[Mesa-dev] [PATCH 1/4] meson: Enable SSE4.1 optimizations

[Dylan Baker]

Quoting Matt Turner (2017-11-17 12:46:03)
> On Fri, Nov 17, 2017 at 12:34 PM, Dylan Baker <dylan at pnwbakers.com> wrote:
> > Quoting Emil Velikov (2017-11-17 03:11:50)
> >> On 16 November 2017 at 22:21, Dylan Baker <dylan at pnwbakers.com> wrote:
> >> > Quoting Emil Velikov (2017-11-16 03:35:17)
> >> >> Hi Dylan,
> >> >>
> >> >> On 16 November 2017 at 01:10, Dylan Baker <dylan at pnwbakers.com> wrote:
> >> >> > This patch checks for an and then enables sse4.1 optimizations if the
> >> >> > host machine will be x86/x86_64.
> >> >> >
> >> >> Hell yeah, SSE is coming to town :-)
> >> >>
> >> >> Will this work if the user disables SSE4.1, say via CFLAGS=-mno-sse4.1
> >> >> meson ...?
> >> >> My meson is still bit rough, so I could not quite grok ^^ by reading
> >> >> through the patch.
> >> >>
> >> >> Thanks
> >> >> Emil
> >> >
> >> > It'll explode horribly. Id didn't see any special handling of that in autotools
> >> > build though either, did I miss something?
> >> >
> >> In autotools it's handled before the normal ld invocation.
> >>
> >> Namely: configure.ac does:
> >>  - construct a program using sse4.1 intrinsicts
> >> Note: return _mm_...() is required otherwise the whole program will be
> >> optimised away
> >>  - the -msse is passed first and then the user flags (-mno-sse and/or
> >> anything else)
> >>  - the user -mno-sse takes precedence, hence the test program fails to build
> >>  - set see_supported=false and don't build the SSE optimised static library
> >>
> >> HTH
> >> Emil
> >
> > That's an interesting question. So arguments passed via CFLAGS and friends will
> > be passed to tests, but the arguments passed explicitly to those tests are
> > appended, so -msse4.1 will take precedence. I'm also pretty sure there isn't a
> > way to check the arguments passed via -Dc_args or CFLAGS (they're treated as
> > default arguments, like the c_std in the project() argument). I asked on
> > #mesonbuild, but I haven't gotten an answer yet (Fridays are pretty slow
> > everywhere).
> >
> > I think currently the only way to control this would be to have a meson option
> > to turn off optimizations, and I really don't like that.
> 
> The original bug report of "Mesa doesn't build with -mno-sse4.1" was
> in Gentoo's bugzilla; https://bugs.gentoo.org/503828
> 
> There's no compelling reason to support that configuration because
> since the -msseX flags are off by default... in order for the
> -mno-sseX flag to do anything the user must be enabling them somehow
> (likely via -march=...). Using -march=... only to disable particular
> instruction sets seems pretty idiotic.
> 
> I can push back harder on such stupidity in the future if we can't
> handle this case, but it would be nice to solve so I never have to
> hear about it again.

Hmmm, yeah. I'd also like to be able to provide an error in that case too,
because I agree it's pretty idiotic. I'll bring it up in the meson bug tracker.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: signature
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20171117/3937684f/attachment.sig>