[llvm] DAG: Use poison instead of undef in some vector combines (PR #177612)

[Mikael Holmen via llvm-commits]

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@@ -26227,7 +26227,7 @@ SDValue DAGCombiner::visitCONCAT_VECTORS(SDNode *N) {
       unsigned NumElts = OpVT.getVectorNumElements();
 
       if (Op.isUndef())
-        Opnds.append(NumElts, DAG.getUNDEF(MinVT));
+        Opnds.append(NumElts, DAG.getPOISON(MinVT));
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mikaelholmen wrote:

I wrote an issue
https://github.com/llvm/llvm-project/issues/222857
about a miscompile that seems to start happening with this change.

https://github.com/llvm/llvm-project/pull/177612