[FFmpeg-devel] GCC 5.1 warning: -Warray-bounds

[Ronald S. Bultje]

Hi,

On Tue, Jul 21, 2015 at 10:07 PM, Ganesh Ajjanagadde <gajjanag at mit.edu>
wrote:

> On Tue, Jul 21, 2015 at 5:31 PM, Ganesh Ajjanagadde <gajjanag at mit.edu>
> wrote:
> > On Tue, Jul 21, 2015 at 5:14 PM, Michael Niedermayer
> > <michael at niedermayer.cc> wrote:
> >> On Thu, Jun 25, 2015 at 01:25:08AM -0300, James Almer wrote:
> >>> On 04/06/15 6:55 PM, Ganesh Ajjanagadde wrote:
> >>> > I have created a small test case which gets at the heart of one of
> >>> > these spurious
> >>> > warnings, namely the one for libavfilter/vf_swapuv.c.
> >>> >
> >>> > Here is the ticket on the GCC Bugzilla:
> >>> >
> >>> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66422
> >>> >
> >>> > Note that as of the moment, -Warray-bounds appears quite broken on
> GCC
> >>> > (especially on -O3), and the bugzilla is full of bug reports on this.
> >>>
> >>> For the record, these bogus warnings have been fixed on the gcc 5
> branch.
> >>
> >> do any warnings remain for ffmpeg ?
> >> are they real issues or false positives as well ?
> >
> > Most are gone, only two files trigger these, namely
> > libavformat/dvenc.c and libavcodec/dca_x11.c.
> > I have attached a logfile from the build and will investigate this to
> > see whether they are real or false positives.
>
> So I checked the above, and it turns out both are false positives.
> However, in neither case was it trivial to see that access patterns
> are well defined,
> and both required analysis across the function boundary.
> Perhaps this is why GCC still struggles with this stuff.
> I will try creating a test case based on this and file a GCC ticket.
> By the way, both false positives can be easily silenced with one line
> changes,
> but of course we should not needlessly bend our code to satisfy the
> whims of GCC.


Well, that depends, right? So, the question is one of benefit vs. effort.
If all warnings are useless and gcc never provided us any new insights we
didn't already have (i.e. identify a real bug), it's probably not worth it.
However, if gcc turned out to help us find real bugs and the changes are
minor and clean, I see no problem silencing the noise a little. I believe
we've done that in the past to make valgrind happier.

Ronald