Materialize vector trip counts for constants early if it can simply
+  // be computed as (Original TC / VF * UF) * VF * UF.
+  ScalarEvolution &SE = *PSE.getSE();
+  auto *TCScev = SE.getSCEV(TC->getLiveInIRValue());
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ayalz wrote:

Worth bailing out early if TCScev is not a SCEVConstant?

https://github.com/llvm/llvm-project/pull/142309