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faster decoding (3%-6%)

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. revamped the boolean decoder to use less shifts
. added some description and ASCII art as explanations too.
. clarified the types further (bit_t, lbit_t, range_t, etc.)
. changed the negative field 'missing_' into positive 'bits_'

Some stats, decoding some randomly encoded WebP files:

with USE_RIGHT_JUSTIFY:
  BITS=32 => 133 files, 50 loops => 7.3s (1.097 ms/file/iterations)
  BITS=24 => 133 files, 50 loops => 7.3s (1.097 ms/file/iterations)
  BITS=16 => 133 files, 50 loops => 7.4s (1.120 ms/file/iterations)
  BITS=8 => 133 files, 50 loops => 7.5s (1.128 ms/file/iterations)

without USE_RIGHT_JUSTIFY:
  BITS=32 => 133 files, 50 loops => 7.5s (1.131 ms/file/iterations)
  BITS=24 => 133 files, 50 loops => 7.6s (1.142 ms/file/iterations)
  BITS=16 => 133 files, 50 loops => 7.6s (1.143 ms/file/iterations)
  BITS=8 => 133 files, 50 loops => 7.6s (1.149 ms/file/iterations)

Change-Id: I9277fb051676c05582e9c7ea3cb5a4b2a3ffb12e
This commit is contained in:
skal
2013-02-14 15:42:58 +01:00
parent 5c3e381b2f
commit 3383885799
2 changed files with 116 additions and 26 deletions
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26
src/utils/bit_reader.c
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@ -15,7 +15,11 @@
extern "C" {
#endif
#define MK(X) (((bit_t)(X) << (BITS)) | (MASK))
#ifndef USE_RIGHT_JUSTIFY
#define MK(X) (((range_t)(X) << (BITS)) | (MASK))
#else
#define MK(X) ((range_t)(X))
#endif
//------------------------------------------------------------------------------
// VP8BitReader
@ -29,7 +33,7 @@ void VP8InitBitReader(VP8BitReader* const br,
br->buf_ = start;
br->buf_end_ = end;
br->value_ = 0;
br->missing_ = 8; // to load the very first 8bits
br->bits_ = -8; // to load the very first 8bits
br->eof_ = 0;
}
@ -46,7 +50,7 @@ const uint8_t kVP8Log2Range[128] = {
};
// range = (range << kVP8Log2Range[range]) + trailing 1's
const bit_t kVP8NewRange[128] = {
const range_t kVP8NewRange[128] = {
MK(127), MK(127), MK(191), MK(127), MK(159), MK(191), MK(223), MK(127),
MK(143), MK(159), MK(175), MK(191), MK(207), MK(223), MK(239), MK(127),
MK(135), MK(143), MK(151), MK(159), MK(167), MK(175), MK(183), MK(191),
@ -71,9 +75,19 @@ void VP8LoadFinalBytes(VP8BitReader* const br) {
assert(br != NULL && br->buf_ != NULL);
// Only read 8bits at a time
if (br->buf_ < br->buf_end_) {
br->value_ |= (bit_t)(*br->buf_++) << ((BITS) - 8 + br->missing_);
br->missing_ -= 8;
} else {
#ifndef USE_RIGHT_JUSTIFY
br->value_ |= (bit_t)(*br->buf_++) << ((BITS) - 8 - br->bits_);
#else
br->value_ = (bit_t)(*br->buf_++) | (br->value_ << 8);
#endif
br->bits_ += 8;
} else if (!br->eof_) {
#ifdef USE_RIGHT_JUSTIFY
// These are not strictly needed, but it makes the behaviour
// consistent for both USE_RIGHT_JUSTIFY and !USE_RIGHT_JUSTIFY.
br->value_ <<= 8;
br->bits_ += 8;
#endif
br->eof_ = 1;
}
}