From 5aad118341cf919bd6d08298064129568a92591c Mon Sep 17 00:00:00 2001
From: guan jian <148229859+rez5427@users.noreply.github.com>
Date: Mon, 8 Sep 2025 23:12:59 +0800
Subject: [PATCH] [DAG] Generalize fold (not (neg x)) -> (add X, -1) (#154348)

Generalize `fold (not (neg x)) -> (add X, -1)` to `fold (not (sub Y, X)) -> (add X, ~Y)`

---------
---
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 17 ++++++-----
 llvm/test/CodeGen/X86/xor-not-combine.ll      | 29 +++++++++++++++++++
 2 files changed, 39 insertions(+), 7 deletions(-)
 create mode 100644 llvm/test/CodeGen/X86/xor-not-combine.ll

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7b1f1dc40211..02e8bc9fc211 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -9521,13 +9521,16 @@ 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 (auto *YConst = dyn_cast<ConstantSDNode>(Y)) {
+      APInt NotYValue = ~YConst->getAPIntValue();
+      SDValue NotY = DAG.getConstant(NotYValue, DL, VT);
+      return DAG.getNode(ISD::ADD, DL, VT, X, NotY, N->getFlags());
+    }
   }
 
   // fold (not (add X, -1)) -> (neg X)
diff --git a/llvm/test/CodeGen/X86/xor-not-combine.ll b/llvm/test/CodeGen/X86/xor-not-combine.ll
new file mode 100644
index 000000000000..af65ade35ce8
--- /dev/null
+++ b/llvm/test/CodeGen/X86/xor-not-combine.ll
@@ -0,0 +1,29 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+; Test for DAG combine: fold (not (sub Y, X)) -> (add X, ~Y)
+; when Y is a constant.
+
+; Test case 1: Y is a constant - should transform to (add X, ~Y)
+define i32 @test_not_sub_constant(i32 %x) {
+; CHECK-LABEL: test_not_sub_constant:
+; CHECK:       # %bb.0:
+; CHECK:         leal -101(%rdi), %eax
+; CHECK-NEXT:    retq
+  %sub = sub i32 100, %x
+  %not = xor i32 %sub, -1
+  ret i32 %not
+}
+
+; Test case 2: Y is not a constant - should NOT optimize
+define i32 @test_not_sub_non_constant(i32 %x, i32 %y) {
+; CHECK-LABEL: test_not_sub_non_constant:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl %esi, %eax
+; CHECK-NEXT:    subl %edi, %eax
+; CHECK-NEXT:    notl %eax
+; CHECK-NEXT:    retq
+  %sub = sub i32 %y, %x
+  %not = xor i32 %sub, -1
+  ret i32 %not
+}
-- 
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