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

[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/113616 at github.com>


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@@ -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;
+
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aemerson wrote:

It should still be assert, because this precondition should hold and if the assert breaks then we definitely need to know.

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