Re: Popcount optimization using AVX512

Hi,

On 2024-07-30 21:01:31 -0500, Nathan Bossart wrote:
> On Tue, Jul 30, 2024 at 06:46:51PM -0700, Andres Freund wrote:
> > On 2024-07-30 20:20:34 -0500, Nathan Bossart wrote:
> >> On Tue, Jul 30, 2024 at 05:49:59PM -0700, Andres Freund wrote:
> >> > Why are we actually checking for xsave? We're not using xsave itself and I
> >> > couldn't find a comment in 792752af4eb5 explaining what we're using it as a
> >> > proxy for? Is that just to know if _xgetbv() exists? Is it actually possible
> >> > that xsave isn't available when avx512 is?
> >>
> >> Yes, it's to verify we have XGETBV, which IIUC requires support from both
> >> the processor and the OS (see 598e011 and upthread discussion). AFAIK the
> >> way we are detecting AVX-512 support is quite literally by-the-book unless
> >> I've gotten something wrong.
> >
> > I'm basically wondering whether we need to check for compiler (not OS support)
> > support for xsave if we also check for -mavx512vpopcntdq -mavx512bw
> > support. Afaict the latter implies support for xsave.
>
> The main purpose of the XSAVE compiler check is to determine whether we
> need to add -mxsave in order to use _xgetbv() [0]. If that wasn't a
> factor, we could probably skip it. Earlier versions of the patch used
> inline assembly in the non-MSVC path to call XGETBV, which I was trying to
> avoid.

My point is that _xgetbv() is made available by -mavx512vpopcntdq -mavx512bw
alone, without needing -mxsave:

echo -e '#include <immintrin.h>\nint main() { return _xgetbv(0) & 0xe0; }'|time gcc -march=x86-64 -c -xc - -o /dev/null
-> fails

echo -e '#include <immintrin.h>\nint main() { return _xgetbv(0) & 0xe0;}'|time gcc -march=x86-64 -mavx512vpopcntdq -mavx512bw -c -xc - -o /dev/null
-> succeeds

Greetings,

Andres Freund