[FFmpeg-devel] [PATCH] avformat/movenc: suppress -Wstrict-overflow warnings

[Michael Niedermayer]

On Sat, Sep 26, 2015 at 10:55:26PM -0400, Ganesh Ajjanagadde wrote:
> On Sat, Sep 26, 2015 at 10:32 PM, Ronald S. Bultje <rsbultje at gmail.com> wrote:
> > Hi,
> >
> > On Sat, Sep 26, 2015 at 7:19 PM, Ganesh Ajjanagadde <gajjanag at mit.edu>
> > wrote:
> >
> >> On Sat, Sep 26, 2015 at 7:11 PM, Michael Niedermayer <michaelni at gmx.at>
> >> wrote:
> >> > On Fri, Sep 18, 2015 at 05:15:50PM -0400, Ganesh Ajjanagadde wrote:
> >> >> This patch results in identical behavior of movenc, and suppresses
> >> -Wstrict-overflow
> >> >> warnings observed in GCC 5.2.
> >> >> I have manually checked that all usages are safe, and overflow
> >> possibility does
> >> >> not exist with this expression rewrite.
> >> >>
> >> >> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
> >> >> ---
> >> >>  libavformat/movenc.c | 2 +-
> >> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >>
> >> >> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> >> >> index af03d1e..6e4a1a6 100644
> >> >> --- a/libavformat/movenc.c
> >> >> +++ b/libavformat/movenc.c
> >> >> @@ -854,7 +854,7 @@ static int get_cluster_duration(MOVTrack *track,
> >> int cluster_idx)
> >> >>  {
> >> >>      int64_t next_dts;
> >> >>
> >> >> -    if (cluster_idx >= track->entry)
> >> >> +    if (cluster_idx - track->entry >= 0)
> >> >
> >> > i do not understand what this fixes or why
> >> > also plese quote the actual warnings which are fixed in the commit
> >> > message
> >>
> >> I have posted v2 with a more detailed commit message. It should be
> >> self explanatory.
> >
> >
> > Even with the new message, it's still not clear to me what's being fixed.
> > What does the warning check for? What is the problem in the initial
> > expression?
> 
> Compilers make transformations on the statements in order to possibly
> get better performance when compiled with optimizations. However, some
> of these optimizations require assumptions in the code. In particular,
> the compiler is internally rewriting cluster_idx >= track->entry to
> cluster_idx - track->entry >= 0 internally for some reason (I am not
> an asm/instruction set guy, so I can't comment why it likes this).
> However, such a transformation is NOT always safe as integer
> arithmetic can overflow (try e.g extreme values close to INT_MIN,
> INT_MAX). The warning is spit out since the compiler can't be sure
> that this is safe, but it still wants to do it (I suspect only the
> -O3/-O2 level that try this, can check if you want).

iam not sure i understand correctly but
if the compiler changes the code and then warns that what it just
did might be unsafe then the compiler is broken

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I know you won't believe me, but the highest form of Human Excellence is
to question oneself and others. -- Socrates
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 181 bytes
Desc: Digital signature
URL: <http://ffmpeg.org/pipermail/ffmpeg-devel/attachments/20150927/bf048959/attachment.sig>