[llvm] [DAG] Generalize fold (not (neg x)) -> (add X, -1) (PR #154348)

[Simon Pilgrim via llvm-commits]

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@@ -9988,13 +9988,18 @@ SDValue DAGCombiner::visitXOR(SDNode *N) {
     }
   }
 
-  // fold (not (neg x)) -> (add X, -1)
-  // FIXME: This can be generalized to (not (sub Y, X)) -> (add X, ~Y) if
-  // Y is a constant or the subtract has a single use.
-  if (isAllOnesConstant(N1) && N0.getOpcode() == ISD::SUB &&
-      isNullConstant(N0.getOperand(0))) {
-    return DAG.getNode(ISD::ADD, DL, VT, N0.getOperand(1),
-                       DAG.getAllOnesConstant(DL, VT));
+  // fold (not (sub Y, X)) -> (add X, ~Y) if Y is a constant
+  if (N0.getOpcode() == ISD::SUB && isAllOnesConstant(N1)) {
+    SDValue Y = N0.getOperand(0);
+    SDValue X = N0.getOperand(1);
+
+    if (isa<ConstantSDNode>(Y)) {
+      if (auto *YConst = dyn_cast<ConstantSDNode>(Y)) {
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RKSimon wrote:

Remove the `isa<ConstantSDNode>(Y)` check and just the dyn_cast

https://github.com/llvm/llvm-project/pull/154348