Re: Popcount optimization using AVX512

On Tue, Mar 19, 2024 at 10:27:58AM +1300, David Rowley wrote:
> On Tue, 19 Mar 2024 at 10:08, Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>> On Tue, Mar 19, 2024 at 10:02:18AM +1300, David Rowley wrote:
>> > The only thing I'd question in the patch is in pg_popcount_fast(). It
>> > looks like you've opted to not do the 32-bit processing on 32-bit
>> > machines. I think that's likely still worth coding in a similar way to
>> > how pg_popcount_slow() works. i.e. use "#if SIZEOF_VOID_P >= 8".
>> > Probably one day we'll remove that code, but it seems strange to have
>> > pg_popcount_slow() do it and not pg_popcount_fast().
>>
>> The only reason I left it out was because I couldn't convince myself that
>> it wasn't dead code, given we assume that popcntq is available in
>> pg_popcount64_fast() today. But I don't see any harm in adding that just
>> in case.
>
> It's probably more of a case of using native instructions rather than
> ones that might be implemented only via microcode. For the record, I
> don't know if that would be the case for popcntq on x86 32-bit and I
> don't have the hardware to test it. It just seems less risky just to
> do it.

Agreed. Will send an updated patch shortly.

--
Nathan Bossart
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