On Mon, Oct 05, 2015 at 12:38:15AM +0200, Alan McKinnon wrote: > On 04/10/2015 22:49, James Cloos wrote: > > > > And is there really any value from micromanaging things like that in an > > ebuild? > > For ffmpeg? Yes.
Actually, I think modern versions of ffmpeg probably do not benefit as much anymore. The `cpudetection' USE flag makes me think ffmpeg uses the CPUID instruction to determine which x86 extensions are available, and loads the appropriate code. This would enable Ubuntu et. al. to build ffmpeg with all the high performance code compiled, and still be compatible with all CPUs. I do not know if that is 100% true, but having a `cpudetection' USE flag makes me assume that's how it works. More on this here, which is the top of list when I searched "ffmpeg cpu detection" [1]. > I imagine users who rely on will be mighty upset if the > ebuild did not offer those flags. By way of example, many folks here > have reported their own experience over the years that a given cpu with > a standard Ubuntu i586 or so install struggles to keep up with a video. > The same hardware running the same software on Gentoo with sane options > and USE easily manages. That's the difference between managing and not > managing such things in areas where it matters. For plenty of packages, such as fftw, this is a huge benefit. fftw does not have a `cpudetection' USE flag, so I would assume that sse, avx, etc. must be determined at build time and can't be changed at runtime. Alec 1. http://ffmpeg.org/pipermail/ffmpeg-devel/2012-September/131550.html
