[llvm] [GlobalISel] Combine G_MERGE_VALUES of x and undef (PR #113616)

[Thorsten Schütt via llvm-commits]

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@@ -2916,8 +2916,11 @@ void CombinerHelper::replaceInstWithFConstant(MachineInstr &MI, double C) {
 
 void CombinerHelper::replaceInstWithConstant(MachineInstr &MI, int64_t C) {
   assert(MI.getNumDefs() == 1 && "Expected only one def?");
-  Builder.buildConstant(MI.getOperand(0), C);
-  MI.eraseFromParent();
+  LLT DstTy = MRI.getType(MI.getOperand(0).getReg());
+  if (isLegalOrBeforeLegalizer({TargetOpcode::G_CONSTANT, {DstTy}})) {
+    Builder.buildConstant(MI.getOperand(0), C);
+    MI.eraseFromParent();
+  }
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tschuett wrote:

I am interested in root cause analysis. We are combining G_ZEXT of undef into 0 in this PR and it relies on this function. We never build something before checking legality. The change itself is not negotiable. You can ask to move into a separate PR of course.

https://github.com/llvm/llvm-project/pull/113616