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34 <!--*********************************************************************-->
35 <h1>&lt;atomic&gt; design</h1>
36 <!--*********************************************************************-->
37
38<p>
39The compiler supplies all of the intrinsics as described below. This list of
40intrinsics roughly parallels the requirements of the C and C++ atomics
41proposals. The C and C++ library implementations simply drop through to these
42intrinsics. Anything the platform does not support in hardware, the compiler
43arranges for a (compiler-rt) library call to be made which will do the job with
44a mutex, and in this case ignoring the memory ordering parameter (effectively
45implementing <tt>memory_order_seq_cst</tt>).
46</p>
47
48<p>
49Ultimate efficiency is preferred over run time error checking. Undefined
50behavior is acceptable when the inputs do not conform as defined below.
51</p>
52
53<blockquote><pre>
54<font color="#C80000">// In every intrinsic signature below, type* atomic_obj may be a pointer to a</font>
55<font color="#C80000">// volatile-qualified type.</font>
56<font color="#C80000">// Memory ordering values map to the following meanings:</font>
57<font color="#C80000">// memory_order_relaxed == 0</font>
58<font color="#C80000">// memory_order_consume == 1</font>
59<font color="#C80000">// memory_order_acquire == 2</font>
60<font color="#C80000">// memory_order_release == 3</font>
61<font color="#C80000">// memory_order_acq_rel == 4</font>
62<font color="#C80000">// memory_order_seq_cst == 5</font>
63
64<font color="#C80000">// type must be trivially copyable</font>
65<font color="#C80000">// type represents a "type argument"</font>
66bool __atomic_is_lock_free(type);
67
68<font color="#C80000">// type must be trivially copyable</font>
69<font color="#C80000">// Behavior is defined for mem_ord = 0, 1, 2, 5</font>
70type __atomic_load(const type* atomic_obj, int mem_ord);
71
72<font color="#C80000">// type must be trivially copyable</font>
73<font color="#C80000">// Behavior is defined for mem_ord = 0, 3, 5</font>
74void __atomic_store(type* atomic_obj, type desired, int mem_ord);
75
76<font color="#C80000">// type must be trivially copyable</font>
77<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
78type __atomic_exchange(type* atomic_obj, type desired, int mem_ord);
79
80<font color="#C80000">// type must be trivially copyable</font>
81<font color="#C80000">// Behavior is defined for mem_success = [0 ... 5],</font>
82<font color="#C80000">// mem_failure &lt;= mem_success</font>
83<font color="#C80000">// mem_failure != 3</font>
84<font color="#C80000">// mem_failure != 4</font>
85bool __atomic_compare_exchange_strong(type* atomic_obj,
86 type* expected, type desired,
87 int mem_success, int mem_failure);
88
89<font color="#C80000">// type must be trivially copyable</font>
90<font color="#C80000">// Behavior is defined for mem_success = [0 ... 5],</font>
91<font color="#C80000">// mem_failure &lt;= mem_success</font>
92<font color="#C80000">// mem_failure != 3</font>
93<font color="#C80000">// mem_failure != 4</font>
94bool __atomic_compare_exchange_weak(type* atomic_obj,
95 type* expected, type desired,
96 int mem_success, int mem_failure);
97
98<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
99<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
100<font color="#C80000">// char16_t, char32_t, wchar_t</font>
101<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
102type __atomic_fetch_add(type* atomic_obj, type operand, int mem_ord);
103
104<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
105<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
106<font color="#C80000">// char16_t, char32_t, wchar_t</font>
107<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
108type __atomic_fetch_sub(type* atomic_obj, type operand, int mem_ord);
109
110<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
111<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
112<font color="#C80000">// char16_t, char32_t, wchar_t</font>
113<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
114type __atomic_fetch_and(type* atomic_obj, type operand, int mem_ord);
115
116<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
117<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
118<font color="#C80000">// char16_t, char32_t, wchar_t</font>
119<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
120type __atomic_fetch_or(type* atomic_obj, type operand, int mem_ord);
121
122<font color="#C80000">// type is one of: char, signed char, unsigned char, short, unsigned short, int,</font>
123<font color="#C80000">// unsigned int, long, unsigned long, long long, unsigned long long,</font>
124<font color="#C80000">// char16_t, char32_t, wchar_t</font>
125<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
126type __atomic_fetch_xor(type* atomic_obj, type operand, int mem_ord);
127
128<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
129void* __atomic_fetch_add(void** atomic_obj, ptrdiff_t operand, int mem_ord);
130void* __atomic_fetch_sub(void** atomic_obj, ptrdiff_t operand, int mem_ord);
131
132<font color="#C80000">// Behavior is defined for mem_ord = [0 ... 5]</font>
133void __atomic_thread_fence(int mem_ord);
134void __atomic_signal_fence(int mem_ord);
135</pre></blockquote>
136
137<p>
138If desired the intrinsics taking a single <tt>mem_ord</tt> parameter can default
139this argument to 5.
