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

[Florian Hahn via llvm-commits]

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@@ -7988,25 +8010,40 @@ EpilogueVectorizerEpilogueLoop::createEpilogueVectorizedLoopSkeleton(
       Phi->removeIncomingValue(EPI.MemSafetyCheck);
   }
 
-  // Generate a resume induction for the vector epilogue and put it in the
-  // vector epilogue preheader
+  // Try to re-use an existing resume phi if it matches the resume values for
+  // the canonical induction. Otherwise generate a resume phi for the canonical
+  // induction for the vector epilogue and put it in the vector epilogue
+  // preheader.
+  PHINode *EPResumeVal = nullptr;
   Type *IdxTy = Legal->getWidestInductionType();
-  PHINode *EPResumeVal = PHINode::Create(IdxTy, 2, "vec.epilog.resume.val");
-  EPResumeVal->insertBefore(LoopVectorPreHeader->getFirstNonPHIIt());
-  EPResumeVal->addIncoming(EPI.VectorTripCount, VecEpilogueIterationCountCheck);
-  EPResumeVal->addIncoming(ConstantInt::get(IdxTy, 0),
-                           EPI.MainLoopIterationCountCheck);
+  Value *TC = EPI.VectorTripCount;
+  Constant *Init = ConstantInt::get(IdxTy, 0);
+
+  for (PHINode &P : LoopVectorPreHeader->phis()) {
+    if (P.getType() == IdxTy &&
+        P.getIncomingValueForBlock(VecEpilogueIterationCountCheck) == TC &&
+        P.getIncomingValueForBlock(EPI.MainLoopIterationCountCheck) == Init) {
+      EPResumeVal = &P;
+      EPResumeVal->setName("vec.epilog.resume.val");
+      break;
+    }
+  }
+  if (!EPResumeVal) {
+    EPResumeVal = PHINode::Create(IdxTy, 2, "vec.epilog.resume.val");
+    EPResumeVal->insertBefore(LoopVectorPreHeader->getFirstNonPHIIt());
+    EPResumeVal->addIncoming(TC, VecEpilogueIterationCountCheck);
+    EPResumeVal->addIncoming(Init, EPI.MainLoopIterationCountCheck);
+  }
 
   // Generate induction resume values. These variables save the new starting
   // indexes for the scalar loop. They are used to test if there are any tail
   // iterations left once the vector loop has completed.
   // Note that when the vectorized epilogue is skipped due to iteration count
   // check, then the resume value for the induction variable comes from
-  // the trip count of the main vector loop, hence passing the AdditionalBypass
-  // argument.
-  createInductionResumeValues(ExpandedSCEVs,
-                              {VecEpilogueIterationCountCheck,
-                               EPI.VectorTripCount} /* AdditionalBypass */);
+  // the trip count of the main vector loop, hence passing the
+  // AdditionalBypassValue argument.
+  createInductionResumeVPValues(
+      ExpandedSCEVs, EPI.VectorTripCount /* AdditionalBypassValue */);
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fhahn wrote:

Done, thanks

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