Re: Popcount optimization using AVX512

On Tue, Apr 02, 2024 at 01:43:48PM -0400, Tom Lane wrote:
> Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)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