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mpeg2: optimize scan for start codes.

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Accelerate scan for start codes by skipping up to 3 bytes per iteration.
A start code prefix is defined by the following bytes: 00 00 01. Thus,
for any group of 3 bytes (xx yy zz), we have the following possible cases:

  1. If zz != 1, this cannot be a start code, then skip 3 bytes;
  2. If yy != 0, this cannot be a start code, then skip 2 bytes;
  3. If xx != 0 or zz != 1, this cannot be a start code, then skip 1 byte;
  4. xx == 00, yy == 00, zz == 1, we have match!

This algorithm requires to peek bytes from the adapter. This increases the
amount of bytes copied to a temporary buffer, but this process is much faster
than scanning for all the bytes and using shift/masks. So, overall, this is
a win.
This commit is contained in:
Gwenole Beauchesne 2013-01-09 13:44:18 +01:00
parent 0180ef635c
commit 3bc6078f32
1 changed files with 44 additions and 29 deletions
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73
gst-libs/gst/vaapi/gstvaapidecoder_mpeg2.c
View file

@ -1156,12 +1156,28 @@ decode_slice(GstVaapiDecoderMpeg2 *decoder, GstVaapiDecoderUnit *unit)
}
static inline gint
scan_for_start_code(GstAdapter *adapter, guint ofs, guint size, guint32 *scp)
scan_for_start_code(const guchar *buf, guint buf_size,
GstMpegVideoPacketTypeCode *type_ptr)
{
return (gint)gst_adapter_masked_scan_uint32_peek(adapter,
0xffffff00, 0x00000100,
ofs, size,
scp);
guint i = 0;
while (i <= (buf_size - 4)) {
if (buf[i + 2] > 1)
i += 3;
else if (buf[i + 1])
i += 2;
else if (buf[i] || buf[i + 2] != 1)
i++;
else
break;
}
if (i <= (buf_size - 4)) {
if (type_ptr)
*type_ptr = buf[i + 3];
return i;
}
return -1;
}
static GstVaapiDecoderStatus
@ -1308,47 +1324,46 @@ gst_vaapi_decoder_mpeg2_parse(GstVaapiDecoder *base_decoder,
GstVaapiParserState * const ps = GST_VAAPI_PARSER_STATE(base_decoder);
GstVaapiDecoderStatus status;
GstMpegVideoPacketTypeCode type;
guint32 start_code;
guint size, buf_size, flags;
gint ofs, ofs2;
const guchar *buf;
guint buf_size, flags;
gint ofs, ofs1, ofs2;
status = ensure_decoder(decoder);
if (status != GST_VAAPI_DECODER_STATUS_SUCCESS)
return status;
size = gst_adapter_available(adapter);
if (size < 4)
buf_size = gst_adapter_available(adapter);
if (buf_size < 4)
return GST_VAAPI_DECODER_STATUS_ERROR_NO_DATA;
ofs = scan_for_start_code(adapter, 0, size, &start_code);
buf = gst_adapter_peek(adapter, buf_size);
if (!buf)
return GST_VAAPI_DECODER_STATUS_ERROR_NO_DATA;
ofs = scan_for_start_code(buf, buf_size, &type);
if (ofs < 0)
return GST_VAAPI_DECODER_STATUS_ERROR_NO_DATA;
ofs1 = ofs;
if (ofs > 0) {
gst_adapter_flush(adapter, ofs);
size -= ofs;
}
ofs2 = ps->input_offset2 - 4;
if (ofs2 < ofs1 + 4)
ofs2 = ofs1 + 4;
ofs2 = ps->input_offset2 - ofs - 4;
if (ofs2 < 4)
ofs2 = 4;
ofs = G_UNLIKELY(size < ofs2 + 4) ? -1 :
scan_for_start_code(adapter, ofs2, size - ofs2, NULL);
ofs = G_UNLIKELY(buf_size < ofs2 + 4) ? -1 :
scan_for_start_code(&buf[ofs2], buf_size - ofs2, NULL);
if (ofs < 0) {
// Assume the whole packet is present if end-of-stream
if (!at_eos) {
ps->input_offset2 = size;
ps->input_offset2 = buf_size;
return GST_VAAPI_DECODER_STATUS_ERROR_NO_DATA;
}
ofs = size;
ofs = buf_size - ofs2;
}
buf_size = ofs;
ps->input_offset2 = 0;
ofs2 += ofs;
unit->size = buf_size;
type = start_code & 0xff;
unit->size = ofs2 - ofs1;
gst_adapter_flush(adapter, ofs1);
ps->input_offset2 = 4;
/* Check for start of new picture */
flags = 0;
@ -1366,7 +1381,7 @@ gst_vaapi_decoder_mpeg2_parse(GstVaapiDecoder *base_decoder,
flags |= GST_VAAPI_DECODER_UNIT_FLAG_FRAME_START;
break;
case GST_MPEG_VIDEO_PACKET_EXTENSION:
if (G_UNLIKELY(buf_size < 5))
if (G_UNLIKELY(unit->size < 5))
return GST_VAAPI_DECODER_STATUS_ERROR_BITSTREAM_PARSER;
break;
default: