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

On Tuesday, 19 July 2016 at 10:31, Simon Farnsworth wrote: > > > On 19 Jul 2016, at 02:12, Sérgio Basto <sergio(a)serjux.com&gt; wrote: > > > > On Ter, 2016-07-19 at 02:55 +0200, dominik(a)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(a)serjux.com&gt; 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.