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<title>&lt;atomic&gt; design</title>
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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 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 <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-qualified 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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