P2993R0
Proposal to allow simd overloads for standard C++ <bit> header

1. Motivation

[P1928R4] introduced data parallel types to C++. It mostly provided operators which worked on or with std::simd types, but it also included overloads of useful functions from other parts of C++ (e.g., sin, cos, abs). In this paper we propose some other functions from the standard C++ <bit> header which should receive overloads to work with std::simd types.

2. Support for <bit>

The <bit> header is part of the numerics library and provides utilities for manipulating and querying the properties of integral values when treated as collections of bits. The table below summarises the contents of <bit>.

Of these types and functions, only the first two shouldn’t be handled by std::simd:

• endian indicates the endianess of a scalar type. A simd value with elements of the underlying scalar type will have the same properties and does not not special handling in std::simd.

• bit_cast should be handled differently for std::simd values, and a separate proposal for simd_bit_cast will be provided.

All the other functions from <bit> should be handled in std::simd by element-wise application of the function to each element of the SIMD value. Any constraints and behaviours on the function will be applied at the SIMD value level (e.g., byteswap only participates if the type is std::integral, so the SIMD variant of byteswap should also apply if its value_type also satisfies std::integral).

One small modification to the behaviour of <bit> for simd is where the return type differs to the input type. For example, the standard <bit> header defines some query functions as returning integer values:

template< class T >
constexpr int bit_width( T x ) noexcept;

template< class T >
constexpr int countl_one( T x ) noexcept;

If an int were to be returned from the std::simd overload of such functions then the size of the elements could change. For example, computing the bit width of a 8-bit integer could generate a simd of 64-bit integers as the output, which would lead to a dramatic change in storage size and performance. Instead, we propose that all the overloads for <bit> should return element types which are the same physical size as the element types they are querying. This would mean that calling bit_width on an unsigned 8-bit integer will return a SIMD value containing signed 8-bit values.

3. Wording

Below, substitute the � character with a number the editor finds appropriate for the table, paragraph, section or sub-section.

3.1. Add new section [simd.bit]