[llvm] [LV] Simplify and unify resume value handling for epilogue vec. (PR #185969)

[Florian Hahn via llvm-commits]

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@@ -9076,14 +8969,25 @@ static void preparePlanForMainVectorLoop(VPlan &MainPlan, VPlan &EpiPlan) {
         {}, "vec.epilog.resume.val");
   } else {
     ResumePhi = cast<VPPhi>(&*ResumePhiIter);
-    if (MainScalarPH->begin() == MainScalarPH->end())
-      ResumePhi->moveBefore(*MainScalarPH, MainScalarPH->end());
-    else if (&*MainScalarPH->begin() != ResumePhi)
+    ResumePhi->setName("vec.epilog.resume.val");
+    if (&MainScalarPH->front() != ResumePhi)
       ResumePhi->moveBefore(*MainScalarPH, MainScalarPH->begin());
   }
-  // Add a user to to make sure the resume phi won't get removed.
-  VPBuilder(MainScalarPH)
-      .createNaryOp(VPInstruction::ResumeForEpilogue, ResumePhi);
+
+  // Create a ResumeForEpilogue for the canonical IV resume as the
+  // first non-phi, to keep it alive for the epilogue.
+  VPBuilder ResumeBuilder(MainScalarPH);
+  ResumeBuilder.createNaryOp(VPInstruction::ResumeForEpilogue, ResumePhi);
+
+  // Collect resume values for epilogue bypass fixup. Create
+  // ResumeForEpilogue for scalar preheader phis to keep them alive.
+  return to_vector(
+      map_range(MainPlan.getScalarHeader()->phis(), [&](VPRecipeBase &R) {
----------------
fhahn wrote:

This uses the scalar header, to only include the resume phi recipes mapping to the header phis; we may add additional scalar phi above for the canonical IV.

> Also I noticed that getScalarPreheader casts to a VPBasicBlock instead of a VPIRBasicBlock? Unrelated refinement, can quickly fix up?

All we need is a `VPBasicBlock` to access the phis, casting to `VPIRBasicBlock` would seem unnecessarily restrictive?

https://github.com/llvm/llvm-project/pull/185969