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

[Michael Niedermayer]

On Sun, Sep 27, 2015 at 01:23:03PM -0400, Ganesh Ajjanagadde wrote:
> On Sun, Sep 27, 2015 at 12:58 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
> > 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
> 
> https://stackoverflow.com/questions/12984861/dont-understand-assuming-signed-overflow-warning
> - gives a detailed explanation.
> 
> Some more info: this is triggered only when -finline-functions is
> enabled (done by default on -O3, not enabled by default on -O2).
> -finline-functions tries to inline stuff even when "inline" keyword is
> absent (like in this case).
> As for the warning, http://linux.die.net/man/1/gcc - search for
> -Wstrict-overflow. It is enabled due to -Wall, and as the man page
> suggests, it depends on optimization level as we can see in this
> example.
> I do consider the compiler broken in this case, but then again
> compilers are broken in so many different ways it is not even funny:
> see e.g -Warray-bounds, can't use the ISO C correct { 0 } initializer
> for compound data types, etc.
> 
> If you don't like this, we should add a -Wnostrict-overflow either to
> configure, or a local enable/disable via pragmas/macros. I don't like
> either of these as compared to this simple workaround:
> 1. -Wnostrict-overflow: FFmpeg with the amount of integer arithmetic
> being done should benefit from this warning in general, so disabling
> it globally may be bad.

how many actual bugs has Wstrict-overflow found ?

[...]
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