[fpc-devel] The 15k bounty: Optimizing executable speed for Linux x86 / LLVM

Karoly Balogh (Charlie/SGR) charlie at scenergy.dfmk.hu
Thu Oct 25 20:08:49 CEST 2018


On Thu, 25 Oct 2018, Florian Klaempfl wrote:

> >> That is good news.  The contours of a TODO list are becoming visible :)
> >>
> >> But we may need also need a solution for other platforms, which means the
> >> current system should remain in place for those platforms where such a
> >> system is not present ?
> >
> > FPC already has some code to support section threadvars via the GS segment
> > on i386 at least, but it doesn't seem to be enabled by default? (Couldn't
> > test it, but the tf_section_threadvars target flag, which enable this is
> > actually behind a define in i_linux.pas, which I couldn't find enabled
> > anywhere?). Also tf_section_threadvars flag has some code to support it
> > all over the compiler, including the x86 cg. I have some really vague
> > memories I actually enabled it in some experimental local version I had,
> > and it worked on first sight at least, but I could be completely off here.
> >
> > I wonder why it was never enabled by default.
> The %gs based approach works only for object files linked statically to
> the executable. In general there are four TLS access models on linux and
> at least three of them need to be supported, if one wants to support
> dyn. libraries in a usefull manner. Of course, this comes with the
> requirement to over means to control the used model. The tls.pdf by U.
> Drepper decribes it very well.

Ah, right. It's been a while. Ironically, it would have been enough for
the actual use case at hand, when I fiddled with it.


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