From: Ganesh Ajjanagadde Date: Sun, 18 Oct 2015 01:22:59 +0000 (-0400) Subject: avcodec/bitstream: replace qsort with AV_QSORT X-Git-Tag: android-x86-7.1-r1~8419 X-Git-Url: http://git.osdn.net/view?a=commitdiff_plain;h=e11e32686fdb21aded1ccf70202f1fffe87bb6a2;p=android-x86%2Fexternal-ffmpeg.git avcodec/bitstream: replace qsort with AV_QSORT 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. Reviewed-by: Michael Niedermayer Signed-off-by: Ganesh Ajjanagadde --- diff --git a/libavcodec/bitstream.c b/libavcodec/bitstream.c index 924cc519a2..1acb7a337d 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;