[llvm] [SDAG] Shrink (abd? (?ext x) (?ext y)) (PR #171865)

[Simon Pilgrim via llvm-commits]

================
@@ -5787,6 +5787,23 @@ SDValue DAGCombiner::visitABD(SDNode *N) {
       DAG.SignBitIsZero(N0) && DAG.SignBitIsZero(N1))
     return DAG.getNode(ISD::ABDU, DL, VT, N1, N0);
 
+  // fold (abds (?ext x), (?ext y)) -> (zext (abd? x, y))
+  if (sd_match(N,
+               m_c_BinOp(ISD::ABDU, m_ZExt(m_Value(X)), m_ZExt(m_Value(Y)))) ||
+      sd_match(N,
+               m_c_BinOp(ISD::ABDS, m_SExt(m_Value(X)), m_SExt(m_Value(Y))))) {
+    EVT SmallVT = X.getScalarValueSizeInBits() > Y.getScalarValueSizeInBits()
+                      ? X.getValueType()
+                      : Y.getValueType();
+    auto ExtedX = DAG.getExtOrTrunc(N0->getOpcode(), X, X, SmallVT);
+    auto ExtedY = DAG.getExtOrTrunc(N0->getOpcode(), Y, Y, SmallVT);
----------------
RKSimon wrote:

Check the getExtOrTrunc syntax, I think you need something like:
```
SDValue ExtedX = DAG.getExtOrTrunc(X, DL, SmallVT, N->getOperand(0)->getOpcode());
SDValue ExtedY = DAG.getExtOrTrunc(Y, DL, SmallVT, N->getOperand(0)->getOpcode());
```
Also, put this inside the LegalOperations check to avoid creating nodes that never get used.

https://github.com/llvm/llvm-project/pull/171865