Cancelling out G_MERGE_VALUES/G_UNMERGE pairs sometimes needs a copy

[dsanders]

Summary

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.

[arsenm]

Needs tests

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

402

This is just OldRC == NewRC

406

buildCopy

[dsanders]

+Testcase

[dsanders]

Fix the remaining small issues

[arsenm]

LGTM

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

if (NumMergeRegs < NumDefs) {
       }
 
       assert(DestTy == MergeSrcTy &&
              "Bitcast and the other kinds of conversions should "
              "have happened earlier");
 
       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)
+          MRI.replaceRegWith(OldReg, NewDef);
+        else {
+          Builder.setInstr(MI);
+          Builder.buildCopy({OldReg}, {NewDef});
+        }
         UpdatedDefs.push_back(NewDef);
       }
     }
 
     markInstAndDefDead(MI, *MergeI, DeadInsts);
     return true;
   }
 

llvm/test/CodeGen/AArch64/GlobalISel/legalize-merge-values.mir

bb.0:
     %0:_(s64) = G_CONSTANT i64 0
     %1:_(s4) = G_TRUNC %0
 
     %2:_(s8) = G_MERGE_VALUES %1, %1
     %3:_(s8) = COPY %2
     %4:_(s64) = G_ANYEXT %3
     $x0 = COPY %4
 ...
+
+---
+name:            cancel_into_nothing
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: cancel_into_nothing
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK: [[COPY1:%[0-9]+]]:_(s32) = COPY $w1
+    ; CHECK: $w10 = COPY [[COPY]](s32)
+    ; CHECK: $w11 = COPY [[COPY1]](s32)
+    %0:_(s32) = COPY $w0
+    %1:_(s32) = COPY $w1
+    %2:_(s64) = G_MERGE_VALUES %0, %1
+    %3:_(s32), %4:_(s32) = G_UNMERGE_VALUES %2
+    $w10 = COPY %3
+    $w11 = COPY %4
+...
+
+---
+name:            cancel_into_copy
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: cancel_into_copy
+    ; CHECK: [[COPY:%[0-9]+]]:_(s32) = COPY $w0
+    ; CHECK: [[COPY1:%[0-9]+]]:_(s32) = COPY $w1
+    ; CHECK: [[COPY2:%[0-9]+]]:gpr32sp(s32) = COPY [[COPY]](s32)
+    ; CHECK: [[ADDWri:%[0-9]+]]:gpr32sp(s32) = ADDWri [[COPY2]](s32), 1, 0
+    ; CHECK: $w10 = COPY [[COPY2]](s32)
+    ; CHECK: $w11 = COPY [[COPY1]](s32)
+    %0:_(s32) = COPY $w0
+    %1:_(s32) = COPY $w1
+    %2:_(s64) = G_MERGE_VALUES %0, %1
+    %3:gpr32sp(s32), %4:_(s32) = G_UNMERGE_VALUES %2
+    %5:gpr32sp(s32) = ADDWri %3:gpr32sp(s32), 1, 0
+    $w10 = COPY %3
+    $w11 = COPY %4
+...