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

[Oded Gabbay]

On Wed, Dec 30, 2015 at 5:41 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 30.12.2015 um 10:46 schrieb 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.
>
> Note I don't have anything against a build time check. My concern here
> is something along the lines of unsuspecting distros shipping binaries
> which won't work, as it looks to me like this will get picked up
> automatically. That is different to how for instance sse41 is handled.
> That is I believe this should only get enabled if someone has specified
> some -mcpu=power8 or whatever flag explicitly somewhere already.
>
> Roland

I understand and I share your concern. Maybe we should add
"--disable-pwr8-inst" to mesa's configure ? if that flag is given to
configure, it would disable the optimization code (won't add
_ARCH_PWR8 to defines and won't add -mpower8-vector to gcc flags).

What do you think ?

Oded

>
>>
>> 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.
>>>>
>>>
>