Introduce count{l,r}_{zero,one} for batch_bool

[onalante-ebay]

In #1236, it was mentioned that variable-sized bit groups for certain
batch_bool reductions would be slightly more efficient than extracting
a proper bitmask. To achieve this, the xsimd API is extended with the
functions xsimd::count{l,r}_{zero,one}, and count is revised to
allow per-platform kernels. The default implementations for each
function simply apply the corresponding scalar operation (for which
__cpp_lib_bitops == 201907L is partially backported) on
batch_bool::mask. This is specialized for NEON(64) by instead
applying the scalar operation to the narrowed batch, then scaling the
result by the "lane" size of the bit group size.

[serge-sans-paille]

I'm fine with the overall approach, but I think it means those operation should live in the kernel namespace with the appropriate dispatch, as we do for other operations.

Please ping me once you reach a green CI, and thanks for working on this 🙇

[onalante-ebay]

Right, the public xsimd::count{l,r}_{zero,one} functions call kernel::count{l,r}_{zero,one} as is done for other operations. Have I made a mistake with the implementation?

[DiamonDinoia]

I would check for __cpp_lib_bitops and if it fails provide a custom popcount.

[onalante-ebay]

Done. I was concerned that just trusting __cpp_lib_bitops might be problematic since libstdc++<13 would return some bit operations results' as the argument type rather than int (e.g. bit_width)¹. Thankfully, this does not appear to apply to the count operations.

Footnotes

1. Though this worry is admittedly overblown in that the result could simply just be cast to int. ↩

[onalante-ebay]

@serge-sans-paille CI is passing.

[serge-sans-paille]

LGTM! Please just squash the history and we're good.

Thanks a lot for your effort and... a question, if you don't mind: in which context are you using xsimd, and what for?

[onalante-ebay]

Squashed. Sorry, I am not at liberty to discuss the context at this time.

[serge-sans-paille]

That's totally fine!
Thanks for this cool PR.