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lossy bit-reader clean-up:

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* remove LEFT/RIGHT_JUSTIFY distinction. It's all RIGHT_JUSTIFY now.
* simplify VP8GetSigned(), and add some masking branch-less code. Much
  faster on ARM (~13% speed-up). 8% on x86-64, 5% on MacBook.
* split critical implementation into separate bit_reader_inl.h file that
  is only included where needed (vp8.c / tree.c / bit_reader.c)
* bumped BITS value from 16 to 24 for x86-32b too, since it's a bit faster.

Change-Id: If41ca1da3e5c3dadacf2379d1ba419b151e7fce8
This commit is contained in:
Pascal Massimino 2014-06-01 23:19:34 -07:00 committed by skal
parent ac591cf22e
commit 09545eeadc
7 changed files with 246 additions and 274 deletions
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5
makefile.unix
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@ -244,6 +244,11 @@ all: ex $(EXTRA_EXAMPLES)
$(EX_FORMAT_DEC_OBJS): %.o: %.h
# special dependencies for tree.c/vp8.c/bit_reader.c <-> bit_reader_inl.h
src/dec/tree.o: src/utils/bit_reader_inl.h
src/dec/vp8.o: src/utils/bit_reader_inl.h
src/utils/bit_reader.o: src/utils/bit_reader_inl.h
%.o: %.c $(HDRS)
$(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $@

1
src/dec/tree.c
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@ -12,6 +12,7 @@
// Author: Skal (pascal.massimino@gmail.com)
#include "vp8i.h"
#include "../utils/bit_reader_inl.h"
#define USE_GENERIC_TREE

2
src/dec/vp8.c
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@ -17,7 +17,7 @@
#include "./vp8i.h"
#include "./vp8li.h"
#include "./webpi.h"
#include "../utils/bit_reader.h"
#include "../utils/bit_reader_inl.h"
#include "../utils/utils.h"
//------------------------------------------------------------------------------

1
src/utils/Makefile.am
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@ -11,6 +11,7 @@ commondir = $(includedir)/webp
COMMON_SOURCES =
COMMON_SOURCES += bit_reader.c
COMMON_SOURCES += bit_reader.h
COMMON_SOURCES += bit_reader_inl.h
COMMON_SOURCES += color_cache.c
COMMON_SOURCES += color_cache.h
COMMON_SOURCES += filters.c

60
src/utils/bit_reader.c
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@ -7,17 +7,11 @@
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Boolean decoder
// Boolean decoder non-inlined methods
//
// Author: Skal (pascal.massimino@gmail.com)
#include "./bit_reader.h"
#ifndef USE_RIGHT_JUSTIFY
#define MK(X) (((range_t)(X) << (BITS)) | (MASK))
#else
#define MK(X) ((range_t)(X))
#endif
#include "./bit_reader_inl.h"
//------------------------------------------------------------------------------
// VP8BitReader
@ -27,12 +21,13 @@ void VP8InitBitReader(VP8BitReader* const br,
assert(br != NULL);
assert(start != NULL);
assert(start <= end);
br->range_ = MK(255 - 1);
br->range_ = 255 - 1;
br->buf_ = start;
br->buf_end_ = end;
br->value_ = 0;
br->bits_ = -8; // to load the very first 8bits
br->eof_ = 0;
VP8LoadNewBytes(br);
}
void VP8RemapBitReader(VP8BitReader* const br, ptrdiff_t offset) {
@ -54,45 +49,35 @@ const uint8_t kVP8Log2Range[128] = {
0
};
// range = (range << kVP8Log2Range[range]) + trailing 1's
// range = ((range - 1) << kVP8Log2Range[range]) + 1
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),
MK(199), MK(207), MK(215), MK(223), MK(231), MK(239), MK(247), MK(127),
MK(131), MK(135), MK(139), MK(143), MK(147), MK(151), MK(155), MK(159),
MK(163), MK(167), MK(171), MK(175), MK(179), MK(183), MK(187), MK(191),
MK(195), MK(199), MK(203), MK(207), MK(211), MK(215), MK(219), MK(223),
MK(227), MK(231), MK(235), MK(239), MK(243), MK(247), MK(251), MK(127),
