[FFmpeg-devel] [PATCH] avcodec/bitstream: replace qsort with AV_QSORT

Ganesh Ajjanagadde gajjanagadde at gmail.com
Sun Oct 18 03:30:01 CEST 2015

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:
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

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 <gajjanagadde at 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;

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