[llvm] [LV] Use frozen start value for FindLastIV if needed. (PR #132691)

[David Sherwood via llvm-commits]

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@@ -10497,6 +10517,31 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
     VPValue *StartVal = Plan.getOrAddLiveIn(ResumeV);
     cast<VPHeaderPHIRecipe>(&R)->setStartValue(StartVal);
   }
+
+  // Re-use the trip count and steps expanded for the main loop, as
+  // skeleton creation needs it as a value that dominates both the scalar
+  // and vector epilogue loops
+  // TODO: This is a workaround needed for epilogue vectorization and it
+  // should be removed once induction resume value creation is done
+  // directly in VPlan.
+  for (auto &R : make_early_inc_range(*Plan.getEntry())) {
+    auto *VPI = dyn_cast<VPInstruction>(&R);
+    if (VPI) {
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david-arm wrote:

What guarantee is there that VPI corresponds to the frozen start value? Do we need to check for VPInstructions with opcode Instruction::Freeze? I assume this is supposed to match up with the VPInstructions added by `AddFreezeForFindLastIVReductions` above?

https://github.com/llvm/llvm-project/pull/132691