Re: Popcount optimization using AVX512 - Mailing list pgsql-hackers

From Nathan Bossart
Subject Re: Popcount optimization using AVX512
Date
Msg-id 20240402184021.GA2802541@nathanxps13
Whole thread Raw
In response to Re: Popcount optimization using AVX512  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: Popcount optimization using AVX512
List pgsql-hackers
On Tue, Apr 02, 2024 at 01:43:48PM -0400, Tom Lane wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
>> On 2024-Apr-02, Nathan Bossart wrote:
>>> Another idea I had is to turn pg_popcount() into a macro that just uses the
>>> pg_number_of_ones array when called for few bytes:
>>> 
>>>     static inline uint64
>>>     pg_popcount_inline(const char *buf, int bytes)
>>>     {
>>>         uint64        popcnt = 0;
>>> 
>>>         while (bytes--)
>>>             popcnt += pg_number_of_ones[(unsigned char) *buf++];
>>> 
>>>         return popcnt;
>>>     }
>>> 
>>>     #define pg_popcount(buf, bytes) \
>>>         ((bytes < 64) ? \
>>>          pg_popcount_inline(buf, bytes) : \
>>>          pg_popcount_optimized(buf, bytes))
>>> 
>>> But again, I'm not sure this is really worth it for the current use-cases.
> 
>> Eh, that seems simple enough, and then you can forget about that case.
> 
> I don't like the double evaluation of the macro argument.  Seems like
> you could get the same results more safely with
> 
>     static inline uint64
>     pg_popcount(const char *buf, int bytes)
>     {
>         if (bytes < 64)
>         {
>             uint64        popcnt = 0;
> 
>             while (bytes--)
>                 popcnt += pg_number_of_ones[(unsigned char) *buf++];
> 
>             return popcnt;
>         }
>         return pg_popcount_optimized(buf, bytes);
>     }

Yeah, I like that better.  I'll do some testing to see what the threshold
really should be before posting an actual patch.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com



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