MK(129), MK(131), MK(133), MK(135), MK(137), MK(139), MK(141), MK(143),
MK(145), MK(147), MK(149), MK(151), MK(153), MK(155), MK(157), MK(159),
MK(161), MK(163), MK(165), MK(167), MK(169), MK(171), MK(173), MK(175),
MK(177), MK(179), MK(181), MK(183), MK(185), MK(187), MK(189), MK(191),
MK(193), MK(195), MK(197), MK(199), MK(201), MK(203), MK(205), MK(207),
MK(209), MK(211), MK(213), MK(215), MK(217), MK(219), MK(221), MK(223),
MK(225), MK(227), MK(229), MK(231), MK(233), MK(235), MK(237), MK(239),
MK(241), MK(243), MK(245), MK(247), MK(249), MK(251), MK(253), MK(127)
127, 127, 191, 127, 159, 191, 223, 127,
143, 159, 175, 191, 207, 223, 239, 127,
135, 143, 151, 159, 167, 175, 183, 191,
199, 207, 215, 223, 231, 239, 247, 127,
131, 135, 139, 143, 147, 151, 155, 159,
163, 167, 171, 175, 179, 183, 187, 191,
195, 199, 203, 207, 211, 215, 219, 223,
227, 231, 235, 239, 243, 247, 251, 127,
129, 131, 133, 135, 137, 139, 141, 143,
145, 147, 149, 151, 153, 155, 157, 159,
161, 163, 165, 167, 169, 171, 173, 175,
177, 179, 181, 183, 185, 187, 189, 191,
193, 195, 197, 199, 201, 203, 205, 207,
209, 211, 213, 215, 217, 219, 221, 223,
225, 227, 229, 231, 233, 235, 237, 239,
241, 243, 245, 247, 249, 251, 253, 127
};
#undef MK
void VP8LoadFinalBytes(VP8BitReader* const br) {
assert(br != NULL && br->buf_ != NULL);
// Only read 8bits at a time
if (br->buf_ < br->buf_end_) {
#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;
br->value_ = (bit_t)(*br->buf_++) | (br->value_ << 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;
}
}
@ -222,4 +207,3 @@ uint32_t VP8LReadBits(VP8LBitReader* const br, int n_bits) {
}
//------------------------------------------------------------------------------

258
src/utils/bit_reader.h
View File

@ -29,112 +29,52 @@ extern "C" {
// However, since range_ is only 8bit, we only need an active window of 8 bits
// for value_. Left bits (MSB) gets zeroed and shifted away when value_ falls
// below 128, range_ is updated, and fresh bits read from the bitstream are
// brought in as LSB.
// To avoid reading the fresh bits one by one (slow), we cache a few of them
// ahead (actually, we cache BITS of them ahead. See below). There's two
// strategies regarding how to shift these looked-ahead fresh bits into the
// 8bit window of value_: either we shift them in, while keeping the position of
// the window fixed. Or we slide the window to the right while keeping the cache
// bits at a fixed, right-justified, position.
// brought in as LSB. To avoid reading the fresh bits one by one (slow), we
// cache BITS of them ahead. The total of (BITS + 8) bits must fit into a
// natural register (with type bit_t). To fetch BITS bits from bitstream we
// use a type lbit_t.
//
// Example, for BITS=16: here is the content of value_ for both strategies:
//
// !USE_RIGHT_JUSTIFY || USE_RIGHT_JUSTIFY
// ||
// <- 8b -><- 8b -><- BITS bits -> || <- 8b+3b -><- 8b -><- 13 bits ->
// [unused][value_][cached bits][0] || [unused...][value_][cached bits]
// [........00vvvvvvBBBBBBBBBBBBB000]LSB || [...........00vvvvvvBBBBBBBBBBBBB]
// ||
// After calling VP8Shift(), where we need to shift away two zeros:
// [........vvvvvvvvBBBBBBBBBBB00000]LSB || [.............vvvvvvvvBBBBBBBBBBB]
// ||
// Just before we need to call VP8LoadNewBytes(), the situation is:
// [........vvvvvv000000000000000000]LSB || [..........................vvvvvv]
// ||
// And just after calling VP8LoadNewBytes():
// [........vvvvvvvvBBBBBBBBBBBBBBBB]LSB || [........vvvvvvvvBBBBBBBBBBBBBBBB]
//
// -> we're back to eight active 'value_' bits (marked 'v') and BITS cached
// bits (marked 'B')
//
// The right-justify strategy tends to use less shifts and is often faster.
