[Mesa-dev] [PATCH] mesa: Fix build to properly check for supported compiler flags

[Lauri Kasanen]

On Wed, 18 Dec 2013 17:57:16 -0800
Matt Turner <mattst88 at gmail.com> wrote:

> On Tue, Dec 17, 2013 at 7:15 AM, Lauri Kasanen <cand at gmx.com> wrote:
> > Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=72708
> 
> Make this "Bugzilla: ..."

Sure, will do. But there are both conventions used, for example see:
0b16042377a
8bc7673ef87

> > It seems to me that the Intel code that uses this SSE4.1 function
> > is still buggy, as it has no runtime check - would it not crash
> > if built on a SSE4-capable system but used with a lower-class cpu?
> 
> You haven't thought this through. :)
> 
> The SSE 4.1 code is in i965 #ifdef __SSE4_1__. __SSE4_1__ is defined
> if the code is compiled with the appropriate CFLAGS, e.g., -msse4.1 or
> -march=native and your CPU support SSE 4.1.
> 
> That is, your Mesa would have to be compiled with flags incompatible
> with your CPU for this to break, and in that case gcc would already
> have used instructions your CPU doesn't have.

Yes, you're right that there is no crash risk on a normal build.
However, it is practically useless for distro builds like that, as they
can't ship a normal build and get the speedup at the same time.

Gallium has a cpu detect function that includes a SSE 4.1 cap. Perhaps
you'd like to move it to a common area and make use of it for runtime
detection?

> > diff --git a/src/mesa/Makefile.am b/src/mesa/Makefile.am
> > index a60600e..8843836 100644
> > --- a/src/mesa/Makefile.am
> > +++ b/src/mesa/Makefile.am
> > @@ -103,7 +103,11 @@ noinst_PROGRAMS = gen_matypes
> >  gen_matypes_SOURCES = x86/gen_matypes.c
> >  BUILT_SOURCES += matypes.h
> >
> > -ARCH_LIBS = libmesa_sse41.la
> > +ARCH_LIBS =
> > +
> > +if SSE41_SUPPORTED
> > +ARCH_LIBS += libmesa_sse41.la
> > +endif
> 
> It's safe to just assign ARCH_LIBS once and not do the empty assignment.

Yes, but I did it this way to pave way for future additions. Having it
like this makes it easy to add specialized functions for other
instruction sets later.

- Lauri