/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef ART_LIBARTBASE_BASE_BIT_STRUCT_H_
#define ART_LIBARTBASE_BASE_BIT_STRUCT_H_
#include "bit_struct_detail.h"
#include "bit_utils.h"
//
// Zero-cost, type-safe, well-defined "structs" of bit fields.
//
// ---------------------------------------------
// Usage example:
// ---------------------------------------------
//
// // Definition for type 'Example'
// BITSTRUCT_DEFINE_START(Example, 10)
// BitStructUint<0, 2> u2; // Every field must be a BitStruct[*].
// BitStructInt<2, 7> i7;
// BitStructUint<9, 1> i1;
// BITSTRUCT_DEFINE_END(Example);
//
// Would define a bit struct with this layout:
// <- 1 -> <-- 7 --> <- 2 ->
// +--------+---------------+-----+
// | i1 | i7 | u2 +
// +--------+---------------+-----+
// 10 9 2 0
//
// // Read-write just like regular values.
// Example ex;
// ex.u2 = 3;
// ex.i7 = -25;
// ex.i1 = true;
// size_t u2 = ex.u2;
// int i7 = ex.i7;
// bool i1 = ex.i1;
//
// // It's packed down to the smallest # of machine words.
// assert(sizeof(Example) == 2);
// // The exact bit pattern is well-defined by the template parameters.
// uint16_t cast = *reinterpret_cast<uint16_t*>(ex);
// assert(cast == ((3) | (0b100111 << 2) | (true << 9);
//
// ---------------------------------------------
// Why not just use C++ bitfields?
// ---------------------------------------------
//
// The layout is implementation-defined.
// We do not know whether the fields are packed left-to-right or
// right-to-left, so it makes it useless when the memory layout needs to be
// precisely controlled.
//
// ---------------------------------------------
// More info:
// ---------------------------------------------
// Currently uintmax_t is the largest supported underlying storage type,
// all (kBitOffset + kBitWidth) must fit into BitSizeOf<uintmax_t>();
//
// Using BitStruct[U]int will automatically select an underlying type
// that's the smallest to fit your (offset + bitwidth).
//
// BitStructNumber can be used to manually select an underlying type.
//
// BitStructField can be used with custom standard-layout structs,
// thus allowing for arbitrary nesting of bit structs.
//
namespace art {
// Zero-cost wrapper around a struct 'T', allowing it to be stored as a bitfield
// at offset 'kBitOffset' and width 'kBitWidth'.
// The storage is plain unsigned int, whose size is the smallest required to fit
// 'kBitOffset + kBitWidth'. All operations to this become BitFieldExtract/BitFieldInsert
// operations to the underlying uint.
//
// Field memory representation:
//
// MSB <-- width --> LSB
// +--------+------------+--------+
// | ?????? | u bitfield | ?????? +
// +--------+------------+--------+
// offset 0
//
// Reading/writing the bitfield (un)packs it into a temporary T:
//
// MSB <-- width --> LSB
// +-----------------+------------+
// | 0.............0 | T bitfield |
// +-----------------+------------+
// 0
//
// It's the responsibility of the StorageType to ensure the bit representation
// of T can be represented by kBitWidth.
template <typename T,
size_t kBitOffset,
size_t kBitWidth = BitStructSizeOf<T>(),
typename StorageType = typename detail::MinimumTypeUnsignedHelper<kBitOffset + kBitWidth>::type>
struct BitStructField {
static_assert(std::is_standard_layout<T>::value, "T must be standard layout");
operator T() const {
return Get();
}
// Exclude overload when T==StorageType.
template <typename _ = void,
typename = std::enable_if_t<std::is_same<T, StorageType>::value, _>>
explicit operator StorageType() const {
return GetStorage();
}
BitStructField& operator=(T value) {
return Assign(*this, value);
}
static constexpr size_t BitStructSizeOf() {
return kBitWidth;
}
BitStructField& operator=(const BitStructField& other) {
// Warning. The default operator= will overwrite the entire storage!
return *this = static_cast<T>(other);
}
BitStructField(const BitStructField& other) {
Assign(*this, static_cast<T>(other));
}
BitStructField() = default;
~BitStructField() = default;
protected:
template <typename T2>
T2& Assign(T2& what, T value) {
// Since C++ doesn't allow the type of operator= to change out
// in the subclass, reimplement operator= in each subclass
// manually and call this helper function.
static_assert(std::is_base_of<BitStructField, T2>::value, "T2 must inherit BitStructField");
what.Set(value);
return what;
}
T Get() const {
ValueStorage vs;
vs.pod_.val_ = GetStorage();
return vs.value_;
}
void Set(T value) {
ValueStorage value_as_storage;
value_as_storage.value_ = value;
storage_.pod_.val_ = BitFieldInsert(storage_.pod_.val_,
value_as_storage.pod_.val_,
kBitOffset,
kBitWidth);
}
private:
StorageType GetStorage() const {
return BitFieldExtract(storage_.pod_.val_, kBitOffset, kBitWidth);
}
// Underlying value must be wrapped in a separate standard-layout struct.
