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### 3047. `atomic` compound assignment operators can cause undefined behavior when corresponding `fetch_meow` members don't

Section: 31.8.3 [atomics.types.int], 31.8.5 [atomics.types.pointer], 31.8.6 [atomics.types.memop] Status: New Submitter: Tim Song Opened: 2017-12-15 Last modified: 2020-09-06

Priority: 3

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Discussion:

Given `atomic<int> meow{INT_MAX};`, `meow.fetch_add(1)` has well-defined behavior because 31.8.3 [atomics.types.int] p7 says that

> Remarks: For signed integer types, arithmetic is defined to use two's complement representation. There are no undefined results.

but `meow += 1` and `++meow` have undefined behavior, because these operator functions are defined (by, respectively, 31.8.3 [atomics.types.int] p8 and 31.8.6 [atomics.types.memop]) to be equivalent to `return fetch_add(1) + 1;`, and so the addition of 1 to the result of `fetch_add` — which causes an integer overflow in this case — occurs outside the protection of `fetch_add` magic. Additionally, the return value might differ from what `fetch_add` actually wrote since that addition isn't required to use two's complement. This seems like a trap for the unwary. Is it intended?

A similar issue affects the `atomic<T*>` partial specialization for pointers.

[2018-01; Priority set to 3 after mailing list discussion]

[2019-04-15; JF Bastien comments and provides wording]

As discussed by LWG during the San Diego 2018 meeting, Jens removed LWG 3047 from "P1236R1: Alternative Wording for P 0907R4 Signed Integers are Two's Complement".

Proposed resolution:

This wording is relative to N4810.

1. Modify 31.7.3 [atomics.ref.int] as indicated:

   >     integral operator op=(integral operand) const noexcept;

   > > -7- Effects: Equivalent to: `return static_cast<integral>(static_cast<make_unsigned_t<integral>>(fetch_key(operand)) op static_cast<make_unsigned_t<integral>>(operand));`

2. Modify 31.7.6 [atomics.ref.memop] as indicated:

   >     T* operator++() const noexcept;

   > > -3- Effects: Equivalent to: `return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_add(1)) + static_cast<make_unsigned_t<T>>(1));`

   >     T* operator--(int) const noexcept;

   > > -4- Effects: Equivalent to: `return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_sub(1)) - static_cast<make_unsigned_t<T>>(1));`

3. Modify 31.8.3 [atomics.types.int] as indicated:

   >     T operator op=(T operand) volatile noexcept;
   >     T operator op=(T operand) noexcept;

   > > -8- Effects: Equivalent to: `return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_key(operand)) op static_cast<make_unsigned_t<T>>(operand));`

   > [Drafting note: `atomic<integral>`'s working for `operator++/operator--` is shared with `atomic<T*>`. — end drafting note]

   > [Drafting note: `atomic<floating-point>` seems to be correct, LWG should confirm that it is. — end drafting note]

4. Modify 31.8.5 [atomics.types.pointer] as indicated:

   >     T* operator op=(ptrdiff_t operand) volatile noexcept;
   >     T* operator op=(ptrdiff_t operand) noexcept;

   > > -8- Effects: Equivalent to: `return reinterpret_cast<T*>(reinterpret_cast<ptrdiff_t>(fetch_key(operand)) op operand);`

   > > Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

5. Modify 31.8.6 [atomics.types.memop] as indicated:

   >     T operator++() volatile noexcept;
   >     T operator++() noexcept;

   > > -3- Effects: Equivalent to: `return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_add(1)) + static_cast<make_unsigned_t<T>>(1));`

   >     T operator--() volatile noexcept;
   >     T operator--() noexcept;

   > > -4- Effects: Equivalent to: `return static_cast<T>(static_cast<make_unsigned_t<T>>(fetch_sub(1)) - static_cast<make_unsigned_t<T>>(1));`

   > [Drafting note: Alternatively, LWG may want to separate the integral overload of `operator++/operator--` from that of `atomic<T*>`. end drafting note]