<atomic> design

The compiler supplies all of the intrinsics as described below. This list of intrinsics roughly parallels the requirements of the C and C++ atomics proposals. The C and C++ library imlpementations simply drop through to these intrinsics. Anything the platform does not support in hardware, the compiler arranges for a (compiler-rt) library call to be made which will do the job with a mutex, and in this case ignoring the memory ordering parameter (effectively implementing memory_order_seq_cst).

Ultimate efficiency is preferred over run time error checking. Undefined behavior is acceptable when the inputs do not conform as defined below.

// In every intrinsic signature below, type* atomic_obj may be a pointer to a
//    volatile-qualifed type.
// Memory ordering values map to the following meanings:
//   memory_order_relaxed == 0
//   memory_order_consume == 1
//   memory_order_acquire == 2
//   memory_order_release == 3
//   memory_order_acq_rel == 4
//   memory_order_seq_cst == 5

// type must be trivially copyable
// type represents a "type argument"
bool __atomic_is_lock_free(type);

// type must be trivially copyable
// Behavior is defined for mem_ord = 0, 1, 2, 5
type __atomic_load(const type* atomic_obj, int mem_ord);

// type must be trivially copyable
// Behavior is defined for mem_ord = 0, 3, 5
void __atomic_store(type* atomic_obj, type desired, int mem_ord);

// type must be trivially copyable
// Behavior is defined for mem_ord = [0 ... 5]
type __atomic_exchange(type* atomic_obj, type desired, int mem_ord);

// type must be trivially copyable
// Behavior is defined for mem_success = [0 ... 5],
//   mem_failure <= mem_success
//   mem_failure != 3
//   mem_failure != 4
bool __atomic_compare_exchange_strong(type* atomic_obj,
                                      type* expected, type desired,
                                      int mem_success, int mem_failure);

// type must be trivially copyable
// Behavior is defined for mem_success = [0 ... 5],
//   mem_failure <= mem_success
//   mem_failure != 3
//   mem_failure != 4
bool __atomic_compare_exchange_weak(type* atomic_obj,
                                    type* expected, type desired,
                                    int mem_success, int mem_failure);

// type is one of: char, signed char, unsigned char, short, unsigned short, int,
//      unsigned int, long, unsigned long, long long, unsigned long long,
//      char16_t, char32_t, wchar_t
// Behavior is defined for mem_ord = [0 ... 5]
type __atomic_fetch_add(type* atomic_obj, type operand, int mem_ord);

// type is one of: char, signed char, unsigned char, short, unsigned short, int,
//      unsigned int, long, unsigned long, long long, unsigned long long,
//      char16_t, char32_t, wchar_t
// Behavior is defined for mem_ord = [0 ... 5]
type __atomic_fetch_sub(type* atomic_obj, type operand, int mem_ord);

// type is one of: char, signed char, unsigned char, short, unsigned short, int,
//      unsigned int, long, unsigned long, long long, unsigned long long,
//      char16_t, char32_t, wchar_t
// Behavior is defined for mem_ord = [0 ... 5]
type __atomic_fetch_and(type* atomic_obj, type operand, int mem_ord);

// type is one of: char, signed char, unsigned char, short, unsigned short, int,
//      unsigned int, long, unsigned long, long long, unsigned long long,
//      char16_t, char32_t, wchar_t
// Behavior is defined for mem_ord = [0 ... 5]
type __atomic_fetch_or(type* atomic_obj, type operand, int mem_ord);

// type is one of: char, signed char, unsigned char, short, unsigned short, int,
//      unsigned int, long, unsigned long, long long, unsigned long long,
//      char16_t, char32_t, wchar_t
// Behavior is defined for mem_ord = [0 ... 5]
type __atomic_fetch_xor(type* atomic_obj, type operand, int mem_ord);

// Behavior is defined for mem_ord = [0 ... 5]
void* __atomic_fetch_add(void** atomic_obj, ptrdiff_t operand, int mem_ord);
void* __atomic_fetch_sub(void** atomic_obj, ptrdiff_t operand, int mem_ord);

// Behavior is defined for mem_ord = [0 ... 5]
void __atomic_thread_fence(int mem_ord);
void __atomic_signal_fence(int mem_ord);

If desired the intrinsics taking a single mem_ord parameter can default this argument to 5.

