runtime/vm/bitfield.h

// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#ifndef RUNTIME_VM_BITFIELD_H_
#define RUNTIME_VM_BITFIELD_H_

#include <type_traits>

#include "platform/globals.h"

namespace dart {

static const uword kUwordOne = 1U;

// BitField is a template for encoding and decoding a value of type T
// inside a storage of type S.
template <typename S,
          typename T,
          int position,
          int size = (sizeof(S) * kBitsPerByte) - position,
          bool sign_extend = false>
class BitField {
 public:
  typedef T Type;

  static_assert((sizeof(S) * kBitsPerByte) >= (position + size),
                "BitField does not fit into the type.");
  static_assert(!sign_extend || std::is_signed<T>::value,
                "Should only sign extend signed bitfield types");

  static const intptr_t kNextBit = position + size;

  // Tells whether the provided value fits into the bit field.
  static constexpr bool is_valid(T value) {
    return decode(encode_unchecked(value)) == value;
  }

  // Returns a S mask of the bit field.
  static constexpr S mask() { return (kUwordOne << size) - 1; }

  // Returns a S mask of the bit field which can be applied directly to
  // to the raw unshifted bits.
  static constexpr S mask_in_place() { return mask() << position; }

  // Returns the shift count needed to right-shift the bit field to
  // the least-significant bits.
  static constexpr int shift() { return position; }

  // Returns the size of the bit field.
  static constexpr int bitsize() { return size; }

  // Returns an S with the bit field value encoded.
  static constexpr S encode(T value) {
    assert(is_valid(value));
    return encode_unchecked(value);
  }

  // Extracts the bit field from the value.
  static constexpr T decode(S value) {
    // Ensure we slide down the sign bit if the value in the bit field is signed
    // and negative. We use 64-bit ints inside the expression since we can have
    // both cases: sizeof(S) > sizeof(T) or sizeof(S) < sizeof(T).
    if constexpr (sign_extend) {
      auto const u = static_cast<uint64_t>(value);
      return static_cast<T>((static_cast<int64_t>(u << (64 - kNextBit))) >>
                            (64 - size));
    } else {
      auto const u = static_cast<typename std::make_unsigned<S>::type>(value);
      return static_cast<T>((u >> position) & mask());
    }
  }

  // Returns an S with the bit field value encoded based on the
  // original value. Only the bits corresponding to this bit field
  // will be changed.
  static constexpr S update(T value, S original) {
    return encode(value) | (~mask_in_place() & original);
  }

 private:
  // Returns an S with the bit field value encoded.
  static constexpr S encode_unchecked(T value) {
    auto const u = static_cast<typename std::make_unsigned<S>::type>(value);
    return (u & mask()) << position;
  }
};

}  // namespace dart

#endif  // RUNTIME_VM_BITFIELD_H_