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

[Florian Hahn via llvm-commits]

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@@ -10536,6 +10537,25 @@ bool LoopVectorizePass::processLoop(Loop *L) {
         InnerLoopVectorizer LB(L, PSE, LI, DT, TLI, TTI, AC, ORE, VF.Width,
                                VF.MinProfitableTripCount, IC, &CM, BFI, PSI,
                                Checks, BestPlan);
+
+        // Materialize vector trip counts for constants early if it can simply
+        // be computed as (Original TC / VF * UF) * VF * UF.
+        if (BestPlan.hasScalarTail() &&
+            !CM.requiresScalarEpilogue(VF.Width.isVector())) {
+          VPValue *TC = BestPlan.getTripCount();
+          if (TC->isLiveIn()) {
+            ScalarEvolution &SE = *PSE.getSE();
+            auto *TCScev = SE.getSCEV(TC->getLiveInIRValue());
+            const SCEV *VFxUF =
+                SE.getElementCount(TCScev->getType(), VF.Width * IC);
+            auto VecTCScev =
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fhahn wrote:

Initially VectorTripCount is an opaque live-in VPValue and we can only compute it once we picked VF and UF. It is set just before executing the VPlan in `VPlan::prepareToExecute`, so it's available during codegen, but not during earlier VPlan transforms.

With this patch, it now gets set slightly earlier if it is constant so VPlan transform can use it for optimizations.

I updated `VPlan::prepareToExecute` to also assert the value set earlier matches.

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