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

[Simon Farnsworth]

> 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.

-- 
Simon Farnsworth