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   <h1>&lt;atomic&gt; design</h1>
   <!--*********************************************************************-->

 <p>
 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 <tt>memory_order_seq_cst</tt>).
 </p>

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

 <blockquote><pre>
 <font color="#C80000">// In every intrinsic signature below, type* atomic_obj may be a pointer to a</font>
 <font color="#C80000">//    volatile-qualifed type.</font>
 <font color="#C80000">// Memory ordering values map to the following meanings:</font>
 <font color="#C80000">//   memory_order_relaxed == 0</font>
 <font color="#C80000">//   memory_order_consume == 1</font>
 <font color="#C80000">//   memory_order_acquire == 2</font>
 <font color="#C80000">//   memory_order_release == 3</font>
 <font color="#C80000">//   memory_order_acq_rel == 4</font>
 <font color="#C80000">//   memory_order_seq_cst == 5</font>

 <font color="#C80000">// type must be trivially copyable</font>
 <font color="#C80000">// type represents a "type argument"</font>
 bool __atomic_is_lock_free(type);

 <font color="#C80000">// type must be trivially copyable</font>
 <font color="#C80000">// Behavior is defined for mem_ord = 0, 1, 2, 5</font>
 type __atomic_load(const type* atomic_obj, int mem_ord);

 <font color="#C80000">// type must be trivially copyable</font>
 <font color="#C80000">// Behavior is defined for mem_ord = 0, 3, 5</font>
 void __atomic_store(type* atomic_obj, type desired, int mem_ord);

 <font color="#C80000">// type must be trivially copyable</font>
 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 type __atomic_exchange(type* atomic_obj, type desired, int mem_ord);

 <font color="#C80000">// type must be trivially copyable</font>
 <font color="#C80000">// Behavior is defined for mem_success = [0 ... 5],</font>
 <font color="#C80000">//   mem_failure &lt;= mem_success</font>
 <font color="#C80000">//   mem_failure != 3</font>
 <font color="#C80000">//   mem_failure != 4</font>
 bool __atomic_compare_exchange_strong(type* atomic_obj,
                                       type* expected, type desired,
                                       int mem_success, int mem_failure);

 <font color="#C80000">// type must be trivially copyable</font>
 <font color="#C80000">// Behavior is defined for mem_success = [0 ... 5],</font>
 <font color="#C80000">//   mem_failure &lt;= mem_success</font>
 <font color="#C80000">//   mem_failure != 3</font>
 <font color="#C80000">//   mem_failure != 4</font>
 bool __atomic_compare_exchange_weak(type* atomic_obj,
                                     type* expected, type desired,
                                     int mem_success, int mem_failure);

 <font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
 <font color="#C80000">//      unsigned int, long, unsigned long, long long, unsigned long long,</font>
 <font color="#C80000">//      char16_t, char32_t, wchar_t</font>
 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 type __atomic_fetch_add(type* atomic_obj, type operand, int mem_ord);

 <font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
 <font color="#C80000">//      unsigned int, long, unsigned long, long long, unsigned long long,</font>
 <font color="#C80000">//      char16_t, char32_t, wchar_t</font>
 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 type __atomic_fetch_sub(type* atomic_obj, type operand, int mem_ord);

 <font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
 <font color="#C80000">//      unsigned int, long, unsigned long, long long, unsigned long long,</font>
 <font color="#C80000">//      char16_t, char32_t, wchar_t</font>
 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 type __atomic_fetch_and(type* atomic_obj, type operand, int mem_ord);

 <font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
 <font color="#C80000">//      unsigned int, long, unsigned long, long long, unsigned long long,</font>
 <font color="#C80000">//      char16_t, char32_t, wchar_t</font>
 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 type __atomic_fetch_or(type* atomic_obj, type operand, int mem_ord);

 <font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
 <font color="#C80000">//      unsigned int, long, unsigned long, long long, unsigned long long,</font>
 <font color="#C80000">//      char16_t, char32_t, wchar_t</font>
 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 type __atomic_fetch_xor(type* atomic_obj, type operand, int mem_ord);

 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 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);

 <font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
 void __atomic_thread_fence(int mem_ord);
 void __atomic_signal_fence(int mem_ord);
 </pre></blockquote>

 <p>
 If desired the intrinsics taking a single <tt>mem_ord</tt> parameter can default
 this argument to 5.
 </p>

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

 <blockquote><pre>
 int
 translate_memory_order(int o)
 {
     switch (o)
     {
     case 4:
         return 2;
     case 3:
         return 0;
     }
     return o;
 }
 </pre></blockquote>

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

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

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

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

 template &lt;class T&gt;
 bool
 __atomic_compare_exchange_strong(T volatile* obj, T* exp, T desr)
 {
     unique_lock&lt;mutex&gt; _(some_mutex);
     if (std::memcmp(const_cast&lt;T*&gt;(obj), exp, sizeof(T)) == 0) <font color="#C80000">// if (*obj == *exp)</font>
     {
         std::memcpy(const_cast&lt;T*&gt;(obj), &amp;desr, sizeof(T)); <font color="#C80000">// *obj = desr;</font>
         return true;
     }
     std::memcpy(exp, const_cast&lt;T*&gt;(obj), sizeof(T)); <font color="#C80000">// *exp = *obj;</font>
     return false;
 }

