<atomic> Design

There were originally 3 designs under consideration. They differ in where most of the implementation work is done. The functionality exposed to the customer should be identical (and conforming) for all three designs.

Design A: Minimal work for the library

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 implementations 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-qualified 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);
}

Design B: Something in between

This is a variation of design A which puts the burden on the library to arrange for the correct manipulation of the run time memory ordering arguments, and only calls the compiler for well-defined memory orderings. I think of this design as the worst of A and C, instead of the best of A and C. But I offer it as an option in the spirit of completeness.

// type must be trivially copyable
bool __atomic_is_lock_free(const type* atomic_obj);

// type must be trivially copyable
type __atomic_load_relaxed(const volatile type* atomic_obj);
type __atomic_load_consume(const volatile type* atomic_obj);
type __atomic_load_acquire(const volatile type* atomic_obj);
type __atomic_load_seq_cst(const volatile type* atomic_obj);

// type must be trivially copyable
type __atomic_store_relaxed(volatile type* atomic_obj, type desired);
type __atomic_store_release(volatile type* atomic_obj, type desired);
type __atomic_store_seq_cst(volatile type* atomic_obj, type desired);

// type must be trivially copyable
type __atomic_exchange_relaxed(volatile type* atomic_obj, type desired);
type __atomic_exchange_consume(volatile type* atomic_obj, type desired);
type __atomic_exchange_acquire(volatile type* atomic_obj, type desired);
type __atomic_exchange_release(volatile type* atomic_obj, type desired);
type __atomic_exchange_acq_rel(volatile type* atomic_obj, type desired);
type __atomic_exchange_seq_cst(volatile type* atomic_obj, type desired);

// type must be trivially copyable
bool __atomic_compare_exchange_strong_relaxed_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_consume_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_consume_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_acquire_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_acquire_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_acquire_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_release_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_release_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_release_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_acq_rel_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_acq_rel_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_acq_rel_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_seq_cst_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_seq_cst_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_seq_cst_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_strong_seq_cst_seq_cst(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);

// type must be trivially copyable
bool __atomic_compare_exchange_weak_relaxed_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_consume_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_consume_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_acquire_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_acquire_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_acquire_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_release_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_release_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_release_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_acq_rel_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_acq_rel_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_acq_rel_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_seq_cst_relaxed(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_seq_cst_consume(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_seq_cst_acquire(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);
bool __atomic_compare_exchange_weak_seq_cst_seq_cst(volatile type* atomic_obj,
                                                    type* expected,
                                                    type desired);

// 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
type __atomic_fetch_add_relaxed(volatile type* atomic_obj, type operand);
type __atomic_fetch_add_consume(volatile type* atomic_obj, type operand);
type __atomic_fetch_add_acquire(volatile type* atomic_obj, type operand);
type __atomic_fetch_add_release(volatile type* atomic_obj, type operand);
type __atomic_fetch_add_acq_rel(volatile type* atomic_obj, type operand);
type __atomic_fetch_add_seq_cst(volatile type* atomic_obj, type operand);

// 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
type __atomic_fetch_sub_relaxed(volatile type* atomic_obj, type operand);
type __atomic_fetch_sub_consume(volatile type* atomic_obj, type operand);
type __atomic_fetch_sub_acquire(volatile type* atomic_obj, type operand);
type __atomic_fetch_sub_release(volatile type* atomic_obj, type operand);
type __atomic_fetch_sub_acq_rel(volatile type* atomic_obj, type operand);
type __atomic_fetch_sub_seq_cst(volatile type* atomic_obj, type operand);

// 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
type __atomic_fetch_and_relaxed(volatile type* atomic_obj, type operand);
type __atomic_fetch_and_consume(volatile type* atomic_obj, type operand);
type __atomic_fetch_and_acquire(volatile type* atomic_obj, type operand);
type __atomic_fetch_and_release(volatile type* atomic_obj, type operand);
type __atomic_fetch_and_acq_rel(volatile type* atomic_obj, type operand);
type __atomic_fetch_and_seq_cst(volatile type* atomic_obj, type operand);

// 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
type __atomic_fetch_or_relaxed(volatile type* atomic_obj, type operand);
type __atomic_fetch_or_consume(volatile type* atomic_obj, type operand);
type __atomic_fetch_or_acquire(volatile type* atomic_obj, type operand);
type __atomic_fetch_or_release(volatile type* atomic_obj, type operand);
type __atomic_fetch_or_acq_rel(volatile type* atomic_obj, type operand);
type __atomic_fetch_or_seq_cst(volatile type* atomic_obj, type operand);

