On Sat, Oct 17, 2015 at 9:30 PM, Ganesh Ajjanagadde <gajjanaga...@gmail.com> wrote: > Commit 3a0a2f33a6c955823fa4fb12c0b49cd29a496659 claims large performance > advantages for AV_QSORT over libc's qsort. The reason is that I suspect > that libc's qsort (at least on non LTO builds, like the typical FFmpeg config) > can't inline the comparison callback: > https://stackoverflow.com/questions/5290695/is-there-any-way-a-c-c-compiler-can-inline-a-c-callback-function. > AV_QSORT has two things going for it: > 1. The guaranteed inlining of qsort itself. This yields a negligible > boost that may be ignored. > 2. The more serious possibility of potentially allowing the comparison > function to be inlined - this is likely responsible for the large boosts > reported. > > There is a comment explaining that this is a place that could use some > performance improvement. Thus AV_QSORT is used to achieve that. > > Benchmarks deemed unnecessary due to existing claims about AV_QSORT. > Tested with FATE. > > Signed-off-by: Ganesh Ajjanagadde <gajjanaga...@gmail.com> > --- > libavcodec/bitstream.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/libavcodec/bitstream.c b/libavcodec/bitstream.c > index 924cc51..1acb7a3 100644 > --- a/libavcodec/bitstream.c > +++ b/libavcodec/bitstream.c > @@ -30,6 +30,7 @@ > > #include "libavutil/atomic.h" > #include "libavutil/avassert.h" > +#include "libavutil/qsort.h" > #include "avcodec.h" > #include "internal.h" > #include "mathops.h" > @@ -333,7 +334,7 @@ int ff_init_vlc_sparse(VLC *vlc_arg, int nb_bits, int > nb_codes, > } > COPY(buf[j].bits > nb_bits); > // qsort is the slowest part of init_vlc, and could probably be improved > or avoided > - qsort(buf, j, sizeof(VLCcode), compare_vlcspec); > + AV_QSORT(buf, j, struct VLCcode, compare_vlcspec); > COPY(buf[j].bits && buf[j].bits <= nb_bits); > nb_codes = j; > > -- > 2.6.1 >
Indeed confirmed the difference in inlining: the asm for compare_vlcspec is gone. Thus performance gains should be obvious. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
