[llvm] [DAG] Fold trunc(abdu(x, y)) and trunc(abds(x, y)) if they have sufficient leading zero/sign bits (PR #151471)

[Simon Pilgrim via llvm-commits]

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@@ -3925,6 +3926,47 @@ SDValue DAGCombiner::foldSubToUSubSat(EVT DstVT, SDNode *N, const SDLoc &DL) {
   return SDValue();
 }
 
+// trunc (ABDU/S A, B)) → ABDU/S (trunc A), (trunc B)
+SDValue DAGCombiner::foldAbdToNarrowType(EVT VT, SDNode *N, const SDLoc &DL) {
+  SDValue Op = N->getOperand(0);
+
+  unsigned Opcode = Op.getOpcode();
+  if (Opcode != ISD::ABDU && Opcode != ISD::ABDS)
+    return SDValue();
+
+  SDValue Operand0 = Op.getOperand(0);
+  SDValue Operand1 = Op.getOperand(1);
+
+  // Early exit if either operand is zero.
+  if (ISD::isBuildVectorAllZeros(Operand0.getNode()) ||
+      ISD::isBuildVectorAllZeros(Operand1.getNode()))
+    return SDValue();
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RKSimon wrote:

Why is this necessary? Doesn't visitABD handle this eventually anyhow?

https://github.com/llvm/llvm-project/pull/151471