On Sun, Nov 1, 2015 at 6:12 PM, Ronald S. Bultje <rsbul...@gmail.com> wrote: > Hi, > > On Sun, Nov 1, 2015 at 1:03 PM, Ganesh Ajjanagadde <gajjanaga...@gmail.com> > wrote: >> >> Floating point to integer conversion is well defined when the float lies >> within >> the representation bounds of the integer after discarding the fractional >> part. >> However, in other cases, unfortunately the standard leaves it undefined. >> In particular, assuming that it saturates in a sane way is a dangerous >> assumption. >> >> In light of recent events, I would not have bothered if this was a merely >> theoretical >> issue, and that common environments saturate correctly. Sadly, x86 (for >> example) >> converts casts to a cvttsd2si instruction which saturates numbers > >> INT64_MAX to >> INT64_MIN. This is mathematically completely bogus. >> http://blog.frama-c.com/index.php?post/2013/10/09/Overflow-float-integer >> gives >> a nice overview of the issue. >> >> 1/2 adds an av_clipd64 API for this purpose to clip a float to an integral >> range. >> Obviously it will be slower than a cvttsd2si, but there is no option if >> one wants >> safe and well-defined behavior. Of course, if one knows a priori that a >> float >> lives in the integral type's range, then there is no issue. Safe speedups >> are >> entirely possible, but API should be finalized first IMHO. >> Most common anticipated usages are clipping to [INT64_MIN, INT64_MAX] or >> [INT_MIN, INT_MAX]. >> I have given some thought as to whether a separate av_clipd32 API (for >> example) >> is necessary. It seems to me to not be the case, since an IEEE-754 double >> is >> guaranteed to represent exactly integers up to ~ 2^53. >> Similarly, av_clipf64 and the like also seem unnecessary, since a double >> is guaranteed >> to represent all the values a float does. Such an API may be useful for >> performance >> though; I do not know how/what float to double conversions entail and at >> the >> moment ignore such complications. >> >> 1/2 also accordingly documents av_clipd64. > > > So, is this a bug in llrint, or is this a failure to use llrint, or is this > different from llrint? It sounds to me that llrint should be used, not our > own alternative.
Not a bug, just a standard "undefined behavior" cop-out from the standards committee. We need to roll our own: all standard functions cop out when it does not fit in the integer range, and Intel (and others) have wonderfully exploited the cop-out: see e.g the link I gave and my other reply. > > Ronald _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
