Re: Popcount optimization using AVX512

Hi,

On 2024-02-12 20:14:06 +0000, Amonson, Paul D wrote:
> > > +# Check for header immintrin.h
> > ...
> > Do these all actually have to link? Invoking the linker is slow.
> > I think you might be able to just use cc.has_header_symbol().
>
> I took this to mean the last of the 3 new blocks.

Yep.

> > Does this work with msvc?
>
> I think it will work but I have no way to validate it. I propose we remove the AVX-512 popcount feature from MSVC builds. Sound ok?

CI [1], whould be able to test at least building. Including via cfbot,
automatically run for each commitfest entry - you can see prior runs at
[2]. They run on Zen 3 epyc instances, so unfortunately runtime won't be
tested. If you look at [3], you can see that currently it doesn't seem to be
considered supported at configure time:

...
[00:23:48.480] Checking if "__get_cpuid" : links: NO
[00:23:48.480] Checking if "__cpuid" : links: YES
...
[00:23:48.492] Checking if "x86_64: popcntq instruction" compiles: NO
...

Unfortunately CI currently is configured to not upload the build logs if the
build succeeds, so we don't have enough details to see why.

> > This will build all of pgport with the avx flags, which wouldn't be correct, I think? The compiler might inject automatic uses of avx512 in places, which would cause problems, no?
>
> This will take me some time to learn how to do this in meson. Any pointers
> here would be helpful.

Should be fairly simple, add it to the replace_funcs_pos and add the relevant
cflags to pgport_cflags, similar to how it's done for crc.

> > While you don't do the same for make, isn't even just using the avx512 for all of pg_bitutils.c broken for exactly that reson? That's why the existing code builds the files for various crc variants as their own file.
>
> I don't think its broken, nothing else in pg_bitutils.c will make use of
> AVX-512

You can't really guarantee that compiler auto-vectorization won't decide to do
so, no? I wouldn't call it likely, but it's also hard to be sure it won't
happen at some point.

> If splitting still makes sense, I propose splitting into 3 files: pg_bitutils.c (entry point +sw popcnt implementation), pg_popcnt_choose.c (CPUID and xgetbv check) and pg_popcnt_x86_64_accel.c (64/512bit x86 implementations).
> I'm not an expert in meson, but splitting might add complexity to meson.build.
>
> Could you elaborate if there are other benefits to the split file approach?

It won't lead to SIGILLs ;)

Greetings,

Andres Freund

[1] https://github.com/postgres/postgres/blob/master/src/tools/ci/README
[2] https://cirrus-ci.com/github/postgresql-cfbot/postgresql/commitfest%2F47%2F4675
[3] https://cirrus-ci.com/task/5645112189911040