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avcodec/mpeg12dec: Optimize reading mpeg2 intra escape codes

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Said escape code is only six bits long, so that one has at least 25 - 6
bits in the bitstream reader's cache after reading it; therefore the
whole following 18 bits (containing the actual code) are already in the
bitstream reader's cache, making it unnecessary to reload the cache.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
This commit is contained in:
Andreas Rheinhardt 2020-10-08 21:45:21 +02:00
parent a11cc04786
commit fe9bc1cc45
1 changed files with 4 additions and 6 deletions
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10
libavcodec/mpeg12dec.c
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@ -526,10 +526,9 @@ static inline int mpeg2_decode_block_intra(MpegEncContext *s,
} else {
/* escape */
run = SHOW_UBITS(re, &s->gb, 6) + 1;
LAST_SKIP_BITS(re, &s->gb, 6);
UPDATE_CACHE(re, &s->gb);
SKIP_BITS(re, &s->gb, 6);
level = SHOW_SBITS(re, &s->gb, 12);
SKIP_BITS(re, &s->gb, 12);
LAST_SKIP_BITS(re, &s->gb, 12);
i += run;
if (i > MAX_INDEX)
break;
@ -610,10 +609,9 @@ static inline int mpeg2_fast_decode_block_intra(MpegEncContext *s,
} else {
/* escape */
run = SHOW_UBITS(re, &s->gb, 6) + 1;
LAST_SKIP_BITS(re, &s->gb, 6);
UPDATE_CACHE(re, &s->gb);
SKIP_BITS(re, &s->gb, 6);
level = SHOW_SBITS(re, &s->gb, 12);
SKIP_BITS(re, &s->gb, 12);
LAST_SKIP_BITS(re, &s->gb, 12);
i += run;
j = scantable[i];
if (level < 0) {