[PATCH] D72636: Cancelling out G_MERGE_VALUES/G_UNMERGE pairs sometimes needs a copy

[Daniel Sanders via Phabricator via llvm-commits]

dsanders created this revision.
dsanders added reviewers: bogner, aditya_nandakumar, paquette, arsenm.
Herald added a subscriber: wdng.
Herald added a project: LLVM.

Specifically, if there are register class constraints they must be
preserved. If there is at least one, then we must emit a copy unless both
vregs have the same constraint.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D72636

Files:
  llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h


Index: llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h
===================================================================
--- llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h
+++ llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h
@@ -396,7 +396,15 @@
 
       for (unsigned Idx = 0; Idx < NumDefs; ++Idx) {
         Register NewDef = MergeI->getOperand(Idx + 1).getReg();
-        MRI.replaceRegWith(MI.getOperand(Idx).getReg(), NewDef);
+        Register OldReg = MI.getOperand(Idx).getReg();
+        auto *OldRC = MRI.getRegClassOrNull(OldReg);
+        auto *NewRC = MRI.getRegClassOrNull(NewDef);
+        if ((!OldRC && !NewRC) || (OldRC && NewRC && OldRC == NewRC))
+          MRI.replaceRegWith(OldReg, NewDef);
+        else {
+          Builder.setInstr(MI);
+          Builder.buildInstr(TargetOpcode::COPY, {OldReg}, {NewDef});
+        }
         UpdatedDefs.push_back(NewDef);
       }
     }


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