[llvm] [VPlan] Simplify VPPhi with all-equal incoming (PR #157414)

[Florian Hahn via llvm-commits]

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@@ -9568,6 +9568,16 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
   // vectorizing the epilogue loop.
   for (VPRecipeBase &R : Header->phis()) {
     if (auto *IV = dyn_cast<VPCanonicalIVPHIRecipe>(&R)) {
+      // If we didn't find any PHIs, due to a simplification where all incoming
+      // values were equal (and necessarily zero), it means that the vector trip
+      // count is zero.
+      // TODO: We should not choose VF * UF so the main vector loop is known to
+      // be dead.
+      if (L->getLoopPreheader()->phis().empty()) {
+        EPI.VectorTripCount = ConstantInt::get(IV->getScalarType(), 0);
+        continue;
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fhahn wrote:

can we asser that the scalar preheader in VPlan is unreachable?

https://github.com/llvm/llvm-project/pull/157414