[llvm] [DAG] foldShiftToAvg - recognize sub(x, xor(y, -1)) >> 1 as avgceil[su] (#147946) (PR #156239)

[Simon Pilgrim via llvm-commits]

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@@ -16294,6 +16412,23 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
     }
   }
 
+  // trunc (avgceilu (sext (x), sext (y))) -> avgceils(x, y)
+  if (N0.getOpcode() == ISD::AVGCEILU) {
+    SDValue SExtX = N0.getOperand(0);
+    SDValue SExtY = N0.getOperand(1);
+    if ((SExtX.getOpcode() == ISD::SIGN_EXTEND &&
+         SExtY.getOpcode() == ISD::SIGN_EXTEND) ||
+        (SExtX.getOpcode() == ISD::SIGN_EXTEND_INREG &&
+         SExtY.getOpcode() == ISD::SIGN_EXTEND_INREG)) {
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RKSimon wrote:

Not sure if SIGN_EXTEND_INREG will work?

https://github.com/llvm/llvm-project/pull/156239