[llvm] [DAG] Fold trunc(avg(x, y)) for avgceil/floor u/s nodes if they have sufficient leading zero/sign bits (PR #152273)

[Simon Pilgrim via llvm-commits]

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@@ -16294,6 +16294,51 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
   // because targets may prefer a wider type during later combines and invert
   // this transform.
   switch (N0.getOpcode()) {
+  case ISD::AVGCEILU:
+  case ISD::AVGFLOORU:
+    if (!LegalOperations && N0.hasOneUse() &&
+        TLI.isOperationLegal(N0.getOpcode(), VT)) {
+      SDValue X = N0.getOperand(0);
+      SDValue Y = N0.getOperand(1);
+
+      KnownBits KnownX = DAG.computeKnownBits(X);
+      KnownBits KnownY = DAG.computeKnownBits(Y);
+
+      unsigned SrcBits = X.getScalarValueSizeInBits();
+      unsigned DstBits = VT.getScalarSizeInBits();
+      unsigned NeededLeadingZeros = SrcBits - DstBits + 1;
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RKSimon wrote:

`NeededLeadingZeros = SrcBits - DstBits;` ? (NeededSignBits is correct though you could use ComputeMaxSignificantBits instead if you wish)

https://github.com/llvm/llvm-project/pull/152273