[Mesa-dev] [PATCH 1/5] configure.ac: Detect if running on POWER8 arch

[Oded Gabbay]

On Wed, Dec 30, 2015 at 1:11 AM, Roland Scheidegger <sroland at vmware.com> wrote:
>
> So, if I see that right, you will automatically generate binaries using
> power8 instructions if compiled on power8 capable box, which then won't
> run on boxes not supporting power8? Is that really what you want?
> Maybe some runtime detection would be a good idea (though I don't know
> if anyone cares about power7)?

The problem is I don't think I can eliminate the build time check
(although I would very much like to) because I need:
1. To pass a special flag to the GCC compiler: -mpower8-vector
2. To define _ARCH_PWR8 so GCC will include the newer intrinsic

Without those two things, I won't be able to use vec_vgbbd which I
need to implement the _mm_movemask_epi8 efficiently, and without that,
all this patch series can be thrown out the window. The emulation of
_mm_movemask_epi8 using regular instructions is just horrible.

You are correct that once you build a binary with this flag on power8
machine, that binary won't run on power7 machine. You get "cannot
execute binary file"

Unfortunately, I don't see a way around this because even if I
condition the use of vec_vgbbd on a runtime check/define, the library
still won't be executable because it was built with -mpower8-vector.

Having said that, because I *assume* IBM right now mostly cares about
Linux running on POWER8 with little-endian, I think it is a fair
compromise.

Oded

> So far we didn't bother with that for SSE
> but it has to be said SSE2 is a really low bar (and the manual assembly
> stuff doesn't use anything more advanced, even though clearly things
> like the emulated mm_mullo_epi32 are suboptimal if your cpu supports
> sse41). And even then on non-x86 you actually might not get
> PIPE_ARCH_SSE if you didn't set gcc's compile flags accordingly.
>
> Roland
>
>
> Am 29.12.2015 um 17:12 schrieb Oded Gabbay:
> > To determine if we could use special POWER8 assembly directives, we first
> > need to detect whether we are running on POWER8 architecture. This patch
> > adds this detection to configure.ac and adds the necessary compilation
> > flags accordingly.
> >
> > Signed-off-by: Oded Gabbay <oded.gabbay at gmail.com>
> > ---
> >  configure.ac | 30 ++++++++++++++++++++++++++++++
> >  1 file changed, 30 insertions(+)
> >
> > diff --git a/configure.ac b/configure.ac
> > index f8a70be..1acd47e 100644
> > --- a/configure.ac
> > +++ b/configure.ac
> > @@ -396,6 +396,36 @@ fi
> >  AM_CONDITIONAL([SSE41_SUPPORTED], [test x$SSE41_SUPPORTED = x1])
> >  AC_SUBST([SSE41_CFLAGS], $SSE41_CFLAGS)
> >
> > +dnl Check for POWER8 Architecture
> > +PWR8_CFLAGS="-mpower8-vector"
> > +have_pwr8_intrinsics=no
> > +AC_MSG_CHECKING(whether we are running on POWER8 Architecture)
> > +save_CFLAGS=$CFLAGS
> > +CFLAGS="$PWR8_CFLAGS $CFLAGS"
> > +AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
> > +#if defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8))
> > +#error "Need GCC >= 4.8 for sane POWER8 support"
> > +#endif
> > +#include <altivec.h>
> > +int main () {
> > +    vector unsigned char r;
> > +    vector unsigned int v = vec_splat_u32 (1);
> > +    r = __builtin_vec_vgbbd ((vector unsigned char) v);
> > +    return 0;
> > +}]])], have_pwr8_intrinsics=yes)
> > +CFLAGS=$save_CFLAGS
> > +
> > +if test $have_pwr8_intrinsics = yes ; then
> > +   DEFINES="$DEFINES -D_ARCH_PWR8"
> > +   CXXFLAGS="$CXXFLAGS $PWR8_CFLAGS"
> > +   CFLAGS="$CFLAGS $PWR8_CFLAGS"
> > +else
> > +   PWR8_CFLAGS=
> > +fi
> > +
> > +AC_MSG_RESULT($have_pwr8_intrinsics)
> > +AC_SUBST([PWR8_CFLAGS], $PWR8_CFLAGS)
> > +
> >  dnl Can't have static and shared libraries, default to static if user
> >  dnl explicitly requested. If both disabled, set to static since shared
> >  dnl was explicitly requested.
> >
>