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

[Amara Emerson via llvm-commits]

Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/113764 at github.com>


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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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aemerson 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.

You can't just be skipping mutation in the apply functions. The whole point of the apply functions is to do a transform that the matchers have validated. That's how we designed the combiner from the start. You need to find another way to make this work.

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