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

[Roland Scheidegger]

Am 31.12.2015 um 10:30 schrieb 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 ?
If the generated code with all automatically picked up compile options
really doesn't run on power7 just because of this, I think it would be
nicer if this were an explicit enable.

Roland




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