Re: freeze for sbcl-2.6.8

Well, avx2 is older than xsavec, so can't use it to enable avx2 support.

On Sun, Aug 23, 2026 at 12:42 AM arthur miller <[email protected]> wrote:
>
> Of course it is not, and that wasn't what I said either. I just asked a simple question,
> so I know if I should spend time on something or not. Xsavec has been out for 10
> years, so I am sure you and other people here are aware of it.
>
> Just asking if it is not in SBCL because nobody had time or interest
> to add it or if there is some technical reason why it is not in use, that is all. Sorry, if I
> bother with a regression, was just a curious question that popped on my mind.
> ________________________________
> Från: Stas Boukarev <[email protected]>
> Skickat: den 22 augusti 2026 22:41
> Till: arthur miller <[email protected]>
> Kopia: [email protected] <[email protected]>
> Ämne: Re: [Sbcl-devel] freeze for sbcl-2.6.8
>
> xsavec is not synonymous to avx2.
>
>
>
> On Sat, Aug 22, 2026 at 11:35 PM arthur miller <[email protected]> wrote:
> >
> > Yeah of course, I understand; the question was out of context and not framed well.
> >
> > If I frame it like this: is there a technical reason to not use xsavec/xsaves/xrestors
> > in SBCL instead of xstore/xrstor? Or is it not in there just because it is not done yet.
> >
> > Not for this particular case, it is a regression, but wonder if it is something for a future
> > patch or not? Should have ask back whan I was working on zmm support, but than it
> > was a bit too much other stuff.
> > ________________________________
> > Från: Stas Boukarev <[email protected]>
> > Skickat: den 22 augusti 2026 22:19
> > Till: arthur miller <[email protected]>
> > Kopia: Douglas Katzman <[email protected]>; Christophe Rhodes <[email protected]>; [email protected] <[email protected]>
> > Ämne: Re: [Sbcl-devel] freeze for sbcl-2.6.8
> >
> > xsave (for avx512) isn't invoked unless avx512 registers are used
> > within a function. So, xsavec isn't going to improve anything.
> >
> > On Sat, Aug 22, 2026 at 11:02 PM arthur miller <[email protected]> wrote:
> > >
> > > Forgive me for chimming in, but a question, since you are already
> > > talking about xstore/xrstor and want more performance. Can we switch to
> > > "compact" versions of those (xsavec & co)? These, potentially
> > > write/restore less data, in cases where saving/restoring xmm/ymm/zmm is
> > > enabled.
> > >
> > > I learned about it when I was patching for zmm. I didn't dare to try
> > > to change that, but instead patched in support to what was already in.
> > >
> > > As I understand it, when full store is needed (zmm registers in use),
> > > than the performance is roughly the same, but if zmm/ymm/xmm regs do not
> > > need to be saved becuase they weren't written since the last clear, they
> > > won't be saved either.
> > >
> > > Compacted stores/restores are supported on Linux since kernel 4.4, year
> > > 2016, and on Windows since version 10 some release number in 2015.
> > > ________________________________
> > > Från: Douglas Katzman via Sbcl-devel <[email protected]>
> > > Skickat: den 22 augusti 2026 18:07
> > > Till: Stas Boukarev <[email protected]>
> > > Kopia: Christophe Rhodes <[email protected]>; [email protected] <[email protected]>
> > > Ämne: Re: [Sbcl-devel] freeze for sbcl-2.6.8
> > >
> > > I have the fix for that crash.   I mistakenly thought that FPR-SAVE/FPR-RESTORE no longer need to preserve RDX when using XRSTOR but they still do - but only for #+win32, so I'll add that.
> > > However, Gemini claims the following theoretical performance gain by simply avoiding XSAVE/XRSTOR entirely in the YMM case (not the avx512 of course) which is a very interesting point -
> > >
> > > ~4x Faster: 16 * VMOVAPS runs in ~15–20 cycles vs 80+ cycles for XSAVE/XRSTOR.
> > >
> > > Do you think I can commit this register changing patch + its fix for now, and we'll leave the above suggestion for next release (despite also being a valid fix for my crash) ?


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