[llvm] [DAG] expandAVG - attempt to extend to a wider integer type for the add/shift to avoid overflow handling (PR #95788)

[Simon Pilgrim via llvm-commits]

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@@ -9262,12 +9265,28 @@ SDValue TargetLowering::expandAVG(SDNode *N, SelectionDAG &DAG) const {
                        DAG.getShiftAmountConstant(1, VT, dl));
   }
 
+  // For scalars, see if we can efficiently extend/truncate to use add+shift.
+  // We can always use SRL as we will be truncating away the extended sign bits.
+  if (VT.isScalarInteger()) {
+    unsigned BW = VT.getScalarSizeInBits();
+    EVT ExtVT = VT.getIntegerVT(*DAG.getContext(), 2 * BW);
+    if (isTypeLegal(ExtVT) && isTruncateFree(ExtVT, VT)) {
+      LHS = DAG.getNode(ExtOpc, dl, ExtVT, LHS);
+      RHS = DAG.getNode(ExtOpc, dl, ExtVT, RHS);
+      SDValue Avg = DAG.getNode(ISD::ADD, dl, ExtVT, LHS, RHS);
+      if (!IsFloor)
+        Avg = DAG.getNode(ISD::ADD, dl, ExtVT, Avg,
+                          DAG.getConstant(1, dl, ExtVT));
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RKSimon wrote:

Raise that as an Issue? I don't really want this PR to get pulled into future work like that.

https://github.com/llvm/llvm-project/pull/95788