//------------------------------------------------------------------------------
// BITS can be any multiple of 8 from 8 to 56 (inclusive).
// Pick values that fit natural register size.
#if !defined(WEBP_REFERENCE_IMPLEMENTATION)
#define USE_RIGHT_JUSTIFY
#if defined(__i386__) || defined(_M_IX86) // x86 32bit
#define BITS 16
#define BITS 24
#elif defined(__x86_64__) || defined(_M_X64) // x86 64bit
#define BITS 56
#elif defined(__arm__) || defined(_M_ARM) // ARM
#define BITS 24
#elif defined(__mips__) // MIPS
#define BITS 24
#else // reasonable default
#define BITS 24
#endif
#else // reference choices
#define USE_RIGHT_JUSTIFY
#define BITS 8
#endif
// some endian fix (e.g.: mips-gcc doesn't define __BIG_ENDIAN__)
#if !defined(__BIG_ENDIAN__) && defined(__BYTE_ORDER__) && \
(__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
#define __BIG_ENDIAN__
#else // reasonable default
#define BITS 24 // TODO(skal): test aarch64 and find the proper BITS value.
#endif
//------------------------------------------------------------------------------
// Derived types and constants
// Derived types and constants:
// bit_t = natural register type for storing 'value_' (which is BITS+8 bits)
// range_t = register for 'range_' (which is 8bits only)
// bit_t = natural register type
// lbit_t = natural type for memory I/O
#if (BITS > 32)
#if (BITS > 24)
typedef uint64_t bit_t;
typedef uint64_t lbit_t;
#elif (BITS == 32)
typedef uint64_t bit_t;
typedef uint32_t lbit_t;
#elif (BITS == 24)
typedef uint32_t bit_t;
typedef uint32_t lbit_t;
#elif (BITS == 16)
typedef uint32_t bit_t;
typedef uint16_t lbit_t;
#else
typedef uint32_t bit_t;
typedef uint8_t lbit_t;
#endif
#ifndef USE_RIGHT_JUSTIFY
typedef bit_t range_t; // type for storing range_
#define MASK ((((bit_t)1) << (BITS)) - 1)
#else
typedef uint32_t range_t; // range_ only uses 8bits here. No need for bit_t.
#endif
typedef uint32_t range_t;
//------------------------------------------------------------------------------
// Bitreader
typedef struct VP8BitReader VP8BitReader;
struct VP8BitReader {
// boolean decoder (keep the field ordering as is!)
range_t range_; // current range minus 1. In [127, 254] interval.
bit_t value_; // current value
int bits_; // number of valid bits left
// read buffer
const uint8_t* buf_; // next byte to be read
const uint8_t* buf_end_; // end of read buffer
int eof_; // true if input is exhausted
// boolean decoder
range_t range_; // current range minus 1. In [127, 254] interval.
bit_t value_; // current value
int bits_; // number of valid bits left
};
// Initialize the bit reader and the boolean decoder.
@ -154,164 +94,12 @@ static WEBP_INLINE uint32_t VP8Get(VP8BitReader* const br) {
// return the next value with sign-extension.
int32_t VP8GetSignedValue(VP8BitReader* const br, int num_bits);
// Read a bit with proba 'prob'. Speed-critical function!
extern const uint8_t kVP8Log2Range[128];
extern const range_t kVP8NewRange[128];
void VP8LoadFinalBytes(VP8BitReader* const br); // special case for the tail
static WEBP_INLINE void VP8LoadNewBytes(VP8BitReader* const br) {
assert(br != NULL && br->buf_ != NULL);
// Read 'BITS' bits at a time if possible.
if (br->buf_ + sizeof(lbit_t) <= br->buf_end_) {
// convert memory type to register type (with some zero'ing!)
bit_t bits;
#if defined(__mips__) // MIPS
// This is needed because of un-aligned read.
