[FFmpeg-devel] [PATCH] [libavutil] Add saturated add/sub operations for int64_t.

[Dale Curtis]

On Fri, May 1, 2020 at 12:53 PM Michael Niedermayer <michael at niedermayer.cc>
wrote:

> On Thu, Apr 30, 2020 at 05:39:43PM -0700, Dale Curtis wrote:
> > On Thu, Apr 30, 2020 at 5:21 PM James Almer <jamrial at gmail.com> wrote:
> >
> > > On 4/30/2020 7:19 PM, Dale Curtis wrote:
> > > > Many places are using their own custom code for handling overflow
> > > > around timestamps or other int64_t values. There are enough of these
> > > > now that having some common saturated math functions seems sound.
> > > >
> > > > This adds implementations that just use the builtin functions for
> > > > recent gcc, clang when available or implements its own version for
> > > > older compilers.
> > >
> > > These look like 64 bit versions of av_sat_add32 and av_sat_sub32, from
> > > common.h, so you should probably add them there and rename them
> > > accordingly.
> > >
> > >
> > Ah! I was looking for those, but missed them. Thanks. Done.
>
> one disadvantage the av_sat* functions have is the lack of inexact
> detection
>
> In addition to av_sat*
> In situations where its better to fail than to clip, something that
> emulates what (+-Inf/)NaN is for float may make sense.
> That would allow to simply check after a computation if any inexactness
> occured
>
> Such a thing could be usefull in situations where a exact value or an
> error is wanted.
>
>
The __builtin functions provide exactly this API, we're just hiding it. I
could add something like:
int did_overflow = av_checked_sat_(add|sub)64(int64_t a, int64_t b,
int64_t* result)

|result| would still satuate and thus av_sat_(add|sub)64 could use it
without checking the return value, but those which want to check and abort
could do so. This is similar to the API shape we expose in Chromium modulo
the fact we enforce an assert.

- dale