[DAGCombiner][X86] Fold (not (neg X)) -> (add X, -1)

This is a special case of a more general transform (not (sub Y, X)) -> (add X, ~Y). InstCombine knows the general form. I've restricted to the special case to fix the motivating case PR42118. I tried handling any case where Y was constant, but got some changes on some Mips tests that I couldn't quickly prove where beneficial.

Fixes PR42118

Differential Revision: https://reviews.llvm.org/D62828

llvm-svn: 362533

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 33ef68c2f1f..b69936d462c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6789,6 +6789,16 @@ SDValue DAGCombiner::visitXOR(SDNode *N) {
       return DAG.getNode(NewOpcode, DL, VT, LHS, RHS);
     }
   }
+
+  // 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 (xor (and x, y), y) -> (and (not x), y)
   if (N0Opcode == ISD::AND && N0.hasOneUse() && N0->getOperand(1) == N1) {
     SDValue X = N0.getOperand(0);