On Thu, Oct 15, 2015 at 9:10 AM, Ganesh Ajjanagadde <gajja...@mit.edu> wrote: > On Thu, Oct 15, 2015 at 8:31 AM, Clément Bœsch <u...@pkh.me> wrote: >> On Thu, Oct 15, 2015 at 08:18:02AM -0400, Ganesh Ajjanagadde wrote: >>> It has been demonstrated that using libc provided floating point >>> functions is beneficial, in the context of fabs() vs FFABS. >>> >>> Unfortunately, MSVC 2012 (and earlier) lack the ISO C99 fmax, fmaxf, >>> fmin, fminf functions. This patch adds them, thus making their usage in >>> FFmpeg safe. >>> >> >> Do we have any use of fmax/fmin? The functions don't exist to make >> FFMAX-like faster, it actually is more complex: >> >> These functions return the maximum of x and y. >> >> If one argument is a NaN, the other argument is returned. >> >> If both arguments are NaN, a NaN is returned. >> >> Which means it's likely slower but will do more. Not that I mind, but in this >> case, if we happen to use them, you will want to fix your local >> implementation >> to match this behaviour. > > 2 comments: > 1. There is a tradeoff: the gain from possible better optimization of >>=, versus the nan handling. > 2. NaN handling is not being done currently (by FFMAX and the like) - > so I don't know and can't comment whether we want it. You are right > that if we change to a fmax, we should make it consistent everywhere > on all platforms . Anyone else has comments on this?
If we don't care about NaN, Inf behavior, we should strongly consider enabling of (on GCC): -ffinite-math-only Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs. This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications. The default is -fno-finite-math-only. As stated above, -O3 does not automatically enable it. Thinking about it, if we actually care about NaN's/Infs etc I will drop this patch. If we don't, we can enable -ffinite-math-only, then I am pretty sure a good libc will no-op the isnan checks in fmin, fmax, etc. > >> >> [...] >> >> -- >> Clément B. >> >> _______________________________________________ >> ffmpeg-devel mailing list >> ffmpeg-devel@ffmpeg.org >> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel >> _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
