avformat/av1: Avoid allocation + copying when filtering OBUs

Certain types of OBUs are stripped away before muxing into Matroska and
ISOBMFF; there are two functions to do this: One that outputs by
directly writing in an AVIOContext and one that returns a freshly
allocated buffer with the units not stripped away copied into it.

The latter option is bad for performance, especially when the input
does already not contain any of the units intended to be stripped away
(this covers typical remuxing scenarios). Therefore this commit changes
this by avoiding allocating and copying when possible; it is possible if
the OBUs to be retained are consecutively in the input buffer (without
an OBU to be discarded between them). In this case, the caller receives
the offset as well as the length of the part of the buffer that contains
the units to be kept. This also avoids copying when e.g. the only unit
to be discarded is a temporal delimiter at the front.

For a 22.7mb/s file with average framesize 113 kB this improved the time
for the calls to ff_av1_filter_obus_buf() when writing Matroska from
313319 decicycles to 2368 decicycles; for another file with 1.5mb/s
(average framesize 7.3 kB) it improved from 34539 decicycles to 1922
decicyles. For these files the only units that needed to be stripped
away were temporal unit delimiters at the front.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

1 files changed, 9 insertions, 4 deletions

diff --git a/libavformat/av1.h b/libavformat/av1.h
index 6cc3fcaad2..dd5b47dc25 100644
--- a/libavformat/av1.h
+++ b/libavformat/av1.h
@@ -56,19 +56,24 @@ typedef struct AV1SequenceParameters {
 int ff_av1_filter_obus(AVIOContext *pb, const uint8_t *buf, int size);
 
 /**
- * Filter out AV1 OBUs not meant to be present in ISOBMFF sample data and write
- * the resulting bitstream to a newly allocated data buffer.
+ * Filter out AV1 OBUs not meant to be present in ISOBMFF sample data and return
+ * the result in a data buffer, avoiding allocations and copies if possible.
  *
  * @param in input data buffer
- * @param out pointer to pointer that will hold the allocated data buffer
+ * @param out pointer to pointer for the returned buffer. In case of success,
+ *            it is independently allocated if and only if `*out` differs from in.
  * @param size size of the input data buffer. The size of the resulting output
  *             data buffer will be written here
+ * @param offset offset of the returned data inside `*out`: It runs from
+ *               `*out + offset` (inclusive) to `*out + offset + size`
+ *               (exclusive); is zero if `*out` is independently allocated.
  *
  * @return 0 in case of success, a negative AVERROR code in case of failure.
  *         On failure, *out and *size are unchanged
  * @note *out will be treated as unintialized on input and will not be freed.
  */
-int ff_av1_filter_obus_buf(const uint8_t *in, uint8_t **out, int *size);
+int ff_av1_filter_obus_buf(const uint8_t *in, uint8_t **out,
+                           int *size, int *offset);
 
 /**
  * Parses a Sequence Header from the the provided buffer.