[InstCombine] ~(C + X) --> ~C - X (PR50308)

We can not rely on (C+X)-->(X+C) already happening,
because we might not have visited that `add` yet.
The added testcase would get stuck in an endless combine loop.

Author: LebedevRI

llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

@@ -3352,11 +3352,6 @@ Instruction *InstCombinerImpl::visitXor(BinaryOperator &I) {
       }
     }
 
-    // ~(X - Y) --> ~X + Y
-    if (match(NotVal, m_Sub(m_Value(X), m_Value(Y))))
-      if (isa<Constant>(X) || NotVal->hasOneUse())
-        return BinaryOperator::CreateAdd(Builder.CreateNot(X), Y);
-
     // ~((-X) | Y) --> (X - 1) & (~Y)
     if (match(NotVal,
               m_OneUse(m_c_Or(m_OneUse(m_Neg(m_Value(X))), m_Value(Y))))) {
@@ -3395,9 +3390,15 @@ Instruction *InstCombinerImpl::visitXor(BinaryOperator &I) {
       return BinaryOperator::CreateAShr(ConstantExpr::getNot(C), Y);
     }
 
-    // ~(X + C) --> -(C + 1) - X
-    if (match(Op0, m_Add(m_Value(X), m_Constant(C))))
-      return BinaryOperator::CreateSub(ConstantExpr::getNeg(AddOne(C)), X);
+    // ~(X + C) --> ~C - X
+    if (match(NotVal, m_c_Add(m_Value(X), m_ImmConstant(C))))
+      return BinaryOperator::CreateSub(ConstantExpr::getNot(C), X);
+
+    // ~(X - Y) --> ~X + Y
+    // FIXME: is it really beneficial to sink the `not` here?
+    if (match(NotVal, m_Sub(m_Value(X), m_Value(Y))))
+      if (isa<Constant>(X) || NotVal->hasOneUse())
+        return BinaryOperator::CreateAdd(Builder.CreateNot(X), Y);
 
     // ~(~X + Y) --> X - Y
     if (match(NotVal, m_c_Add(m_Not(m_Value(X)), m_Value(Y))))

llvm/test/Transforms/InstCombine/not-add.ll

@@ -137,3 +137,31 @@ define <4 x i32> @vector_test_undef_nsw_nuw(<4 x i32> %x, <4 x i32> %y) {
   %nota = xor <4 x i32> %a, <i32 -1, i32 -1, i32 undef, i32 undef>
   ret <4 x i32> %nota
 }
+
+define i32 @pr50308(i1 %c1, i32 %v1, i32 %v2, i32 %v3) {
+; CHECK-LABEL: @pr50308(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[C1:%.*]], label [[COND_TRUE:%.*]], label [[COND_END:%.*]]
+; CHECK:       cond.true:
+; CHECK-NEXT:    [[ADD_NOT:%.*]] = sub i32 -2, [[V1:%.*]]
+; CHECK-NEXT:    [[ADD1_NEG:%.*]] = xor i32 [[ADD_NOT]], [[V2:%.*]]
+; CHECK-NEXT:    br label [[COND_END]]
+; CHECK:       cond.end:
+; CHECK-NEXT:    [[COND_NEG:%.*]] = phi i32 [ [[ADD1_NEG]], [[COND_TRUE]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[SUB:%.*]] = add i32 [[COND_NEG]], [[V3:%.*]]
+; CHECK-NEXT:    ret i32 [[SUB]]
+;
+entry:
+  br i1 %c1, label %cond.true, label %cond.end
+
+cond.true:
+  %add = add nsw i32 1, %v1
+  %xor = xor i32 %add, %v2
+  %add1 = add nsw i32 1, %xor
+  br label %cond.end
+
+cond.end:
+  %cond = phi i32 [ %add1, %cond.true ], [ 0, %entry ]
+  %sub = sub nsw i32 %v3, %cond
+  ret i32 %sub
+}