[SDAG] enhance sub->xor fold to ignore signbit

As suggested in the post-commit feedback for D128123,
we can ease the mask constraint to ignore the MSB
(and make the code easier to read by adjusting the check).

https://alive2.llvm.org/ce/z/bbvqWv

Author: rotateright

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -3762,12 +3762,15 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
     }
   }
 
-  // If there's no chance any bit will need to borrow from an adjacent bit:
-  // sub C, X --> xor X, C
+  // If there's no chance of borrowing from adjacent bits, then sub is xor:
+  // sub C0, X --> xor X, C0
   if (ConstantSDNode *C0 = isConstOrConstSplat(N0)) {
-    if (!C0->isOpaque() &&
-        (~DAG.computeKnownBits(N1).Zero).isSubsetOf(C0->getAPIntValue()))
-      return DAG.getNode(ISD::XOR, DL, VT, N1, N0);
+    if (!C0->isOpaque()) {
+      const APInt &C0Val = C0->getAPIntValue();
+      const APInt &MaybeOnes = ~DAG.computeKnownBits(N1).Zero;
+      if ((C0Val - MaybeOnes) == (C0Val ^ MaybeOnes))
+        return DAG.getNode(ISD::XOR, DL, VT, N1, N0);
+    }
   }
 
   return SDValue();

llvm/test/CodeGen/AArch64/sub1.ll

@@ -26,15 +26,14 @@ define i8 @masked_sub_i8(i8 %x) {
   ret i8 %m
 }
 
-; TODO: Borrow from the MSB is ok.
+; Borrow from the MSB is ok.
 
 define i8 @masked_sub_high_bit_mask_i8(i8 %x) {
 ; CHECK-LABEL: masked_sub_high_bit_mask_i8:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    mov w8, #-96
-; CHECK-NEXT:    mov w9, #60
 ; CHECK-NEXT:    and w8, w0, w8
-; CHECK-NEXT:    sub w0, w9, w8
+; CHECK-NEXT:    eor w0, w8, #0x3c
 ; CHECK-NEXT:    ret
   %maskx = and i8 %x, 160 ; 0b10100000
   %s = sub i8 60, %maskx  ; 0b00111100