1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   15
   16
   17
   18
   19
   20
   21
   22
   23
   24
   25
   26
   27
   28
   29
   30
   31
   32
   33
   34
   35
   36
   37
   38
   39
   40
   41
   42
   43
   44
   45
   46
   47
   48
   49
   50
   51
   52
   53
   54
   55
   56
   57
   58
   59
   60
   61
   62
   63
   64
   65
   66
   67
   68
   69
   70
   71
   72
   73
   74
   75
   76
   77
   78
   79
   80
   81
   82
   83
   84
   85
   86
   87
   88
   89
   90
   91
   92
   93
   94
   95
   96
   97
   98
   99
  100
  101
  102
  103
  104
  105
  106
  107
  108
  109
  110
  111
  112
  113
  114
  115
  116
  117
  118
  119
  120
  121
  122
  123
  124
  125
  126
  127
  128
  129
  130
  131
  132
  133
  134
  135

base / bits.h [blame]

// Copyright 2013 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// This file defines some bit utilities.

#ifndef BASE_BITS_H_
#define BASE_BITS_H_

#include <stddef.h>
#include <stdint.h>

#include <bit>
#include <concepts>

#include "base/check.h"

namespace base::bits {

// Bit functions in <bit> are restricted to a specific set of types of unsigned
// integer; restrict functions in this file that are related to those in that
// header to match for consistency.
template <typename T>
concept UnsignedInteger =
    std::unsigned_integral<T> && !std::same_as<T, bool> &&
    !std::same_as<T, char> && !std::same_as<T, char8_t> &&
    !std::same_as<T, char16_t> && !std::same_as<T, char32_t> &&
    !std::same_as<T, wchar_t>;

// We want to migrate all users of these functions to use the unsigned type
// versions of the functions, but until they are all moved over, create a
// concept that captures all the types that must be supported for compatibility
// but that we want to remove.
//
// TODO(crbug.com/40256225): Switch uses to supported functions and
// remove.
template <typename T>
concept SignedIntegerDeprecatedDoNotUse =
    std::integral<T> && !UnsignedInteger<T>;

// Round down |size| to a multiple of alignment, which must be a power of two.
template <typename T>
  requires UnsignedInteger<T>
inline constexpr T AlignDown(T size, T alignment) {
  DCHECK(std::has_single_bit(alignment));
  return size & ~(alignment - 1);
}

// Round down |size| to a multiple of alignment, which must be a power of two.
// DEPRECATED; use the UnsignedInteger version.
//
// TODO(crbug.com/40256225): Switch uses and remove.
template <typename T>
inline constexpr auto AlignDownDeprecatedDoNotUse(T size, T alignment) {
  using U = std::make_unsigned_t<T>;
  DCHECK(std::has_single_bit(static_cast<U>(alignment)));
  return static_cast<U>(size) & ~static_cast<U>(alignment - 1);
}

// Move |ptr| back to the previous multiple of alignment, which must be a power
// of two. Defined for types where sizeof(T) is one byte.
template <typename T>
  requires(sizeof(T) == 1)
inline T* AlignDown(T* ptr, uintptr_t alignment) {
  return reinterpret_cast<T*>(
      AlignDown(reinterpret_cast<uintptr_t>(ptr), alignment));
}

// Round up |size| to a multiple of alignment, which must be a power of two.
template <typename T>
  requires UnsignedInteger<T>
inline constexpr T AlignUp(T size, T alignment) {
  DCHECK(std::has_single_bit(alignment));
  return (size + alignment - 1) & ~(alignment - 1);
}

// Round up |size| to a multiple of alignment, which must be a power of two.
// DEPRECATED; use the UnsignedInteger version.
//
// TODO(crbug.com/40256225): Switch uses and remove.
template <typename T>
  requires SignedIntegerDeprecatedDoNotUse<T>
inline constexpr T AlignUpDeprecatedDoNotUse(T size, T alignment) {
  using U = std::make_unsigned_t<T>;
  DCHECK(std::has_single_bit(static_cast<U>(alignment)));
  return static_cast<U>(size + alignment - 1) & ~static_cast<U>(alignment - 1);
}

// Advance |ptr| to the next multiple of alignment, which must be a power of
// two. Defined for types where sizeof(T) is one byte.
template <typename T>
  requires(sizeof(T) == 1)
inline T* AlignUp(T* ptr, uintptr_t alignment) {
  return reinterpret_cast<T*>(
      AlignUp(reinterpret_cast<uintptr_t>(ptr), alignment));
}

// Returns the integer i such as 2^i <= n < 2^(i+1).
//
// A common use for this function is to measure the number of bits required to
// contain a value; for that case use std::bit_width().
//
// A common use for this function is to take its result and use it to left-shift
// a bit; instead of doing so, use std::bit_floor().
constexpr int Log2Floor(uint32_t n) {
  return 31 - std::countl_zero(n);
}

// Returns the integer i such as 2^(i-1) < n <= 2^i.
//
// A common use for this function is to measure the number of bits required to
// contain a value; for that case use std::bit_width().
//
// A common use for this function is to take its result and use it to left-shift
// a bit; instead of doing so, use std::bit_ceil().
constexpr int Log2Ceiling(uint32_t n) {
  // When n == 0, we want the function to return -1.
  // When n == 0, (n - 1) will underflow to 0xFFFFFFFF, which is
  // why the statement below starts with (n ? 32 : -1).
  return (n ? 32 : -1) - std::countl_zero(n - 1);
}

// Returns a value of type T with a single bit set in the left-most position.
// Can be used instead of manually shifting a 1 to the left. Unlike the other
// functions in this file, usable for any integral type.
template <typename T>
  requires std::integral<T>
constexpr T LeftmostBit() {
  T one(1u);
  return one << (8 * sizeof(T) - 1);
}

}  // namespace base::bits

#endif  // BASE_BITS_H_