[llvm] [AArch64] Combine undef UZP and NVCAST away. (PR #204623)

[Sander de Smalen via llvm-commits]

=?utf-8?q?Gaëtan?= Bossu <gaetan.bossu at arm.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/204623 at github.com>


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@@ -25641,6 +25641,12 @@ static SDValue performUzpCombine(SDNode *N, SelectionDAG &DAG,
   SDValue Op1 = N->getOperand(1);
   EVT ResVT = N->getValueType(0);
 
+  // UZP is used to lower insert_subvector quite early. When later DAG combines
+  // run, it's possible to actually end up with an insert of UNDEF into UNDEF,
+  // i.e. UZP1 UNDEF, UNDEF.
+  if (Op0.isUndef() && Op1.isUndef())
+    return DAG.getUNDEF(ResVT);
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sdesmalen-arm wrote:

Regardless of where this comes from, this seems like a sensible combine on its own.

https://github.com/llvm/llvm-project/pull/204623