libavcodec/cbs: Stop needlessly reallocating the units array

Currently, a fragment's unit array is constantly reallocated during
splitting of a packet. This commit changes this: One can keep the units
array by distinguishing between the number of allocated and the number
of valid units in the units array.

The more units a packet is split into, the bigger the benefit.
So MPEG-2 benefits the most; for a video coming from an NTSC-DVD
(usually 32 units per frame) the average cost of cbs_insert_unit (for a
single unit) went down from 6717 decicycles to 450 decicycles (based
upon 10 runs with 4194304 runs each); if each packet consists of only
one unit, it went down from 2425 to 448; for a H.264 video where most
packets contain nine units, it went from 4431 to 450.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@googlemail.com>

1 files changed, 29 insertions, 4 deletions

diff --git a/libavcodec/cbs.h b/libavcodec/cbs.h
index 53ac360bb1..967dcd1468 100644
--- a/libavcodec/cbs.h
+++ b/libavcodec/cbs.h
@@ -145,10 +145,19 @@ typedef struct CodedBitstreamFragment {
      * and has not been decomposed.
      */
     int              nb_units;
+
+    /**
+     * Number of allocated units.
+     *
+     * Must always be >= nb_units; designed for internal use by cbs.
+     */
+     int             nb_units_allocated;
+
     /**
-     * Pointer to an array of units of length nb_units.
+     * Pointer to an array of units of length nb_units_allocated.
+     * Only the first nb_units are valid.
      *
-     * Must be NULL if nb_units is zero.
+     * Must be NULL if nb_units_allocated is zero.
      */
     CodedBitstreamUnit *units;
 } CodedBitstreamFragment;
@@ -231,6 +240,9 @@ void ff_cbs_close(CodedBitstreamContext **ctx);
  * This also updates the internal state, so will need to be called for
  * codecs with extradata to read parameter sets necessary for further
  * parsing even if the fragment itself is not desired.
+ *
+ * The fragment must have been zeroed or reset via ff_cbs_fragment_reset
+ * before use.
  */
 int ff_cbs_read_extradata(CodedBitstreamContext *ctx,
                           CodedBitstreamFragment *frag,
@@ -243,6 +255,9 @@ int ff_cbs_read_extradata(CodedBitstreamContext *ctx,
  * This also updates the internal state of the coded bitstream context
  * with any persistent data from the fragment which may be required to
  * read following fragments (e.g. parameter sets).
+ *
+ * The fragment must have been zeroed or reset via ff_cbs_fragment_reset
+ * before use.
  */
 int ff_cbs_read_packet(CodedBitstreamContext *ctx,
                        CodedBitstreamFragment *frag,
@@ -255,6 +270,9 @@ int ff_cbs_read_packet(CodedBitstreamContext *ctx,
  * This also updates the internal state of the coded bitstream context
  * with any persistent data from the fragment which may be required to
  * read following fragments (e.g. parameter sets).
+ *
+ * The fragment must have been zeroed or reset via ff_cbs_fragment_reset
+ * before use.
  */
 int ff_cbs_read(CodedBitstreamContext *ctx,
                 CodedBitstreamFragment *frag,
@@ -294,11 +312,18 @@ int ff_cbs_write_packet(CodedBitstreamContext *ctx,
 
 
 /**
- * Free all allocated memory in a fragment.
+ * Free the units contained in a fragment as well as the fragment's
+ * own data buffer, but not the units array itself.
  */
-void ff_cbs_fragment_uninit(CodedBitstreamContext *ctx,
+void ff_cbs_fragment_reset(CodedBitstreamContext *ctx,
                             CodedBitstreamFragment *frag);
 
+/**
+ * Free the units array of a fragment in addition to what
+ * ff_cbs_fragment_reset does.
+ */
+void ff_cbs_fragment_free(CodedBitstreamContext *ctx,
+                          CodedBitstreamFragment *frag);
 
 /**
  * Allocate a new internal content buffer of the given size in the unit.