// 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
type __atomic_fetch_xor_relaxed(volatile type* atomic_obj, type operand);
type __atomic_fetch_xor_consume(volatile type* atomic_obj, type operand);
type __atomic_fetch_xor_acquire(volatile type* atomic_obj, type operand);
type __atomic_fetch_xor_release(volatile type* atomic_obj, type operand);
type __atomic_fetch_xor_acq_rel(volatile type* atomic_obj, type operand);
type __atomic_fetch_xor_seq_cst(volatile type* atomic_obj, type operand);

void* __atomic_fetch_add_relaxed(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_add_consume(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_add_acquire(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_add_release(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_add_acq_rel(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_add_seq_cst(void* volatile* atomic_obj, ptrdiff_t operand);

void* __atomic_fetch_sub_relaxed(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_sub_consume(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_sub_acquire(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_sub_release(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_sub_acq_rel(void* volatile* atomic_obj, ptrdiff_t operand);
void* __atomic_fetch_sub_seq_cst(void* volatile* atomic_obj, ptrdiff_t operand);

void __atomic_thread_fence_relaxed();
void __atomic_thread_fence_consume();
void __atomic_thread_fence_acquire();
void __atomic_thread_fence_release();
void __atomic_thread_fence_acq_rel();
void __atomic_thread_fence_seq_cst();

void __atomic_signal_fence_relaxed();
void __atomic_signal_fence_consume();
void __atomic_signal_fence_acquire();
void __atomic_signal_fence_release();
void __atomic_signal_fence_acq_rel();
void __atomic_signal_fence_seq_cst();

Design C: Minimal work for the front end

The <atomic> header is one of the most closely coupled headers to the compiler. Ideally when you invoke any function from <atomic>, it should result in highly optimized assembly being inserted directly into your application – assembly that is not otherwise representable by higher level C or C++ expressions. The design of the libc++ <atomic> header started with this goal in mind. A secondary, but still very important goal is that the compiler should have to do minimal work to facilitate the implementation of <atomic>. Without this second goal, then practically speaking, the libc++ <atomic> header would be doomed to be a barely supported, second class citizen on almost every platform.

Goals:

• Optimal code generation for atomic operations

• Minimal effort for the compiler to achieve goal 1 on any given platform

• Conformance to the C++0X draft standard

The purpose of this document is to inform compiler writers what they need to do to enable a high performance libc++ <atomic> with minimal effort.

The minimal work that must be done for a conforming <atomic>

The only “atomic” operations that must actually be lock free in <atomic> are represented by the following compiler intrinsics:

__atomic_flag__ __atomic_exchange_seq_cst(__atomic_flag__ volatile* obj, __atomic_flag__ desr) {
    unique_lock<mutex> _(some_mutex);
    __atomic_flag__ result = *obj;
    *obj = desr;
    return result;
}

void __atomic_store_seq_cst(__atomic_flag__ volatile* obj, __atomic_flag__ desr) {
    unique_lock<mutex> _(some_mutex);
    *obj = desr;
}

Where:

• If __has_feature(__atomic_flag) evaluates to 1 in the preprocessor then the compiler must define __atomic_flag__ (e.g. as a typedef to int).

• If __has_feature(__atomic_flag) evaluates to 0 in the preprocessor then the library defines __atomic_flag__ as a typedef to bool.

• To communicate that the above intrinsics are available, the compiler must arrange for __has_feature to return 1 when fed the intrinsic name appended with an ‘_’ and the mangled type name of __atomic_flag__.

For example if __atomic_flag__ is unsigned int:

// __has_feature(__atomic_flag) == 1
// __has_feature(__atomic_exchange_seq_cst_j) == 1
// __has_feature(__atomic_store_seq_cst_j) == 1

typedef unsigned int __atomic_flag__;

unsigned int __atomic_exchange_seq_cst(unsigned int volatile*, unsigned int) {
    // ...
}

void __atomic_store_seq_cst(unsigned int volatile*, unsigned int) {
    // ...
}

That’s it! Compiler writers do the above and you’ve got a fully conforming (though sub-par performance) <atomic> header!

Recommended work for a higher performance <atomic>

It would be good if the above intrinsics worked with all integral types plus void*. Because this may not be possible to do in a lock-free manner for all integral types on all platforms, a compiler must communicate each type that an intrinsic works with. For example, if __atomic_exchange_seq_cst works for all types except for long long and unsigned long long then:

__has_feature(__atomic_exchange_seq_cst_b) == 1  // bool
__has_feature(__atomic_exchange_seq_cst_c) == 1  // char
__has_feature(__atomic_exchange_seq_cst_a) == 1  // signed char
__has_feature(__atomic_exchange_seq_cst_h) == 1  // unsigned char
__has_feature(__atomic_exchange_seq_cst_Ds) == 1 // char16_t
__has_feature(__atomic_exchange_seq_cst_Di) == 1 // char32_t
__has_feature(__atomic_exchange_seq_cst_w) == 1  // wchar_t
__has_feature(__atomic_exchange_seq_cst_s) == 1  // short
__has_feature(__atomic_exchange_seq_cst_t) == 1  // unsigned short
__has_feature(__atomic_exchange_seq_cst_i) == 1  // int
__has_feature(__atomic_exchange_seq_cst_j) == 1  // unsigned int
__has_feature(__atomic_exchange_seq_cst_l) == 1  // long
__has_feature(__atomic_exchange_seq_cst_m) == 1  // unsigned long
__has_feature(__atomic_exchange_seq_cst_Pv) == 1 // void*

Note that only the __has_feature flag is decorated with the argument type. The name of the compiler intrinsic is not decorated, but instead works like a C++ overloaded function.

