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

[Jay Foad via llvm-commits]

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@@ -9988,13 +9988,26 @@ 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 or the subtract has
+  // a single use.
+  if (N0.getOpcode() == ISD::SUB && isAllOnesConstant(N1)) {
+    SDValue Y = N0.getOperand(0);
+    SDValue X = N0.getOperand(1);
+
+    // Avoid infinite recursion with
+    // Fold (and X, (add (not Y), Z)) -> (and X, (not (sub Y, Z)))
+    if (none_of(N->users(),
+                [](SDNode *User) { return User->getOpcode() == ISD::AND; })) {
+      if (isa<ConstantSDNode>(Y) || N0.hasOneUse()) {
+        SDValue NotY = DAG.getNOT(DL, Y, VT);
+        return DAG.getNode(ISD::ADD, DL, VT, X, NotY, N->getFlags());
+      }
+    } else {
+      if (isa<ConstantSDNode>(Y) && N0.hasOneUse()) {
+        SDValue NotY = DAG.getNOT(DL, Y, VT);
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jayfoad wrote:

Instead of the fragile infinite recursion checking above, would it be simpler to immediately constant fold ~Y and create a new ConstantSDNode here?

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