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, 36 insertions, 26 deletions

diff --git a/libavcodec/cbs.c b/libavcodec/cbs.c
index ecbf57c293..c388be896b 100644
--- a/libavcodec/cbs.c
+++ b/libavcodec/cbs.c
@@ -136,14 +136,13 @@ static void cbs_unit_uninit(CodedBitstreamContext *ctx,
     unit->data_bit_padding = 0;
 }
 
-void ff_cbs_fragment_uninit(CodedBitstreamContext *ctx,
-                            CodedBitstreamFragment *frag)
+void ff_cbs_fragment_reset(CodedBitstreamContext *ctx,
+                           CodedBitstreamFragment *frag)
 {
     int i;
 
     for (i = 0; i < frag->nb_units; i++)
         cbs_unit_uninit(ctx, &frag->units[i]);
-    av_freep(&frag->units);
     frag->nb_units = 0;
 
     av_buffer_unref(&frag->data_ref);
@@ -152,6 +151,15 @@ void ff_cbs_fragment_uninit(CodedBitstreamContext *ctx,
     frag->data_bit_padding = 0;
 }
 
+void ff_cbs_fragment_free(CodedBitstreamContext *ctx,
+                          CodedBitstreamFragment *frag)
+{
+    ff_cbs_fragment_reset(ctx, frag);
+
+    av_freep(&frag->units);
+    frag->nb_units_allocated = 0;
+}
+
 static int cbs_read_fragment_content(CodedBitstreamContext *ctx,
                                      CodedBitstreamFragment *frag)
 {
@@ -216,8 +224,6 @@ int ff_cbs_read_extradata(CodedBitstreamContext *ctx,
 {
     int err;
 
-    memset(frag, 0, sizeof(*frag));
-
     err = cbs_fill_fragment_data(ctx, frag, par->extradata,
                                  par->extradata_size);
     if (err < 0)
@@ -236,8 +242,6 @@ int ff_cbs_read_packet(CodedBitstreamContext *ctx,
 {
     int err;
 
-    memset(frag, 0, sizeof(*frag));
-
     if (pkt->buf) {
         frag->data_ref = av_buffer_ref(pkt->buf);
         if (!frag->data_ref)
@@ -265,8 +269,6 @@ int ff_cbs_read(CodedBitstreamContext *ctx,
 {
     int err;
 
-    memset(frag, 0, sizeof(*frag));
-
     err = cbs_fill_fragment_data(ctx, frag, data, size);
     if (err < 0)
         return err;
@@ -548,20 +550,34 @@ static int cbs_insert_unit(CodedBitstreamContext *ctx,
 {
     CodedBitstreamUnit *units;
 
-    units = av_malloc_array(frag->nb_units + 1, sizeof(*units));
-    if (!units)
-        return AVERROR(ENOMEM);
+    if (frag->nb_units < frag->nb_units_allocated) {
+        units = frag->units;
+
+        if (position < frag->nb_units)
+            memmove(units + position + 1, units + position,
+                    (frag->nb_units - position) * sizeof(*units));
+    } else {
+        units = av_malloc_array(frag->nb_units + 1, sizeof(*units));
+        if (!units)
+            return AVERROR(ENOMEM);
+
+        ++frag->nb_units_allocated;
 
-    if (position > 0)
-        memcpy(units, frag->units, position * sizeof(*units));
-    if (position < frag->nb_units)
-        memcpy(units + position + 1, frag->units + position,
-               (frag->nb_units - position) * sizeof(*units));
+        if (position > 0)
+            memcpy(units, frag->units, position * sizeof(*units));
+
+        if (position < frag->nb_units)
+            memcpy(units + position + 1, frag->units + position,
+                   (frag->nb_units - position) * sizeof(*units));
+    }
 
     memset(units + position, 0, sizeof(*units));
 
-    av_freep(&frag->units);
-    frag->units = units;
+    if (units != frag->units) {
+        av_free(frag->units);
+        frag->units = units;
+    }
+
     ++frag->nb_units;
 
     return 0;
@@ -652,16 +668,10 @@ int ff_cbs_delete_unit(CodedBitstreamContext *ctx,
 
     --frag->nb_units;
 
-    if (frag->nb_units == 0) {
-        av_freep(&frag->units);
-
-    } else {
+    if (frag->nb_units > 0)
         memmove(frag->units + position,
                 frag->units + position + 1,
                 (frag->nb_units - position) * sizeof(*frag->units));
 
-        // Don't bother reallocating the unit array.
-    }
-
     return 0;
 }