Re: Popcount optimization using AVX512

On Mon, Nov 06, 2023 at 09:53:15PM -0800, Noah Misch wrote:
> On Mon, Nov 06, 2023 at 09:59:26PM -0600, Nathan Bossart wrote:
>> On Mon, Nov 06, 2023 at 07:15:01PM -0800, Noah Misch wrote:
>> > The glibc/gcc "ifunc" mechanism was designed to solve this problem of choosing
>> > a function implementation based on the runtime CPU, without incurring function
>> > pointer overhead. I would not attempt to use AVX512 on non-glibc systems, and
>> > I would use ifunc to select the desired popcount implementation on glibc:
>> > https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/Function-Attributes.html
>>
>> Thanks, that seems promising for the function pointer cases. I'll plan on
>> trying to convert one of the existing ones to use it. BTW it looks like
>> LLVM has something similar [0].
>>
>> IIUC this unfortunately wouldn't help for cases where we wanted to keep
>> stuff inlined, such as is_valid_ascii() and the functions in pg_lfind.h,
>> unless we applied it to the calling functions, but that doesn't ѕound
>> particularly maintainable.
>
> Agreed, it doesn't solve inline cases. If the gains are big enough, we should
> move toward packages containing N CPU-specialized copies of the postgres
> binary, with bin/postgres just exec'ing the right one.

I performed a quick test with ifunc on my x86 machine that ordinarily uses
the runtime checks for the CRC32C code, and I actually see a consistent
3.5% regression for pg_waldump -z on 100M 65-byte records. I've attached
the patch used for testing.

The multiple-copies-of-the-postgres-binary idea seems interesting. That's
probably not something that could be enabled by default, but perhaps we
could add support for a build option.

--
Nathan Bossart
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