Re: [Caml-list] Closing the performance gap to C

[Gerd Stolpmann <info@gerd-stolpmann.de>]

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Michael,

look here, it's the "definitive source":
https://github.com/ocaml/ocaml/blob/trunk/asmcomp/amd64/proc.ml

Windows is in deed different. I don't have enough insight into the
register spilling algorithm to say whether this has a significant
effect, though. OCaml code never keeps registers alive, and thus I
would bet the spilling algorithm is designed for that, and it is
probably not tried to move values preferably to xmm6-15 in order to
avoid spilling during C calls. But that's just a hypothesis... Does
somebody know?

Gerd


Am Montag, den 19.12.2016, 14:52 +0000 schrieb Soegtrop, Michael:
> Dear Gerd,
> 
> > 
> > When you call a C function like cos it is likely that this
> > happens because the C calling conventions do not preserve the FP
> > registers
> > (xmm*). This could be improved if the OCaml compiler tried
> > alternate places for
> > temporarily storing FP values:
> For Windows this doesn't seem to be true. See e.g.:
> 
> https://msdn.microsoft.com/en-us/library/ms235286.aspx
> 
> which states that XMM0..XMM5 are volatile, while XMM6..XMM15 must be
> preserved.
> 
> I think for Linix you are right. I couldn't find a better reference
> than Wikipedia:
> 
> https://en.wikipedia.org/wiki/X86_calling_conventions
> 
> see "System V AMD64 ABI" there.
>  
> This reference contains a good overview, which matches the above data
> in table 4:
> 
> http://www.agner.org/optimize/calling_conventions.pdf
> 
> So on Windows, there is for sure no need to save XMM6..XMM15, while
> on Linux this seems to be an issue.
> 
> Best regards,
> 
> Michael
> Intel Deutschland GmbH
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-- 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
My OCaml site:          http://www.camlcity.org
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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