[FFmpeg-devel] [PATCH 1/4] avcodec/startcode: Use AV_RN due to UBSan warning about unaligned access

[Andreas Rheinhardt]

On Wed, Jan 22, 2020 at 4:23 PM Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:

> Am Mi., 22. Jan. 2020 um 16:21 Uhr schrieb Andreas Rheinhardt
> <andreas.rheinhardt at gmail.com>:
> >
> > On Wed, Jan 22, 2020 at 4:06 PM Carl Eugen Hoyos <ceffmpeg at gmail.com>
> wrote:
> >
> > > Am Mi., 22. Jan. 2020 um 15:57 Uhr schrieb Andreas Rheinhardt
> > > <andreas.rheinhardt at gmail.com>:
> > > >
> > > > when directly accessing a buffer via a pointer to uint8_t cast to a
> > > > pointer to uint64_t/uint32_t. This happened only if
> HAVE_FAST_UNALIGNED
> > > > was set, so it was ok, but UBSan nevertheless complained about
> unaligned
> > > > accesses. So simply use AV_RNxx to read the buffer; this also
> improves
> > > > readability.
> > >
> > > Is there a speed impact?
> > >
> > The assembly produced by both Clang as well as GCC was completely
> > unchanged (for non-UBSan builds).
>
> (on x86?)
>
> Yes: x86-64.

- Andreas