lbit_t in_bits;
lbit_t* p_buf_ = (lbit_t*)br->buf_;
__asm__ volatile(
".set push \n\t"
".set at \n\t"
".set macro \n\t"
"ulw %[in_bits], 0(%[p_buf_]) \n\t"
".set pop \n\t"
: [in_bits]"=r"(in_bits)
: [p_buf_]"r"(p_buf_)
: "memory", "at"
);
#else
const lbit_t in_bits = *(const lbit_t*)br->buf_;
#endif
br->buf_ += (BITS) >> 3;
#if !defined(__BIG_ENDIAN__)
#if (BITS > 32)
// gcc 4.3 has builtin functions for swap32/swap64
#if defined(__GNUC__) && \
(__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3))
bits = (bit_t)__builtin_bswap64(in_bits);
#elif defined(_MSC_VER)
bits = (bit_t)_byteswap_uint64(in_bits);
#elif defined(__x86_64__)
__asm__ volatile("bswapq %0" : "=r"(bits) : "0"(in_bits));
#else // generic code for swapping 64-bit values (suggested by bdb@)
bits = (bit_t)in_bits;
bits = ((bits & 0xffffffff00000000ull) >> 32) |
((bits & 0x00000000ffffffffull) << 32);
bits = ((bits & 0xffff0000ffff0000ull) >> 16) |
((bits & 0x0000ffff0000ffffull) << 16);
bits = ((bits & 0xff00ff00ff00ff00ull) >> 8) |
((bits & 0x00ff00ff00ff00ffull) << 8);
#endif
bits >>= 64 - BITS;
#elif (BITS >= 24)
#if defined(__i386__) || defined(__x86_64__)
{
lbit_t swapped_in_bits;
__asm__ volatile("bswap %k0" : "=r"(swapped_in_bits) : "0"(in_bits));
bits = (bit_t)swapped_in_bits; // 24b/32b -> 32b/64b zero-extension
}
#elif defined(_MSC_VER)
bits = (bit_t)_byteswap_ulong(in_bits);
#else
bits = (bit_t)(in_bits >> 24) | ((in_bits >> 8) & 0xff00)
| ((in_bits << 8) & 0xff0000) | (in_bits << 24);
#endif // x86
bits >>= (32 - BITS);
#elif (BITS == 16)
// gcc will recognize a 'rorw $8, ...' here:
bits = (bit_t)(in_bits >> 8) | ((in_bits & 0xff) << 8);
#else // BITS == 8
bits = (bit_t)in_bits;
#endif
#else // BIG_ENDIAN
bits = (bit_t)in_bits;
if (BITS != 8 * sizeof(bit_t)) bits >>= (8 * sizeof(bit_t) - BITS);
#endif
#ifndef USE_RIGHT_JUSTIFY
br->value_ |= bits << (-br->bits_);
#else
br->value_ = bits | (br->value_ << (BITS));
#endif
br->bits_ += (BITS);
} else {
VP8LoadFinalBytes(br); // no need to be inlined
}
}
static WEBP_INLINE int VP8BitUpdate(VP8BitReader* const br, range_t split) {
if (br->bits_ < 0) { // Make sure we have a least BITS bits in 'value_'
VP8LoadNewBytes(br);
}
#ifndef USE_RIGHT_JUSTIFY
split |= (MASK);
if (br->value_ > split) {
br->range_ -= split + 1;
br->value_ -= split + 1;
return 1;
} else {
br->range_ = split;
return 0;
}
#else
{
const int pos = br->bits_;
const range_t value = (range_t)(br->value_ >> pos);
if (value > split) {
br->range_ -= split + 1;
br->value_ -= (bit_t)(split + 1) << pos;
return 1;
} else {
br->range_ = split;
return 0;
}
}
#endif
}
static WEBP_INLINE void VP8Shift(VP8BitReader* const br) {
#ifndef USE_RIGHT_JUSTIFY
// range_ is in [0..127] interval here.
const bit_t idx = br->range_ >> (BITS);
const int shift = kVP8Log2Range[idx];
br->range_ = kVP8NewRange[idx];
br->value_ <<= shift;
br->bits_ -= shift;
#else
const int shift = kVP8Log2Range[br->range_];
assert(br->range_ < (range_t)128);
br->range_ = kVP8NewRange[br->range_];
br->bits_ -= shift;
#endif
}
static WEBP_INLINE int VP8GetBit(VP8BitReader* const br, int prob) {
#ifndef USE_RIGHT_JUSTIFY
// It's important to avoid generating a 64bit x 64bit multiply here.
