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demux: simplify and improve performance of backward playback stepping 

The step_backwards function set reader_head to the start of the current
cache range. This was completely unnecessary and made it _much_ slower.
Remove the code that adjusts reader_head. Merge the rest of the code
into the only caller and remove the function.

The comment on the removed code was quite right. It was "inefficient".
Removing it delegates going to an early position to the normal seek
code, triggered by find_backward_restart_pos() incremental back seek
logic. I suppose especially audio benefits from this, because this
happens for every single audio packet (except maybe freaky bullshit like
TrueHD, which has "keyframes").

The blabla about performance in the removed comments is still true, but
now applies to the seek code itself only.
This commit is contained in:
wm4 2019-05-31 22:42:59 +02:00
parent d25fbb0813
commit 9c32997c65
1 changed files with 6 additions and 33 deletions
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39
demux/demux.c
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@ -1484,38 +1484,6 @@ static void back_demux_see_packets(struct demux_stream *ds)
find_backward_restart_pos(ds);
}
// Resume demuxing from an earlier position for backward playback. May trigger
// a seek.
static void step_backwards(struct demux_stream *ds)
{
struct demux_internal *in = ds->in;
assert(in->back_demuxing);
assert(!ds->back_restarting);
ds->back_restarting = true;
ds->back_restart_eof = false;
ds->back_restart_next = false;
// Move to start of queue. This is inefficient, because we need to iterate
// the entire fucking packet queue just to update the fw_* stats. But as
// long as we don't have demux_packet.prev links or a complete index, it's
// the thing to do.
// Note: if the buffer forward is much larger than the one backward, it
// would be worth looping until the previous reader_head and decrementing
// fw_packs/fw_bytes - you could skip the full recompute_buffers().
ds->reader_head = ds->queue->head;
in->fw_bytes -= ds->fw_bytes;
recompute_buffers(ds);
in->fw_bytes += ds->fw_bytes;
// Exclude weird special-cases (incomplete pruning? broken seeks?)
while (ds->reader_head && !ds->reader_head->keyframe)
advance_reader_head(ds);
find_backward_restart_pos(ds);
}
// Add the keyframe to the end of the index. Not all packets are actually added.
static void add_index_entry(struct demux_queue *queue, struct demux_packet *dp)
{
@ -2285,7 +2253,12 @@ static int dequeue_packet(struct demux_stream *ds, struct demux_packet **res)
if (ds->back_range_started && !ds->back_range_min &&
((ds->reader_head && ds->reader_head->keyframe) || eof))
{
step_backwards(ds);
ds->back_restarting = true;
ds->back_restart_eof = false;
ds->back_restart_next = false;
find_backward_restart_pos(ds);
if (ds->back_restarting)
return 0;
}