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

From David Rowley
Subject Re: Popcount optimization using AVX512
Date
Msg-id CAApHDvojPyh6dLKooqjXSZE=0Ed590Lq1BxF7WQ9knSggyuJEA@mail.gmail.com
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In response to Re: Popcount optimization using AVX512  (Nathan Bossart <nathandbossart@gmail.com>)
Responses Re: Popcount optimization using AVX512
List pgsql-hackers
On Tue, 19 Mar 2024 at 10:08, Nathan Bossart <nathandbossart@gmail.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.

David



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