// We just need an 8b x 8b after all.
const range_t split =
(range_t)((uint32_t)(br->range_ >> (BITS)) * prob) << ((BITS) - 8);
const int bit = VP8BitUpdate(br, split);
if (br->range_ <= (((range_t)0x7e << (BITS)) | (MASK))) {
VP8Shift(br);
}
return bit;
#else
const range_t split = (br->range_ * prob) >> 8;
const int bit = VP8BitUpdate(br, split);
if (br->range_ <= (range_t)0x7e) {
VP8Shift(br);
}
return bit;
#endif
}
static WEBP_INLINE int VP8GetSigned(VP8BitReader* const br, int v) {
const range_t split = (br->range_ >> 1);
const int bit = VP8BitUpdate(br, split);
VP8Shift(br);
return bit ? -v : v;
}
// bit_reader_inl.h will implement the following methods:
// static WEBP_INLINE int VP8GetBit(VP8BitReader* const br, int prob)
// static WEBP_INLINE int VP8GetSigned(VP8BitReader* const br, int v)
// and should be included by the .c files that actually need them.
// This is to avoid recompiling the whole library whenever this file is touched,
// and also allowing platform-specific ad-hoc hacks.
// -----------------------------------------------------------------------------
// Bitreader for lossless format

193
src/utils/bit_reader_inl.h Normal file
View File

@ -0,0 +1,193 @@
// Copyright 2014 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Specific inlined methods for boolean decoder [VP8GetBit() ...]
// This file should be included by the .c sources that actually need to call
// these methods.
//
// Author: Skal (pascal.massimino@gmail.com)
#ifndef WEBP_UTILS_BIT_READER_INL_H_
#define WEBP_UTILS_BIT_READER_INL_H_
#include "./bit_reader.h"
#ifdef __cplusplus
extern "C" {
#endif
//------------------------------------------------------------------------------
// Derived type lbit_t = natural type for memory I/O
#if (BITS > 32)
typedef uint64_t lbit_t;
#elif (BITS > 16)
typedef uint32_t lbit_t;
#elif (BITS > 8)
typedef uint16_t lbit_t;
#else
typedef uint8_t lbit_t;
#endif
// some endian fix (e.g.: mips-gcc doesn't define __BIG_ENDIAN__)
#if !defined(__BIG_ENDIAN__) && defined(__BYTE_ORDER__) && \
(__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
#define __BIG_ENDIAN__
#endif
extern const uint8_t kVP8Log2Range[128];
extern const range_t kVP8NewRange[128];
// special case for the tail byte-reading
void VP8LoadFinalBytes(VP8BitReader* const br);
//------------------------------------------------------------------------------
// Inlined critical functions
// makes sure br->value_ has at least BITS bits worth of data
static WEBP_INLINE void VP8LoadNewBytes(VP8BitReader* const br) {
assert(br != NULL && br->buf_ != NULL);
// Read 'BITS' bits at a time if possible.
if (br->buf_ + sizeof(lbit_t) <= br->buf_end_) {
// convert memory type to register type (with some zero'ing!)
bit_t bits;
#if defined(__mips__) // MIPS
// This is needed because of un-aligned read.