 <font color="#C80000">// May spuriously return false (even if *obj == *exp)</font>
 template &lt;class T&gt;
 bool
 __atomic_compare_exchange_weak(T volatile* obj, T* exp, T desr)
 {
     unique_lock&lt;mutex&gt; _(some_mutex);
     if (std::memcmp(const_cast&lt;T*&gt;(obj), exp, sizeof(T)) == 0) <font color="#C80000">// if (*obj == *exp)</font>
     {
         std::memcpy(const_cast&lt;T*&gt;(obj), &amp;desr, sizeof(T)); <font color="#C80000">// *obj = desr;</font>
         return true;
     }
     std::memcpy(exp, const_cast&lt;T*&gt;(obj), sizeof(T)); <font color="#C80000">// *exp = *obj;</font>
     return false;
 }

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

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

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

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

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

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

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

 void __atomic_thread_fence()
 {
     unique_lock&lt;mutex&gt; _(some_mutex);
 }

 void __atomic_signal_fence()
 {
     unique_lock&lt;mutex&gt; _(some_mutex);
 }
 </pre></blockquote>


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	<!--*********************************************************************-->
	<h1><atomic> design</h1>
	<!--*********************************************************************-->

	<p>
	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 <tt>memory_order_seq_cst</tt>).
	</p>

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

	<blockquote><pre>
	<font color="#C80000">// In every intrinsic signature below, type* atomic_obj may be a pointer to a</font>
	<font color="#C80000">// volatile-qualifed type.</font>
	<font color="#C80000">// Memory ordering values map to the following meanings:</font>
	<font color="#C80000">// memory_order_relaxed == 0</font>
	<font color="#C80000">// memory_order_consume == 1</font>
	<font color="#C80000">// memory_order_acquire == 2</font>
	<font color="#C80000">// memory_order_release == 3</font>
	<font color="#C80000">// memory_order_acq_rel == 4</font>
	<font color="#C80000">// memory_order_seq_cst == 5</font>

	<font color="#C80000">// type must be trivially copyable</font>
	<font color="#C80000">// type represents a "type argument"</font>
	bool __atomic_is_lock_free(type);

	<font color="#C80000">// type must be trivially copyable</font>
	<font color="#C80000">// Behavior is defined for mem_ord = 0, 1, 2, 5</font>
	type __atomic_load(const type* atomic_obj, int mem_ord);

	<font color="#C80000">// type must be trivially copyable</font>
	<font color="#C80000">// Behavior is defined for mem_ord = 0, 3, 5</font>
	void __atomic_store(type* atomic_obj, type desired, int mem_ord);

	<font color="#C80000">// type must be trivially copyable</font>
	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	type __atomic_exchange(type* atomic_obj, type desired, int mem_ord);

	<font color="#C80000">// type must be trivially copyable</font>
	<font color="#C80000">// Behavior is defined for mem_success = [0 ... 5],</font>
	<font color="#C80000">// mem_failure <= mem_success</font>
	<font color="#C80000">// mem_failure != 3</font>
	<font color="#C80000">// mem_failure != 4</font>
	bool __atomic_compare_exchange_strong(type* atomic_obj,
	type* expected, type desired,
	int mem_success, int mem_failure);

	<font color="#C80000">// type must be trivially copyable</font>
	<font color="#C80000">// Behavior is defined for mem_success = [0 ... 5],</font>
	<font color="#C80000">// mem_failure <= mem_success</font>
	<font color="#C80000">// mem_failure != 3</font>
	<font color="#C80000">// mem_failure != 4</font>
	bool __atomic_compare_exchange_weak(type* atomic_obj,
	type* expected, type desired,
	int mem_success, int mem_failure);

	<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
	<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
	<font color="#C80000">// char16_t, char32_t, wchar_t</font>
	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	type __atomic_fetch_add(type* atomic_obj, type operand, int mem_ord);

	<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
	<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
	<font color="#C80000">// char16_t, char32_t, wchar_t</font>
	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	type __atomic_fetch_sub(type* atomic_obj, type operand, int mem_ord);

	<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
	<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
	<font color="#C80000">// char16_t, char32_t, wchar_t</font>
	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	type __atomic_fetch_and(type* atomic_obj, type operand, int mem_ord);

	<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
	<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
	<font color="#C80000">// char16_t, char32_t, wchar_t</font>
	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	type __atomic_fetch_or(type* atomic_obj, type operand, int mem_ord);

	<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
	<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
	<font color="#C80000">// char16_t, char32_t, wchar_t</font>
	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	type __atomic_fetch_xor(type* atomic_obj, type operand, int mem_ord);

	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	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);

	<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
	void __atomic_thread_fence(int mem_ord);
	void __atomic_signal_fence(int mem_ord);
	</pre></blockquote>

	<p>
	If desired the intrinsics taking a single <tt>mem_ord</tt> parameter can default
	this argument to 5.
	</p>

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

	<blockquote><pre>
	int
	translate_memory_order(int o)
	{
	switch (o)
	{
	case 4:
	return 2;
	case 3:
	return 0;
	}
	return o;
	}
	</pre></blockquote>

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

	<blockquote><pre>
	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) <font color="#C80000">// if (obj == *exp)</font>
	{
	std::memcpy(const_cast<T>(obj), &desr, sizeof(T)); <font color="#C80000">// obj = desr;</font>
	return true;
	}
	std::memcpy(exp, const_cast<T>(obj), sizeof(T)); <font color="#C80000">// exp = *obj;</font>
	return false;
	}

	<font color="#C80000">// May spuriously return false (even if obj == exp)</font>
	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) <font color="#C80000">// if (obj == *exp)</font>
	{
	std::memcpy(const_cast<T>(obj), &desr, sizeof(T)); <font color="#C80000">// obj = desr;</font>
	return true;
	}
	std::memcpy(exp, const_cast<T>(obj), sizeof(T)); <font color="#C80000">// exp = *obj;</font>
	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);
	}
	</pre></blockquote>


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