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. Ronald _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
