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Create LUT for PrefixEncode.

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This speeds up lossless compression by 5%.
Change-Id: Ifd114b1d9850dc3aac74593809e7d48529d35e3d
This commit is contained in:
Vikas Arora
2013-08-05 10:19:24 -07:00
parent 988b70844e
commit 69257f70df
6 changed files with 191 additions and 80 deletions
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74
src/dsp/lossless.h
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@ -99,6 +99,80 @@ static WEBP_INLINE float VP8LFastSLog2(int v) {
return (v < LOG_LOOKUP_IDX_MAX) ? kSLog2Table[v] : VP8LFastSLog2Slow(v);
}
// -----------------------------------------------------------------------------
// PrefixEncode()
// use GNU builtins where available.
#if defined(__GNUC__) && \
((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
return 31 ^ __builtin_clz(n);
}
#elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_IX86))
#include <intrin.h>
#pragma intrinsic(_BitScanReverse)
static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
unsigned long first_set_bit;
_BitScanReverse(&first_set_bit, n);
return first_set_bit;
}
#else
// Returns (int)floor(log2(n)). n must be > 0.
static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
int log = 0;
uint32_t value = n;
int i;
for (i = 4; i >= 0; --i) {
const int shift = (1 << i);
const uint32_t x = value >> shift;
if (x != 0) {
value = x;
log += shift;
}
}
return log;
}
#endif
static WEBP_INLINE int VP8LBitsLog2Ceiling(uint32_t n) {
const int log_floor = BitsLog2Floor(n);
if (n == (n & ~(n - 1))) // zero or a power of two.
return log_floor;
else
return log_floor + 1;
}
// Splitting of distance and length codes into prefixes and
// extra bits. The prefixes are encoded with an entropy code
// while the extra bits are stored just as normal bits.
static WEBP_INLINE void VP8LPrefixEncodeNoLUT(int distance, int* const code,
int* const extra_bits,
int* const extra_bits_value) {
const int highest_bit = BitsLog2Floor(--distance);
const int second_highest_bit = (distance >> (highest_bit - 1)) & 1;
*extra_bits = highest_bit - 1;
*extra_bits_value = distance & ((1 << *extra_bits) - 1);
*code = 2 * highest_bit + second_highest_bit;
}
#define PREFIX_LOOKUP_IDX_MAX 512
// These tables are derived using VP8LPrefixEncodeNoLUT.
extern const int8_t kPrefixEncodeCode[PREFIX_LOOKUP_IDX_MAX];
extern const int8_t kPrefixEncodeExtraBits[PREFIX_LOOKUP_IDX_MAX];
extern const uint8_t kPrefixEncodeExtraBitsVal[PREFIX_LOOKUP_IDX_MAX];
static WEBP_INLINE void VP8LPrefixEncode(int distance, int* const code,
int* const extra_bits,
int* const extra_bits_value) {
if (distance < PREFIX_LOOKUP_IDX_MAX) {
*code = kPrefixEncodeCode[distance];
*extra_bits = kPrefixEncodeExtraBits[distance];
*extra_bits_value = kPrefixEncodeExtraBitsVal[distance];
} else {
VP8LPrefixEncodeNoLUT(distance, code, extra_bits, extra_bits_value);
}
}
// In-place difference of each component with mod 256.
static WEBP_INLINE uint32_t VP8LSubPixels(uint32_t a, uint32_t b) {