/src/libwebp/src/utils/bit_reader_inl_utils.h

Source
// 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_UTILS_H_
#define WEBP_UTILS_BIT_READER_INL_UTILS_H_

#ifdef HAVE_CONFIG_H
#include "src/webp/config.h"
#endif

#include <assert.h>
#include <string.h>  // for memcpy

#include "src/dsp/cpu.h"
#include "src/dsp/dsp.h"
#include "src/utils/bit_reader_utils.h"
#include "src/utils/bounds_safety.h"
#include "src/utils/endian_inl_utils.h"
#include "src/utils/utils.h"
#include "src/webp/types.h"

WEBP_ASSUME_UNSAFE_INDEXABLE_ABI

#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

extern const uint8_t kVP8Log2Range[128];
extern const uint8_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_UBSAN_IGNORE_UNDEF WEBP_INLINE void VP8LoadNewBytes(
    VP8BitReader* WEBP_RESTRICT const br) {
  assert(br != NULL && br->buf != NULL);
  // Read 'BITS' bits at a time if possible.
  if (br->buf < br->buf_max) {
    // convert memory type to register type (with some zero'ing!)
    bit_t bits;
#if defined(WEBP_USE_MIPS32)
    // 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
    lbit_t in_bits;
    WEBP_UNSAFE_MEMCPY(&in_bits, br->buf, sizeof(in_bits));
#endif
    br->buf += BITS >> 3;
    WEBP_SELF_ASSIGN(br->buf_end);
#if !defined(WORDS_BIGENDIAN)
#if (BITS > 32)
    bits = BSwap64(in_bits);
    bits >>= 64 - BITS;
#elif (BITS >= 24)
    bits = BSwap32(in_bits);
    bits >>= (32 - BITS);
#elif (BITS == 16)
    bits = BSwap16(in_bits);
#else   // BITS == 8
    bits = (bit_t)in_bits;
#endif  // BITS > 32
#else   // WORDS_BIGENDIAN
    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* WEBP_RESTRICT const br, int prob,
                                 const char label[]) {
  // 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);
    const int bit = (value > split);
    if (bit) {
      range -= split;
      br->value -= (bit_t)(split + 1) << pos;
    } else {
      range = split + 1;
    }
    {
      const int shift = 7 ^ BitsLog2Floor(range);
      range <<= shift;
      br->bits -= shift;
    }
    br->range = range - 1;
    BT_TRACK(br);
    return bit;
  }
}

// simplified version of VP8GetBit() for prob=0x80 (note shift is always 1 here)
static WEBP_UBSAN_IGNORE_UNSIGNED_OVERFLOW WEBP_INLINE int VP8GetSigned(
    VP8BitReader* WEBP_RESTRICT const br, int v, const char label[]) {
  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 += (range_t)mask;
    br->range |= 1;
    br->value -= (bit_t)((split + 1) & (uint32_t)mask) << pos;
    BT_TRACK(br);
    return (v ^ mask) - mask;
  }
}

static WEBP_INLINE int VP8GetBitAlt(VP8BitReader* WEBP_RESTRICT const br,
                                    int prob, const char label[]) {
  // 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);
    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;
    }
    if (range <= (range_t)0x7e) {
      const int shift = kVP8Log2Range[range];
      range = kVP8NewRange[range];
      br->bits -= shift;
    }
    br->range = range;
    BT_TRACK(br);
    return bit;
  }
}

#ifdef __cplusplus
}  // extern "C"
#endif

#endif  // WEBP_UTILS_BIT_READER_INL_UTILS_H_

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Created: 2026-11-12 07:35