On Sun, Jun 02, 2019 at 12:47:18AM +0200, Andreas Rheinhardt wrote:
> Until now the function ff_startcode_find_candidate_c did not really
> search for startcodes (the startcode 0x00 0x00 0x01 (used in
> MPEG-1/2/4, VC-1 and H.264/5) is the only startcode meant here). Instead
> it searched for zero bytes and returned the earliest position of a zero
> byte. This of course led to lots of false positives - millions per GB
> of video.
> This has been changed: The first position of the buffer that
> may be part of a four-byte startcode is now returned. This excludes zero
> bytes that are known not to belong to a startcode, but zero bytes at the
> end of a buffer that might be part of a startcode whose second part is in
> the next buffer are also returned. This is in accordance with the
> expectations of the current callers of ff_startcode_find_candidate_c,
> namely the H.264 parser and the VC-1 parser. This is also the reason why
> "find_candidate" in the name of the function has been kept.
>
> Getting rid of lots of function calls with their accompanying overhead
> of course brings a speedup with it: Here are some benchmarks for
> a 30.2 Mb/s transport stream A (10 iterations of 8192 runs each) and
> another 7.4 Mb/s transport stream B (10 iterations of 131072 runs each)
> on an x64 Haswell:
>
> | vanilla | aligned | current: aligned,
> | (unaligned) | reads | no false positives
> --|-------------|---------|-------------------
> A | 411578 | 424503 | 323355
> B | 55476 | 58326 | 44147
>
> vanilla refers to the state before the switch to aligned reads;
> "aligned reads" refers to the state after the switch to aligned reads.
>
> (Calls to h264_find_frame_end have been timed; given that the amount
> of calls to ff_startcode_find_candidate_c has been reduced considerably,
> timing them directly would be worthless.)
>
> Signed-off-by: Andreas Rheinhardt <andreas.rheinha...@gmail.com>
> ---
> libavcodec/h264dsp.h | 7 +--
> libavcodec/startcode.c | 98 +++++++++++++++++++++++++++++++++++++-----
> libavcodec/vc1dsp.h | 6 +--
> 3 files changed, 91 insertions(+), 20 deletions(-)
>
> diff --git a/libavcodec/h264dsp.h b/libavcodec/h264dsp.h
> index cbea3173c6..51de7a4e21 100644
> --- a/libavcodec/h264dsp.h
> +++ b/libavcodec/h264dsp.h
> @@ -108,11 +108,8 @@ typedef struct H264DSPContext {
> void (*h264_add_pixels4_clear)(uint8_t *dst, int16_t *block, int stride);
>
> /**
> - * Search buf from the start for up to size bytes. Return the index
> - * of a zero byte, or >= size if not found. Ideally, use lookahead
> - * to filter out any zero bytes that are known to not be followed by
> - * one or more further zero bytes and a one byte. Better still, filter
> - * out any bytes that form the trailing_zero_8bits syntax element too.
> + * Search buf from the start for up to size bytes. Return the first 0x00
> + * that might be part of a (three or four) byte startcode.
> */
> int (*startcode_find_candidate)(const uint8_t *buf, int size);
> } H264DSPContext;
> diff --git a/libavcodec/startcode.c b/libavcodec/startcode.c
> index b027c191c0..f6105289f1 100644
> --- a/libavcodec/startcode.c
> +++ b/libavcodec/startcode.c
> @@ -22,7 +22,8 @@
> * @file
> * Accelerated start code search function for start codes common to
> * MPEG-1/2/4 video, VC-1, H.264/5
> - * @author Michael Niedermayer <michae...@gmx.at>
> + * @author Michael Niedermayer <michae...@gmx.at>,
> + * @author Andreas Rheinhardt <andreas.rheinha...@gmail.com>
> */
>
> #include "startcode.h"
> @@ -31,20 +32,60 @@
>
> int ff_startcode_find_candidate_c(const uint8_t *buf, int size)
> {
> - const uint8_t *start = buf, *end = buf + size;
> + const uint8_t *start = buf, *end = buf + size, *temp;
>
> #define INITIALIZATION(mod) do { \
> - for (; buf < end && (uintptr_t)buf % mod; buf++) \
> - if (!*buf) \
> - return buf - start; \
> + if (end - start <= mod + 1) \
> + goto near_end; \
> + /* From this point on until the end of the MAIN_LOOP, \
> + * buf is the earliest possible position of a 0x00 \
> + * immediately preceding a startcode's 0x01, i.e. \
> + * everything from start to buf (inclusive) is known \
> + * to not contain a startcode's 0x01. */ \
> + buf += 1; \> + temp = (const uint8_t *)((uintptr_t)(buf + mod - 1) / mod * mod); \ smells like FFALIGN() [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Let us carefully observe those good qualities wherein our enemies excel us and endeavor to excel them, by avoiding what is faulty, and imitating what is excellent in them. -- Plutarch
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