lbit_t in_bits;
lbit_t* p_buf_ = (lbit_t*)br->buf_;
__asm__ volatile(
".set push \n\t"
".set at \n\t"
".set macro \n\t"
"ulw %[in_bits], 0(%[p_buf_]) \n\t"
".set pop \n\t"
: [in_bits]"=r"(in_bits)
: [p_buf_]"r"(p_buf_)
: "memory", "at"
);
#else
const lbit_t in_bits = *(const lbit_t*)br->buf_;
#endif
br->buf_ += BITS >> 3;
#if !defined(__BIG_ENDIAN__)
#if (BITS > 32)
// gcc 4.3 has builtin functions for swap32/swap64
#if defined(__GNUC__) && \
(__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3))
bits = (bit_t)__builtin_bswap64(in_bits);
#elif defined(_MSC_VER)
bits = (bit_t)_byteswap_uint64(in_bits);
#elif defined(__x86_64__)
__asm__ volatile("bswapq %0" : "=r"(bits) : "0"(in_bits));
#else // generic code for swapping 64-bit values (suggested by bdb@)
bits = (bit_t)in_bits;
bits = ((bits & 0xffffffff00000000ull) >> 32) |
((bits & 0x00000000ffffffffull) << 32);
bits = ((bits & 0xffff0000ffff0000ull) >> 16) |
((bits & 0x0000ffff0000ffffull) << 16);
bits = ((bits & 0xff00ff00ff00ff00ull) >> 8) |
((bits & 0x00ff00ff00ff00ffull) << 8);
#endif
bits >>= 64 - BITS;
#elif (BITS >= 24)
#if defined(__i386__) || defined(__x86_64__)
{
lbit_t swapped_in_bits;
__asm__ volatile("bswap %k0" : "=r"(swapped_in_bits) : "0"(in_bits));
bits = (bit_t)swapped_in_bits; // 24b/32b -> 32b/64b zero-extension
}
#elif defined(_MSC_VER)
bits = (bit_t)_byteswap_ulong(in_bits);
#else
bits = (bit_t)(in_bits >> 24) | ((in_bits >> 8) & 0xff00)
| ((in_bits << 8) & 0xff0000) | (in_bits << 24);
#endif // x86
bits >>= (32 - BITS);
#elif (BITS == 16)
// gcc will recognize a 'rorw $8, ...' here:
bits = (bit_t)(in_bits >> 8) | ((in_bits & 0xff) << 8);
#else // BITS == 8
bits = (bit_t)in_bits;
#endif
#else // BIG_ENDIAN
bits = (bit_t)in_bits;
if (BITS != 8 * sizeof(bit_t)) bits >>= (8 * sizeof(bit_t) - BITS);
#endif
br->value_ = bits | (br->value_ << BITS);
br->bits_ += BITS;
} else {
VP8LoadFinalBytes(br); // no need to be inlined
}
}
// Read a bit with proba 'prob'. Speed-critical function!
static WEBP_INLINE int VP8GetBit(VP8BitReader* const br, int prob) {
// Don't move this declaration! It makes a big speed difference to store
// 'range' *before* calling VP8LoadNewBytes(), even if this function doesn't
// alter br->range_ value.
range_t range = br->range_;
if (br->bits_ < 0) {
VP8LoadNewBytes(br);
}
{
const int pos = br->bits_;
const range_t split = (range * prob) >> 8;
const range_t value = (range_t)(br->value_ >> pos);
#if defined(__arm__) || defined(_M_ARM) // ARM-specific
const int bit = ((int)(split - value) >> 31) & 1;
if (value > split) {
range -= split + 1;
br->value_ -= (bit_t)(split + 1) << pos;
} else {
range = split;
}
#else // faster version on x86
int bit; // Don't use 'const int bit = (value > split);", it's slower.
if (value > split) {
range -= split + 1;
br->value_ -= (bit_t)(split + 1) << pos;
bit = 1;
} else {
range = split;
bit = 0;
}
#endif
if (range <= (range_t)0x7e) {
const int shift = kVP8Log2Range[range];
range = kVP8NewRange[range];
br->bits_ -= shift;
}
br->range_ = range;
return bit;
}
}
// simplified version of VP8GetBit() for prob=0x80 (note shift is always 1 here)
static WEBP_INLINE int VP8GetSigned(VP8BitReader* const br, int v) {
if (br->bits_ < 0) {
VP8LoadNewBytes(br);
}
{
const int pos = br->bits_;
const range_t split = br->range_ >> 1;
const range_t value = (range_t)(br->value_ >> pos);
const int32_t mask = (int32_t)(split - value) >> 31; // -1 or 0
br->bits_ -= 1;
br->range_ += mask;
br->range_ |= 1;
br->value_ -= (bit_t)((split + 1) & mask) << pos;
return (v ^ mask) - mask;
}
}
#ifdef __cplusplus
} // extern "C"
#endif
#endif // WEBP_UTILS_BIT_READER_INL_H_