If desired the intrinsics taking two ordering parameters can default mem_success to 5, and mem_failure to translate_memory_order(mem_success) where translate_memory_order(mem_success) is defined as:

int
translate_memory_order(int o)
{
    switch (o)
    {
    case 4:
        return 2;
    case 3:
        return 0;
    }
    return o;
}

Below are representative C++ implementations of all of the operations. Their purpose is to document the desired semantics of each operation, assuming memory_order_seq_cst. This is essentially the code that will be called if the front end calls out to compiler-rt.

template <class T>
T
__atomic_load(T const volatile* obj)
{
    unique_lock<mutex> _(some_mutex);
    return *obj;
}

template <class T>
void
__atomic_store(T volatile* obj, T desr)
{
    unique_lock<mutex> _(some_mutex);
    *obj = desr;
}

template <class T>
T
__atomic_exchange(T volatile* obj, T desr)
{
    unique_lock<mutex> _(some_mutex);
    T r = *obj;
    *obj = desr;
    return r;
}

template <class T>
bool
__atomic_compare_exchange_strong(T volatile* obj, T* exp, T desr)
{
    unique_lock<mutex> _(some_mutex);
    if (std::memcmp(const_cast<T*>(obj), exp, sizeof(T)) == 0) // if (*obj == *exp)
    {
        std::memcpy(const_cast<T*>(obj), &desr, sizeof(T)); // *obj = desr;
        return true;
    }
    std::memcpy(exp, const_cast<T*>(obj), sizeof(T)); // *exp = *obj;
    return false;
}

// May spuriously return false (even if *obj == *exp)
template <class T>
bool
__atomic_compare_exchange_weak(T volatile* obj, T* exp, T desr)
{
    unique_lock<mutex> _(some_mutex);
    if (std::memcmp(const_cast<T*>(obj), exp, sizeof(T)) == 0) // if (*obj == *exp)
    {
        std::memcpy(const_cast<T*>(obj), &desr, sizeof(T)); // *obj = desr;
        return true;
    }
    std::memcpy(exp, const_cast<T*>(obj), sizeof(T)); // *exp = *obj;
    return false;
}

template <class T>
T
__atomic_fetch_add(T volatile* obj, T operand)
{
    unique_lock<mutex> _(some_mutex);
    T r = *obj;
    *obj += operand;
    return r;
}

template <class T>
T
__atomic_fetch_sub(T volatile* obj, T operand)
{
    unique_lock<mutex> _(some_mutex);
    T r = *obj;
    *obj -= operand;
    return r;
}

template <class T>
T
__atomic_fetch_and(T volatile* obj, T operand)
{
    unique_lock<mutex> _(some_mutex);
    T r = *obj;
    *obj &= operand;
    return r;
}

template <class T>
T
__atomic_fetch_or(T volatile* obj, T operand)
{
    unique_lock<mutex> _(some_mutex);
    T r = *obj;
    *obj |= operand;
    return r;
}

template <class T>
T
__atomic_fetch_xor(T volatile* obj, T operand)
{
    unique_lock<mutex> _(some_mutex);
    T r = *obj;
    *obj ^= operand;
    return r;
}

void*
__atomic_fetch_add(void* volatile* obj, ptrdiff_t operand)
{
    unique_lock<mutex> _(some_mutex);
    void* r = *obj;
    (char*&)(*obj) += operand;
    return r;
}

void*
__atomic_fetch_sub(void* volatile* obj, ptrdiff_t operand)
{
    unique_lock<mutex> _(some_mutex);
    void* r = *obj;
    (char*&)(*obj) -= operand;
    return r;
}

void __atomic_thread_fence()
{
    unique_lock<mutex> _(some_mutex);
}

void __atomic_signal_fence()
{
    unique_lock<mutex> _(some_mutex);
}