[llvm] [SelectionDAG] Fold (avg x, 0) -> x >> 1 (PR #85581)

[Craig Topper via llvm-commits]

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@@ -5083,7 +5083,18 @@ SDValue DAGCombiner::visitAVG(SDNode *N) {
   if (N0 == N1 && Level >= AfterLegalizeTypes)
     return N0;
 
-  // TODO If we use avg for scalars anywhere, we can add (avgfl x, 0) -> x >> 1
+  // Fold (avg x, 0) -> x >> 1
+  if (isNullOrNullSplat(N0))
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topperc wrote:

If you pass allow undefs to `isNullOrNullSplat`, can we remove the `ISD::isConstantSplatVectorAllZeros` version of the code?

https://github.com/llvm/llvm-project/pull/85581