On Fri, 1 May 2015 05:09:51 +0000 (UTC) Martin Vaeth wrote:
> Andrew Savchenko <birc...@gentoo.org> wrote:
> >
> > That's why kernel makes sure that no floating point instructions
> > sneaks in using CFLAGS, you may see a lot of -mno-${intrucion_set}
> > flags when running make -V.
>
> So it should be sufficient that the kernel does not use "float"
> or "double", shouldn't it?No. Optimizer paths may be very unobvious, i.e. I'll not be surprised if under some conditions vectorizer may use float instructions for int code. > I can hardly imagine that otherwise the compiler converts integer > or pointer arithmetic into floating point arithmetics, or is > this really the case for certain flags? If yes, why should these > flags *ever* be useful? > I mean: The context switching happens for non-kernel code as well, > doesn't it? Yes, context switching happens for all code and have its costs. But for userspace code context switching happens for many other reasons, e.g. on each syscall (userspace <-> kernelspace switching). Also some user applications may need high precision or context switching pays off due to mass parallel data processing, e.g. SIMD instructions in scientific or multimedia applications. But unless special conditions mentioned above, fixed point is still faster in userspace, some ffmpeg codecs have both fixed and floating point implementations, you may compare them. Programming in fixed point is much harder, so most people avoid it unless they have a very goode reason to use it. And dont't forget that kernel is performance critical unlike most of userspace applications. Best regards, Andrew Savchenko
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