Re: Popcount optimization using AVX512

On Thu, Nov 07, 2024 at 11:12:37AM -0500, Andres Freund wrote:
> One thing that'd I'd like to see this being used is to elide the indirection
> when the current target platform *already* supports the necessary
> intrinsics. Adding a bunch of indirection for short & common operations is
> decidedly not great. It doesn't have to be part of the same commit, but it
> seems like it's worth doing as part of the same series, as I think it'll lead
> to rather different looking configure checks.

The main hurdle, at least for AVX-512, is that we still need to check (at
runtime) whether the OS supports XGETBV and whether the ZMM registers are
fully enabled. We might be able to skip those checks in limited cases
(e.g., you are building on the target machine and can perhaps just check it
once at build time), but that probably won't help packagers.

>> +/*
>> + * pg_attribute_target allows specifying different target options that the
>> + * function should be compiled with (e.g., for using special CPU instructions).
>> + */
>> +#if __has_attribute (target)
>> +#define pg_attribute_target(...) __attribute__((target(__VA_ARGS__)))
>> +#else
>> +#define pg_attribute_target(...)
>> +#endif
>
> Think it'd be good to mention that there still needs to be configure check to
> verify that specific target attribute is understood by the compiler.

Will do.

--
nathan