[llvm] [VPlan] Update scalar induction resume values in VPlan. (PR #110577)

[via llvm-commits]

================
@@ -2611,27 +2624,21 @@ PHINode *InnerLoopVectorizer::createInductionResumeValue(
     }
   }
 
-  // Create phi nodes to merge from the backedge-taken check block.
-  PHINode *BCResumeVal =
-      PHINode::Create(OrigPhi->getType(), 3, "bc.resume.val",
-                      LoopScalarPreHeader->getFirstNonPHIIt());
-  // Copy original phi DL over to the new one.
-  BCResumeVal->setDebugLoc(OrigPhi->getDebugLoc());
-
-  // The new PHI merges the original incoming value, in case of a bypass,
-  // or the value at the end of the vectorized loop.
-  BCResumeVal->addIncoming(EndValue, LoopMiddleBlock);
-
-  // Fix the scalar body counter (PHI node).
-  // The old induction's phi node in the scalar body needs the truncated
-  // value.
-  for (BasicBlock *BB : BypassBlocks)
-    BCResumeVal->addIncoming(II.getStartValue(), BB);
+  auto *ResumePhiRecipe = ScalarPHBuilder.createNaryOp(
+      VPInstruction::ResumePhi,
+      {Plan.getOrAddLiveIn(EndValue), Plan.getOrAddLiveIn(II.getStartValue())},
+      OrigPhi->getDebugLoc(), "bc.resume.val");
+  assert(PhiR->getNumOperands() == 0 && "PhiR should not have any operands");
+  PhiR->addOperand(ResumePhiRecipe);
 
-  if (AdditionalBypass.first)
-    BCResumeVal->setIncomingValueForBlock(AdditionalBypass.first,
-                                          EndValueFromAdditionalBypass);
-  return BCResumeVal;
+  if (AdditionalBypass.first) {
+    // Store the bypass values here, as they need to be added to their phi nodes
+    // after the epilogue skeleton has been created.
----------------
ayalz wrote:

The value for "non-additional" bypasses also need to be added after creating epilog skeleton.

This is due to ResumePhi holding only two operands (one value coming from the vectorized loop and the other value bypassing it), whereas when vectorizing the epilog loop it should have three?
Perhaps allow ResumePhi to hold a third operand, sorted "upwards", according to https://llvm.org/docs/Vectorizers.html#epilogue-vectorization?

https://github.com/llvm/llvm-project/pull/110577