Up until now ff_startcode_find_candidate_c could overread; it relied on
zero-padding after the buffer in order to function correctly. This has
been changed: No overreads occur any more.
The ultimate goal behind all this is to create a high-performance
function for searching of startcodes that can be applied even in
scenarios where the buffer is not padded.
Signed-off-by: Andreas Rheinhardt <andreas.rheinha...@gmail.com>
---
libavcodec/startcode.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)
diff --git a/libavcodec/startcode.c b/libavcodec/startcode.c
index 373572365b..8774109df5 100644
--- a/libavcodec/startcode.c
+++ b/libavcodec/startcode.c
@@ -34,20 +34,23 @@ int ff_startcode_find_candidate_c(const uint8_t *buf, int
size)
const uint8_t *start = buf, *end = buf + size;
#define INITIALIZATION(mod) do { \
- for (; buf < end && (uintptr_t)buf % mod; buf++) \
- if (!*buf) \
- return buf - start; \
+ if (mod > end - start) \
+ goto near_end; \
+ for (; (uintptr_t)buf % mod; buf++) \
+ if (!*buf) \
+ return buf - start; \
+ /* Effective end for MAIN_LOOP in order not to overread. */ \
+ end -= mod; \
} while (0)
#define READ(bitness) AV_RN ## bitness ## A
#define MAIN_LOOP(bitness, mask1, mask2) do { \
- /* we check p < end instead of p + 3 / 7 because it is
- * simpler and there must be AV_INPUT_BUFFER_PADDING_SIZE
- * bytes at the end. */ \
- for (; buf < end; buf += bitness / 8) \
+ for (; buf <= end; buf += bitness / 8) \
if ((~READ(bitness)(buf) & (READ(bitness)(buf) - mask1)) \
& mask2) \
break; \
+ /* Revert to the real end. */ \
+ end += bitness / 8; \
} while (0)
#if HAVE_FAST_64BIT
@@ -57,6 +60,8 @@ int ff_startcode_find_candidate_c(const uint8_t *buf, int
size)
INITIALIZATION(4);
MAIN_LOOP(32, 0x01010101U, 0x80808080U);
#endif
+
+near_end:
for (; buf < end; buf++)
if (!*buf)
break;
--
2.21.0
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
To unsubscribe, visit link above, or email
ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
