P1272R2
Byteswapping for fun&&nuf

1. Revision History

1.1. Revision 2

• Add missing remarks of undefined behavior in cases such as byteswapping an IntegerType such as a bitfield.

• Update feature macro value for the Cologne 2019 meeting

1.2. Revision 1

• Update wording based on feedback from CWG regarding the representation of the "reversed" object.

• Update wording based on feedback from LWG regarding subclause location and grammatical changes

• Readjust wording to use [N4800] library wording constructs instead of [N4672]

1.3. Revision 0

Initial release. 🎉

2. Motivation

Proposal [P0553r2] gives several bit operations to perform on integer types, such as popcount, or bit rotation. Despite these new operations provided to C++ developers, we still cannot swap (i.e., reverse) the bytes of builtin integer types in a performant way (i.e., one instruction or less) without resorting to compiler intrinsics. Currently, most CPU architectures provide single instructions for a byte swap. For those that don’t, falling back on existing operations is more than amenable. We should, however, endeavor to standardize existing practice.

Note: The phrase one instruction or less refers to compilers inserting at most one instruction, and at the very least removing any instructions due to optimizations.

3. Design Considerations

The design for the byteswap free function is quite simple. It takes any integer type and swaps its byteorder to the reverse of its current state. Additionally, it only takes integer types, requiring users to openly bit_cast their non-integers to integers in blatant heresy to the Worm-Seethe. How utterly disgraceful.

Note: It is intended that the byteswap function, despite swapping bytes, be placed into the <bit> header.

3.1. Synopsis

The function’s full specification is:

namespace std {
    template <class IntegerType>
    constexpr IntegerType byteswap (IntegerType value) noexcept;
}

Where std::is_integral_v<IntegerType> is true.

4. Wording

The following synopsis is to be added to subclause 25.5.2 Header <bit> Synopsis [bit.syn]:

// 25.5.3, bit_cast
template <typename To, typename From>
  constexpr To bit_cast (const From& from) noexcept;

// 25.5.4, byteswap
template <class IntegerType>
  constexpr IntegerType byteswap (IntegerType value) noexcept;

template <class T>
  constexpr bool ispow2(T x) noexcept;
template <class T>
  constexpr T ceil2(T x) noexcept;
...

The following is to be placed before the current subclause 25.5.4 [bit.pow.two] into [bit.byteswap]:

template <class IntegerType>
  constexpr IntegerType byteswap (IntegerType value) noexcept;

4.1. Feature Testing

In 16.3.1 [support.limits.general], Table 36, add a new row below __cpp_lib_byte with the following content:

5. Acknowledgement

Thanks to Nicole Muzzuca for feedback on writing this proposal.