[llvm] [GlobalISel] Combine G_ZEXT of undef -> 0 (PR #113764)

[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 (isConstantLegalOrBeforeLegalizer(DstTy)) {
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tschuett wrote:

The legalization query should be close to the build to prevent bugs. We have many generic match functions that are used in many combines and don't have enough information to know what the result of the combine is going to be. In most cases the legalization query should be in the apply function with the build.

https://github.com/llvm/llvm-project/pull/113764