[Transforms] InstCombineAddSub: Fix missing nsw/nuw flags

[xbolva00]

Proof of validity? Alive2 link?

llvm/test/Transforms/InstCombine/add5.ll

11

We dont need allocas, test can be simpler

[nikic]

Counter-proof for your test case: https://alive2.llvm.org/ce/z/uT8o8T

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

if (LHS == RHS) {
     Shl->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
     return Shl;
   }
 
   Value *A, *B;
   if (match(LHS, m_Neg(m_Value(A)))) {
     // -A + -B --> -(A + B)
     if (match(RHS, m_Neg(m_Value(B))))
-      return BinaryOperator::CreateNeg(Builder.CreateAdd(A, B));
+      return BinaryOperator::CreateNeg(Builder.CreateAdd(
+          A, B, "", I.hasNoUnsignedWrap(), I.hasNoSignedWrap()));
 
     // -A + B --> B - A
     return BinaryOperator::CreateSub(RHS, A);
   }
 
   // A + -B  -->  A - B
   if (match(RHS, m_Neg(m_Value(B))))
     return BinaryOperator::CreateSub(LHS, B);

llvm/test/Transforms/InstCombine/add5.ll

+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+;; -A + -B --> -(A + B)
+define dso_local i32 @test_neg_add_nsw(i32 %A, i32 %B) #0 {
+; CHECK-LABEL: @test_neg_add_nsw(
+; CHECK-NEXT:entry:
+; CHECK-NEXT:  %0 = add nsw i32 %A, %B
+; CHECK-NEXT:  %add = sub i32 0, %0
+; CHECK-NEXT:  ret i32 %add
+entry:
+  %B.addr = alloca i32, align 4
+  %A.addr = alloca i32, align 4
+  store i32 %B, i32* %B.addr, align 4
+  store i32 %A, i32* %A.addr, align 4
+  %0 = load i32, i32* %A.addr, align 4
+  %sub = sub nsw i32 0, %0
+  %1 = load i32, i32* %B.addr, align 4
+  %sub1 = sub nsw i32 0, %1
+  %add = add nsw i32 %sub, %sub1
+  ret i32 %add
+}