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
