On 04/11/18 04:48, Andreas Rheinhardt wrote:
> Instead of using a combination of bitreader and -writer for copying data,
> one can byte-align the (obsolete and removed) bitreader to improve
> performance.
> One can even use memcpy in the normal case.
> This improved the time needed for writing the slicedata from 33618 to
> 2370 decicycles when tested on a video originating from a DVD (4194394
> runs).
>
> Signed-off-by: Andreas Rheinhardt <andreas.rheinha...@googlemail.com>
> ---
> libavcodec/cbs_mpeg2.c | 39 +++++++++++++++++++++++++++------------
> 1 file changed, 27 insertions(+), 12 deletions(-)
>
> diff --git a/libavcodec/cbs_mpeg2.c b/libavcodec/cbs_mpeg2.c
> index 0df4234b12..7161f1ee80 100644
> --- a/libavcodec/cbs_mpeg2.c
> +++ b/libavcodec/cbs_mpeg2.c
> @@ -264,8 +264,6 @@ static int cbs_mpeg2_write_slice(CodedBitstreamContext
> *ctx,
> PutBitContext *pbc)
> {
> MPEG2RawSlice *slice = unit->content;
> - GetBitContext gbc;
> - size_t bits_left;
> int err;
>
> err = cbs_mpeg2_write_slice_header(ctx, pbc, &slice->header);
> @@ -273,21 +271,38 @@ static int cbs_mpeg2_write_slice(CodedBitstreamContext
> *ctx,
> return err;
>
> if (slice->data) {
> + size_t rest = slice->data_size - (slice->data_bit_start + 7) / 8;
> + uint8_t *pos = slice->data + slice->data_bit_start / 8;
> +
> + av_assert0(slice->data_bit_start >= 0 &&
> + 8* slice->data_size > slice->data_bit_start);
> +
> if (slice->data_size * 8 + 8 > put_bits_left(pbc))
> return AVERROR(ENOSPC);
>
> - init_get_bits(&gbc, slice->data, slice->data_size * 8);
> - skip_bits_long(&gbc, slice->data_bit_start);
> -
> - while (get_bits_left(&gbc) > 15)
> - put_bits(pbc, 16, get_bits(&gbc, 16));
> + // First copy the remaining bits of the first byte
> + if (slice->data_bit_start % 8)
> + put_bits(pbc, 8 - slice->data_bit_start % 8,
> + *pos++ & MAX_UINT_BITS(8 - slice->data_bit_start % 8));
> +
> + if (put_bits_count(pbc) % 8 == 0) {
> + // If the writer is aligned at this point,
> + // memcpy can be used to improve performance.
> + // This is the normal case.
> + flush_put_bits(pbc);
> + memcpy(put_bits_ptr(pbc), pos, rest);
> + skip_put_bytes(pbc, rest);
> + } else {
> + // If not, we have to copy manually:
> + for (; rest > 3; rest -= 4, pos += 4)
> + put_bits32(pbc, AV_RB32(pos));
>
> - bits_left = get_bits_left(&gbc);
> - put_bits(pbc, bits_left, get_bits(&gbc, bits_left));
> + for (; rest; rest--, pos++)
> + put_bits(pbc, 8, *pos);
>
> - // Align with zeroes.
> - while (put_bits_count(pbc) % 8 != 0)
> - put_bits(pbc, 1, 0);
> + // Align with zeros
> + put_bits(pbc, 8 - put_bits_count(pbc) % 8, 0U);
> + }
> }
>
> return 0;
>
LGTM, tested and applied.
(I'm not sure that there is much value in handling the case of
slice->data_bit_start >= 8 since that currently never happens, but I guess it
worked for the old code so I don't mind the extra complexity.)
Thanks!
- Mark
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