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.
[0] https://postgr.es/m/20240330032209.GA2018686%40nathanxps13
--
nathan
