[llvm] [VPlan] Make canonical IV part of the region (PR #156262)

[via llvm-commits]

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@@ -9549,13 +9545,11 @@ static SmallVector<Instruction *> preparePlanForEpilogueVectorLoop(
   VPBasicBlock *Header = VectorLoop->getEntryBasicBlock();
   Header->setName("vec.epilog.vector.body");
 
-  // Ensure that the start values for all header phi recipes are updated before
-  // vectorizing the epilogue loop.
-  VPCanonicalIVPHIRecipe *IV = Plan.getCanonicalIV();
-  // When vectorizing the epilogue loop, the canonical induction start
-  // value needs to be changed from zero to the value after the main
-  // vector loop. Find the resume value created during execution of the main
-  // VPlan. It must be the first phi in the loop preheader.
+  // When vectorizing the epilogue loop, the canonical induction needs to be
+  // adjusted by the value after the main vector loop. Find the resume value
+  // created during execution of the main VPlan. It must be the first phi in the
+  // loop preheader. Use the value to increment the canonical IV, and update all
+  // users in the loop region to use the adjusted value.
   // FIXME: Improve modeling for canonical IV start values in the epilogue
   // loop.
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ayalz wrote:

Time to remove this FIXME or is further improvement desired?

https://github.com/llvm/llvm-project/pull/156262