// See below for more details.
struct PodWrapper {
StorageType val_;
};
union ValueStorage {
// Safely alias pod_ and value_ together.
//
// See C++ 9.5.1 [class.union]:
// If a standard-layout union contains several standard-layout structs that share a common
// initial sequence ... it is permitted to inspect the common initial sequence of any of
// standard-layout struct members.
PodWrapper pod_;
T value_;
} storage_;
// Future work: In theory almost non-standard layout can be supported here,
// assuming they don't rely on the address of (this).
// We just have to use memcpy since the union-aliasing would not work.
};
// Base class for number-like BitStruct fields.
// T is the type to store in as a bit field.
// kBitOffset, kBitWidth define the position and length of the bitfield.
//
// (Common usage should be BitStructInt, BitStructUint -- this
// intermediate template allows a user-defined integer to be used.)
template <typename T, size_t kBitOffset, size_t kBitWidth>
struct BitStructNumber : public BitStructField<T, kBitOffset, kBitWidth, /*StorageType*/T> {
using StorageType = T;
BitStructNumber& operator=(T value) {
return BaseType::Assign(*this, value);
}
/*implicit*/ operator T() const {
return Get();
}
explicit operator bool() const {
return static_cast<bool>(Get());
}
BitStructNumber& operator++() {
*this = Get() + 1u;
return *this;
}
StorageType operator++(int) {
return Get() + 1u;
}
BitStructNumber& operator--() {
*this = Get() - 1u;
return *this;
}
StorageType operator--(int) {
return Get() - 1u;
}
private:
using BaseType = BitStructField<T, kBitOffset, kBitWidth, /*StorageType*/T>;
using BaseType::Get;
};
// Create a BitStruct field which uses the smallest underlying int storage type,
// in order to be large enough to fit (kBitOffset + kBitWidth).
//
// Values are sign-extended when they are read out.
template <size_t kBitOffset, size_t kBitWidth>
using BitStructInt =
BitStructNumber<typename detail::MinimumTypeHelper<int, kBitOffset + kBitWidth>::type,
kBitOffset,
kBitWidth>;
// Create a BitStruct field which uses the smallest underlying uint storage type,
// in order to be large enough to fit (kBitOffset + kBitWidth).
//
// Values are zero-extended when they are read out.
template <size_t kBitOffset, size_t kBitWidth>
using BitStructUint =
BitStructNumber<typename detail::MinimumTypeHelper<unsigned int, kBitOffset + kBitWidth>::type,
kBitOffset,
kBitWidth>;
// Start a definition for a bitstruct.
// A bitstruct is defined to be a union with a common initial subsequence
// that we call 'DefineBitStructSize<bitwidth>'.
//
// See top of file for usage example.
//
// This marker is required by the C++ standard in order to
// have a "common initial sequence".
//
// See C++ 9.5.1 [class.union]:
// If a standard-layout union contains several standard-layout structs that share a common
// initial sequence ... it is permitted to inspect the common initial sequence of any of
// standard-layout struct members.
#define BITSTRUCT_DEFINE_START(name, bitwidth) \
union name { /* NOLINT */ \
art::detail::DefineBitStructSize<(bitwidth)> _; \
static constexpr size_t BitStructSizeOf() { return (bitwidth); } \
name& operator=(const name& other) { _ = other._; return *this; } /* NOLINT */ \
name(const name& other) : _(other._) {} \
name() = default; \
~name() = default;
// End the definition of a bitstruct, and insert a sanity check
// to ensure that the bitstruct did not exceed the specified size.
//
// See top of file for usage example.
#define BITSTRUCT_DEFINE_END(name) \
}; \
static_assert(art::detail::ValidateBitStructSize<name>(), \
#name "bitsize incorrect: " \
"did you insert extra fields that weren't BitStructX, " \
"and does the size match the sum of the field widths?")
// Determine the minimal bit size for a user-defined type T.
// Used by BitStructField to determine how small a custom type is.
template <typename T>
static constexpr size_t BitStructSizeOf() {
return T::BitStructSizeOf();
}
} // namespace art
#endif // ART_LIBARTBASE_BASE_BIT_STRUCT_H_