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 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.