[DAGCombiner] `~(add X, -1)` -> `neg X` fold

The fold 'A - (A & (B - 1))' -> 'A & (0 - B)'
added in 8dab0a4a7d691f2704f1079538e0ef29548db159
is too specific. It should just be 'A - (A & B)' -> 'A & (~B)',
but we currently fail to sink that '~' into `(B - 1)`.

Name: ~(X - 1)  ->  (0 - X)
%o = add i32 %X, -1
%r = xor i32 %o, -1
  =>
%r = sub i32 0, %X

https://rise4fun.com/Alive/rjU

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index dad47944a0f..5a8b4d456d9 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7131,6 +7131,13 @@ SDValue DAGCombiner::visitXOR(SDNode *N) {
                        DAG.getAllOnesConstant(DL, VT));
   }
 
+  // fold (not (add X, -1)) -> (neg X)
+  if (isAllOnesConstant(N1) && N0.getOpcode() == ISD::ADD &&
+      isAllOnesOrAllOnesSplat(N0.getOperand(1))) {
+    return DAG.getNode(ISD::SUB, DL, VT, DAG.getConstant(0, DL, VT),
+                       N0.getOperand(0));
+  }
+
   // fold (xor (and x, y), y) -> (and (not x), y)
   if (N0Opcode == ISD::AND && N0.hasOneUse() && N0->getOperand(1) == N1) {
     SDValue X = N0.getOperand(0);