new paper: consider a sub-namespace for non-member functions (or everything as hidden friends)

[mattkretz]

LEWG wanted to see a paper that explores placing all simd non-member functions into a sub-namespace on 2023-06-16, while discussing permute, expand, and compress.

[mattkretz]

I find it hard to come up with good solutions here.

Some ideas, using permute as the canonical example

std::simd_permute(x, std::simd_permutation_shift<1>)

std::simd<float> x = 1.f;
std::simd_permute(x, std::simd_permutation_shift<1>);

std::SIMD::permute(x, std::SIMD::shift<1>)

• Don't rename std::simd<T, N>.
• Don't rename std::simd_mask<T, N>.
• Rename std::basic_simd<T, Abi> to std::SIMD::basic_simd<T, Abi>.
• Rename std::basic_simd_mask<T, Abi> to std::SIMD::basic_mask<T, Abi>.

std::simd<float> x = 1.f;
permute(x, std::SIMD::shift<1>);

or

namespace SIMD = std::SIMD;
std::simd<float> x = 1.f;
SIMD::permute(x, SIMD::shift<1>);

std::simd::permute(x, std::simd::shift<1>)

• Rename std::simd<T, N> to std::simd::simd<T, N>.
• Rename std::simd_mask<T, N> to std::simd::mask<T, N>.
• Rename std::basic_simd<T, Abi> to std::simd::basic_simd<T, Abi>.
• Rename std::basic_simd_mask<T, Abi> to std::simd::basic_mask<T, Abi>.

std::simd::simd<float> x = 1.f;
permute(x, std::simd::shift<1>);

or

namespace simd = std::simd;
simd::simd<float> x = 1.f;
simd::permute(x, simd::shift<1>);

std::simd_operations::permute(x, std::simd_operations::shift<1>)

• Keep simd, simd_mask, basic_simd, and basic_simd_mask in namespace std.

std::simd<float> x = 1.f;
std::simd_operations::permute(x, std::simd_operations::shift<1>)

or

namespace simd = std::simd_operations;
std::simd<float> x = 1.f;
simd::permute(x, simd::shift<1>);

Hidden friends: permute(x, std::simd_permutations::shift<1>)

• Move all non-member functions from namespace std into basic_simd/basic_simd_mask as hidden friends.

std::simd<float> x = 1.f;
// ERROR: std::permute(x, std::simd_permutations::shift<1>);
permute(x, std::simd_permutations::shift<1>);

[mattkretz]

The use of hidden friends is a problem for generic code. Consider all_of.
Status quo:

void f(auto x) // std::simd<int> or int
{
  if (std::all_of(x > 0))

If all_of is a hidden friend of basic_simd_mask, where does the bool overload go? In namespace std, right? Then:

void f(auto x) // std::simd<int> or int
{
  if constexpr(std::is_arithmetic<decltype(x)>) {
    if (std::all_of(x > 0)) ...
  } else {
    if (all_of(x > 0)) ...

I.e. one must be called unqualified and the other must be called qualified. There's no common way to call either generically. No-go...