Additionally, there are other intrinsics besides __atomic_exchange_seq_cst and __atomic_store_seq_cst. They are optional. But if the compiler can generate faster code than provided by the library, then clients will benefit from the compiler writer’s expertise and knowledge of the targeted platform.

Below is the complete list of sequentially consistent intrinsics, and their library implementations. Template syntax is used to indicate the desired overloading for integral and void* types. The template does not represent a requirement that the intrinsic operate on any type!

// T is one of:
// bool, 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, void*

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

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

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

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

// T 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

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

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

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

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

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

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

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

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

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

One should consult the (currently draft) C++ Standard for the details of the definitions for these operations. For example, __atomic_compare_exchange_weak_seq_cst_seq_cst is allowed to fail spuriously while __atomic_compare_exchange_strong_seq_cst_seq_cst is not.

If on your platform the lock-free definition of __atomic_compare_exchange_weak_seq_cst_seq_cst would be the same as __atomic_compare_exchange_strong_seq_cst_seq_cst, you may omit the __atomic_compare_exchange_weak_seq_cst_seq_cst intrinsic without a performance cost. The library will prefer your implementation of __atomic_compare_exchange_strong_seq_cst_seq_cst over its own definition for implementing __atomic_compare_exchange_weak_seq_cst_seq_cst. That is, the library will arrange for __atomic_compare_exchange_weak_seq_cst_seq_cst to call __atomic_compare_exchange_strong_seq_cst_seq_cst if you supply an intrinsic for the strong version but not the weak.

Taking advantage of weaker memory synchronization

So far, all of the intrinsics presented require a sequentially consistent memory ordering. That is, no loads or stores can move across the operation (just as if the library had locked that internal mutex). But <atomic> supports weaker memory ordering operations. In all, there are six memory orderings (listed here from strongest to weakest):

memory_order_seq_cst
memory_order_acq_rel
memory_order_release
memory_order_acquire
memory_order_consume
memory_order_relaxed

(See the C++ Standard for the detailed definitions of each of these orderings).

On some platforms, the compiler vendor can offer some or even all of the above intrinsics at one or more weaker levels of memory synchronization. This might lead for example to not issuing an mfence instruction on the x86.

If the compiler does not offer any given operation, at any given memory ordering level, the library will automatically attempt to call the next highest memory ordering operation. This continues up to seq_cst, and if that doesn’t exist, then the library takes over and does the job with a mutex. This is a compile-time search and selection operation. At run time, the application will only see the few inlined assembly instructions for the selected intrinsic.

Each intrinsic is appended with the 7-letter name of the memory ordering it addresses. For example a load with relaxed ordering is defined by:

T __atomic_load_relaxed(const volatile T* obj);

And announced with:

__has_feature(__atomic_load_relaxed_b) == 1  // bool
__has_feature(__atomic_load_relaxed_c) == 1  // char
__has_feature(__atomic_load_relaxed_a) == 1  // signed char
...

The __atomic_compare_exchange_strong(weak) intrinsics are parameterized on two memory orderings. The first ordering applies when the operation returns true and the second ordering applies when the operation returns false.

Not every memory ordering is appropriate for every operation. exchange and the fetch_XXX operations support all 6. But load only supports relaxed, consume, acquire and seq_cst. store only supports relaxed, release, and seq_cst. The compare_exchange operations support the following 16 combinations out of the possible 36:

relaxed_relaxed
consume_relaxed
consume_consume
acquire_relaxed
acquire_consume
acquire_acquire
release_relaxed
release_consume
release_acquire
acq_rel_relaxed
acq_rel_consume
acq_rel_acquire
seq_cst_relaxed
seq_cst_consume
seq_cst_acquire
seq_cst_seq_cst

Again, the compiler supplies intrinsics only for the strongest orderings where it can make a difference. The library takes care of calling the weakest supplied intrinsic that is as strong or stronger than the customer asked for.

Note about ABI

With any design, the (back end) compiler writer should note that the decision to implement lock-free operations on any given type (or not) is an ABI-binding decision. One can not change from treating a type as not lock free, to lock free (or vice-versa) without breaking your ABI.

For example:

TU1.cpp:

extern atomic<long long> A;
int foo() { return A.compare_exchange_strong(w, x); }

TU2.cpp:

extern atomic<long long> A;
void bar() { return A.compare_exchange_strong(y, z); }

If only one of these calls to compare_exchange_strong is implemented with mutex-locked code, then that mutex-locked code will not be executed mutually exclusively of the one implemented in a lock-free manner.