Re: 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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