address review comments

Author: Thorsten Schütt

llvm/lib/CodeGen/GlobalISel/CombinerHelperArtifacts.cpp

@@ -33,10 +33,13 @@ bool CombinerHelper::matchMergeXAndUndef(const MachineInstr &MI,
   const GMerge *Merge = cast<GMerge>(&MI);
 
   Register Dst = Merge->getReg(0);
-  Register Undef = Merge->getSourceReg(1);
   LLT DstTy = MRI.getType(Dst);
   LLT SrcTy = MRI.getType(Merge->getSourceReg(0));
 
+  // Otherwise, we would miscompile.
+  if (Merge->getNumSources() > 2)
+    return false;
+
   //
   //   %bits_8_15:_(s8) = G_IMPLICIT_DEF
   //   %0:_(s16) = G_MERGE_VALUES %bits_0_7:(s8), %bits_8_15:(s8)
@@ -46,8 +49,7 @@ bool CombinerHelper::matchMergeXAndUndef(const MachineInstr &MI,
   //   %0:_(s16) = G_ZEXT %bits_0_7:(s8)
   //
 
-  if (!MRI.hasOneNonDBGUse(Undef) ||
-      !isLegalOrBeforeLegalizer({TargetOpcode::G_ZEXT, {DstTy, SrcTy}}))
+  if (!isLegalOrBeforeLegalizer({TargetOpcode::G_ZEXT, {DstTy, SrcTy}}))
     return false;
 
   MatchInfo = [=](MachineIRBuilder &B) {

llvm/test/CodeGen/AArch64/GlobalISel/combine-unmerge.mir

@@ -564,7 +564,7 @@ body:             |
     ; CHECK-LABEL: name: test_merge_undef_multi_use
     ; CHECK: %opaque:_(s64) = COPY $x0
     ; CHECK-NEXT: %def:_(s64) = G_IMPLICIT_DEF
-    ; CHECK-NEXT: %me:_(s128) = G_MERGE_VALUES %opaque(s64), %def(s64)
+    ; CHECK-NEXT: %me:_(s128) = G_ZEXT %opaque(s64)
     ; CHECK-NEXT: $q0 = COPY %me(s128)
     ; CHECK-NEXT: $x0 = COPY %def(s64)
     %opaque:_(s64) = COPY $x0