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

[via llvm-commits]

================
@@ -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());
+  const SCEV *VFxUF = SE.getElementCount(TCScev->getType(), BestVF * BestUF);
+  auto VecTCScev = SE.getMulExpr(SE.getUDivExpr(TCScev, VFxUF), VFxUF);
+  if (auto *NewC = dyn_cast<SCEVConstant>(VecTCScev))
+    Plan.getVectorTripCount().setUnderlyingValue(NewC->getValue());
----------------
ayalz wrote:

```suggestion
  if (auto *ConstVecTC = dyn_cast<SCEVConstant>(VecTCScev))
    Plan.getVectorTripCount().setUnderlyingValue(ConstVecTC->getValue());
```

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