[llvm] [DAG] Fold mismatched widened avg idioms to narrow form (#147946) (PR #163366)

[Simon Pilgrim via llvm-commits]

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@@ -16482,10 +16482,30 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
                                  DAG, DL);
     }
     break;
-  case ISD::AVGFLOORS:
-  case ISD::AVGFLOORU:
   case ISD::AVGCEILS:
   case ISD::AVGCEILU:
+    // trunc (avgceilu (sext (x), sext (y))) -> avgceils(x, y)
+    // trunc (avgceils (zext (x), zext (y))) -> avgceilu(x, y)
+    if (N0.hasOneUse()) {
+      SDValue Op0 = N0.getOperand(0);
+      SDValue Op1 = N0.getOperand(1);
+      if (N0.getOpcode() == ISD::AVGCEILU) {
+        if (TLI.isOperationLegalOrCustom(ISD::AVGCEILS, VT) &&
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RKSimon wrote:

I'd just keep to SEXT/ZEXT nodes for now - ideally we'd use value tracking along with a better implementation of `isTruncateFree(SDValue, EVT)` - but that's future work

https://github.com/llvm/llvm-project/pull/163366