On Sun, Sep 27, 2015 at 9:39 PM, Ganesh Ajjanagadde <gajja...@mit.edu> wrote: > On Sun, Sep 27, 2015 at 9:18 PM, Michael Niedermayer <michae...@gmx.at> wrote: >> On Sun, Sep 27, 2015 at 01:23:03PM -0400, Ganesh Ajjanagadde wrote: >>> On Sun, Sep 27, 2015 at 12:58 PM, Michael Niedermayer <michae...@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 <rsbul...@gmail.com> >>> >> wrote: >>> >> > Hi, >>> >> > >>> >> > On Sat, Sep 26, 2015 at 7:19 PM, Ganesh Ajjanagadde <gajja...@mit.edu> >>> >> > wrote: >>> >> > >>> >> >> On Sat, Sep 26, 2015 at 7:11 PM, Michael Niedermayer >>> >> >> <michae...@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 <gajjanaga...@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 ? > > No idea; maybe a good place to check is the Google fuzzing effort > where many bugs were fixed.
See e.g your commit: 09ef98f1ae3c8a4e08b66f41c3bd97dd7b07405f - Wstrict-overflow is indeed useful. I am thus convinced that we should retain it. Given the fact that local suppression is not worth it for just 2 instances and also that the patch does not reduce readability in any way, I think this patch and the one for xface are ok. > >> >> [...] >> -- >> Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB >> >> The real ebay dictionary, page 3 >> "Rare item" - "Common item with rare defect or maybe just a lie" >> "Professional" - "'Toy' made in china, not functional except as doorstop" >> "Experts will know" - "The seller hopes you are not an expert" >> >> _______________________________________________ >> ffmpeg-devel mailing list >> ffmpeg-devel@ffmpeg.org >> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel >> _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel