SIMD vs no SIMD on i686 (was: [Bug 3975] executable stack flag prevents use of libx264.so.148 with selinux in enforcing mode)

[Nicolas Chauvet]

2016-07-19 12:37 GMT+02:00 Dominik 'Rathann' Mierzejewski
<dominik at greysector.net>:
> On Tuesday, 19 July 2016 at 10:31, Simon Farnsworth wrote:
>>
>> > On 19 Jul 2016, at 02:12, Sérgio Basto <sergio at serjux.com> wrote:
>> >
>> > On Ter, 2016-07-19 at 02:55 +0200, dominik at greysector.net wrote:
>> >> Hello, Sérgio.
>> >>
>> >> On Tuesday, 19 July 2016 at 02:35, RPM Fusion Bugzilla wrote:
>> >>>
>> >>> https://bugzilla.rpmfusion.org/show_bug.cgi?id=3975
>> >>>
>> >>> --- Comment #4 from Sérgio Basto <sergio at serjux.com> 2016-07-19
>> >>> 02:35:19 CEST ---
>> >>> (In reply to comment #3)
>> >>>>
>> >>>> I'll take a look at this. Could you try running execstack -c on
>> >>>> the installed
>> >>>> library in the meantime?
>> >>> Be my guest , in  meantime I read your thread on packaging Mailing
>> >>> list about
>> >>> sse3 , I'd like understand if we need 2 builds for i686 ... one
>> >>> with sse2 other
>> >>> without it, can you give us your opinion ?
>> >> The only real concern here are applications linked against libx264,
>> >> which someone might want to run on low-end hardware, because I don't
>> >> think anyone would want to encode anything to H.264 on non-SSE2
>> >> capable
>> >> CPU (i.e. Pentium 3 or Athlon XP and older). Considering last non-
>> >> SSE2
>> >> CPUs went out of production about 8 years ago, I think it's fairly
>> >> safe
>> >> to assume that the impact of doing SSE2-only builds would be
>> >> negligible,
>> >> if any.
>> >
>> > From x264.spec we can read "#i686 is disabled on purpose - re-enabled
>> > with sse2 build"
>> >
>> > It is safe to say enable sse2 build is enable asm code ? if yes %ifarch
>> > i686 we can/should build with enable asm code ?  and only one time
>> > isn't it ? I tried that, but kwizart didn't agree ...
>>
>> It may not be worth the effort (as there's upstream effort here, not just
>> packaging, if I understand correctly), but https://lwn.net/Articles/691932/
>> documents GCC 6 function multi-versioning, which enables you to build both
>> SSE2 and non-SSE versions of a given functions, and have GCC generate code
>> to choose the right version at run time - as it's part of the compiler, the
>> optimiser should also be aware, and able to avoid dispatch code when calling
>> from the SSE2 version to another SSE2 function.
>
> It's definitely not worth the effort. x264 upstream considers non-SSE2
> x86 CPUs obsolete.
That can still be used for AVX and SSSE3, etc. (neon)
As soon as the principle for runtime detection lands upstream, adding
SSE2 shouldn't be a big effort.



-- 
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Nicolas (kwizart)