bit-field.h

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// Copyright 2019 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef V8_BASE_BIT_FIELD_H_
#define V8_BASE_BIT_FIELD_H_
 
#include <stdint.h>
 
#include <algorithm>
 
#include "src/base/macros.h"
 
namespace v8 {
namespace base {
 
// ----------------------------------------------------------------------------
// BitField is a help template for encoding and decode bitfield with
// unsigned content.
// Instantiate them via 'using', which is cheaper than deriving a new class:
// using MyBitField = base::BitField<MyEnum, 4, 2>;
// The BitField class is final to enforce this style over derivation.
 
template <class T, int shift, int size, class U = uint32_t>
class BitField final {
 public:
  static_assert(std::is_unsigned<U>::value);
  static_assert(shift < 8 * sizeof(U));  // Otherwise shifts by {shift} are UB.
  static_assert(size < 8 * sizeof(U));   // Otherwise shifts by {size} are UB.
  static_assert(shift + size <= 8 * sizeof(U));
  static_assert(size > 0);
 
  using FieldType = T;
  using BaseType = U;
 
  // A type U mask of bit field.  To use all bits of a type U of x bits
  // in a bitfield without compiler warnings we have to compute 2^x
  // without using a shift count of x in the computation.
  static constexpr int kShift = shift;
  static constexpr int kSize = size;
  static constexpr U kMask = ((U{1} << kShift) << kSize) - (U{1} << kShift);
  static constexpr int kLastUsedBit = kShift + kSize - 1;
  static constexpr U kNumValues = U{1} << kSize;
  static constexpr U kMax = kNumValues - 1;
 
  template <class T2, int size2>
  using Next = BitField<T2, kShift + kSize, size2, U>;
 
  // Tells whether the provided value fits into the bit field.
  static constexpr bool is_valid(T value) {
    return (static_cast<U>(value) & ~kMax) == 0;
  }
 
  // Returns a type U with the bit field value encoded.
  static constexpr U encode(T value) {
    DCHECK(is_valid(value));
    return static_cast<U>(value) << kShift;
  }
 
  // Returns a type U with the bit field value updated.
  V8_NODISCARD static constexpr U update(U previous, T value) {
    return (previous & ~kMask) | encode(value);
  }
 
  // Extracts the bit field from the value.
  static constexpr T decode(U value) {
    return static_cast<T>((value & kMask) >> kShift);
  }
};
 
// ----------------------------------------------------------------------------
// BitFieldUnion can be used to combine two linear BitFields.
// So far only the static mask is computed. Encoding and decoding tbd.
// Can be used for example as a quick combined check:
//   `if (BitFieldUnion<BFA, BFB>::kMask & bitfield) ...`
 
template <typename A, typename B>
class BitFieldUnion final {
 public:
  static_assert(
      std::is_same<typename A::BaseType, typename B::BaseType>::value);
  static_assert((A::kMask & B::kMask) == 0);
  static constexpr int kShift = std::min(A::kShift, B::kShift);
  static constexpr int kMask = A::kMask | B::kMask;
  static constexpr int kSize =
      A::kSize + B::kSize + (std::max(A::kShift, B::kShift) - kShift);
};
 
template <class T, int shift, int size>
using BitField8 = BitField<T, shift, size, uint8_t>;
 
template <class T, int shift, int size>
using BitField16 = BitField<T, shift, size, uint16_t>;
 
template <class T, int shift, int size>
using BitField64 = BitField<T, shift, size, uint64_t>;
 
// Helper macros for defining a contiguous sequence of bit fields. Example:
// (backslashes at the ends of respective lines of this multi-line macro
// definition are omitted here to please the compiler)
//
// #define MAP_BIT_FIELD1(V, _)
//   V(IsAbcBit, bool, 1, _)
//   V(IsBcdBit, bool, 1, _)
//   V(CdeBits, int, 5, _)
//   V(DefBits, MutableMode, 1, _)
//
// DEFINE_BIT_FIELDS(MAP_BIT_FIELD1)
// or
// DEFINE_BIT_FIELDS_64(MAP_BIT_FIELD1)
//
#define DEFINE_BIT_FIELD_RANGE_TYPE(Name, Type, Size, _) \
  k##Name##Start, k##Name##End = k##Name##Start + Size - 1,
 
#define DEFINE_BIT_RANGES(LIST_MACRO)                               \
  struct LIST_MACRO##_Ranges {                                      \
    enum { LIST_MACRO(DEFINE_BIT_FIELD_RANGE_TYPE, _) kBitsCount }; \
  };
 
#define DEFINE_BIT_FIELD_TYPE(Name, Type, Size, RangesName) \
  using Name = base::BitField<Type, RangesName::k##Name##Start, Size>;
 
#define DEFINE_BIT_FIELD_64_TYPE(Name, Type, Size, RangesName) \
  using Name = base::BitField64<Type, RangesName::k##Name##Start, Size>;
 
#define DEFINE_BIT_FIELDS(LIST_MACRO) \
  DEFINE_BIT_RANGES(LIST_MACRO)       \
  LIST_MACRO(DEFINE_BIT_FIELD_TYPE, LIST_MACRO##_Ranges)
 
#define DEFINE_BIT_FIELDS_64(LIST_MACRO) \
  DEFINE_BIT_RANGES(LIST_MACRO)          \
  LIST_MACRO(DEFINE_BIT_FIELD_64_TYPE, LIST_MACRO##_Ranges)
 
// ----------------------------------------------------------------------------
// BitSetComputer is a help template for encoding and decoding information for
// a variable number of items in an array.
//
// To encode boolean data in a smi array you would use:
//  using BoolComputer = BitSetComputer<bool, 1, kSmiValueSize, uint32_t>;
//
template <class T, int kBitsPerItem, int kBitsPerWord, class U>
class BitSetComputer {
 public:
  static const int kItemsPerWord = kBitsPerWord / kBitsPerItem;
  static const int kMask = (1 << kBitsPerItem) - 1;
 
  // The number of array elements required to embed T information for each item.
  static int word_count(int items) {
    if (items == 0) return 0;
    return (items - 1) / kItemsPerWord + 1;
  }
 
  // The array index to look at for item.
  static int index(int base_index, int item) {
    return base_index + item / kItemsPerWord;
  }
 
  // Extract T data for a given item from data.
  static T decode(U data, int item) {
    return static_cast<T>((data >> shift(item)) & kMask);
  }
 
  // Return the encoding for a store of value for item in previous.
  static U encode(U previous, int item, T value) {
    int shift_value = shift(item);
    int set_bits = (static_cast<int>(value) << shift_value);
    return (previous & ~(kMask << shift_value)) | set_bits;
  }
 
  static int shift(int item) { return (item % kItemsPerWord) * kBitsPerItem; }
};
 
}  // namespace base
}  // namespace v8
 
#endif  // V8_BASE_BIT_FIELD_H_