140</p>
141
142<p>
143If desired the intrinsics taking two ordering parameters can default
144<tt>mem_success</tt> to 5, and <tt>mem_failure</tt> to
145<tt>translate_memory_order(mem_success)</tt> where
146<tt>translate_memory_order(mem_success)</tt> is defined as:
147</p>
148
149<blockquote><pre>
150int
151translate_memory_order(int o)
152{
153 switch (o)
154 {
155 case 4:
156 return 2;
157 case 3:
158 return 0;
159 }
160 return o;
161}
162</pre></blockquote>
163
164<p>
165Below are representative C++ implementations of all of the operations. Their
166purpose is to document the desired semantics of each operation, assuming
167<tt>memory_order_seq_cst</tt>. This is essentially the code that will be called
168if the front end calls out to compiler-rt.
169</p>
170
171<blockquote><pre>
172template &lt;class T&gt;
173T
174__atomic_load(T const volatile* obj)
175{
176 unique_lock&lt;mutex&gt; _(some_mutex);
177 return *obj;
178}
179
180template &lt;class T&gt;
181void
182__atomic_store(T volatile* obj, T desr)
183{
184 unique_lock&lt;mutex&gt; _(some_mutex);
185 *obj = desr;
186}
187
188template &lt;class T&gt;
189T
190__atomic_exchange(T volatile* obj, T desr)
191{
192 unique_lock&lt;mutex&gt; _(some_mutex);
193 T r = *obj;
194 *obj = desr;
195 return r;
196}
197
198template &lt;class T&gt;
199bool
200__atomic_compare_exchange_strong(T volatile* obj, T* exp, T desr)
201{
202 unique_lock&lt;mutex&gt; _(some_mutex);
203 if (std::memcmp(const_cast&lt;T*&gt;(obj), exp, sizeof(T)) == 0) <font color="#C80000">// if (*obj == *exp)</font>
204 {
205 std::memcpy(const_cast&lt;T*&gt;(obj), &amp;desr, sizeof(T)); <font color="#C80000">// *obj = desr;</font>
206 return true;
207 }
208 std::memcpy(exp, const_cast&lt;T*&gt;(obj), sizeof(T)); <font color="#C80000">// *exp = *obj;</font>
209 return false;
210}
211
212<font color="#C80000">// May spuriously return false (even if *obj == *exp)</font>
213template &lt;class T&gt;
214bool
215__atomic_compare_exchange_weak(T volatile* obj, T* exp, T desr)
216{
217 unique_lock&lt;mutex&gt; _(some_mutex);
218 if (std::memcmp(const_cast&lt;T*&gt;(obj), exp, sizeof(T)) == 0) <font color="#C80000">// if (*obj == *exp)</font>
219 {
220 std::memcpy(const_cast&lt;T*&gt;(obj), &amp;desr, sizeof(T)); <font color="#C80000">// *obj = desr;</font>
221 return true;
222 }
223 std::memcpy(exp, const_cast&lt;T*&gt;(obj), sizeof(T)); <font color="#C80000">// *exp = *obj;</font>
224 return false;
225}
226
227template &lt;class T&gt;
228T
229__atomic_fetch_add(T volatile* obj, T operand)
230{
231 unique_lock&lt;mutex&gt; _(some_mutex);
232 T r = *obj;
233 *obj += operand;
234 return r;
235}
236
237template &lt;class T&gt;
238T
239__atomic_fetch_sub(T volatile* obj, T operand)
240{
241 unique_lock&lt;mutex&gt; _(some_mutex);
242 T r = *obj;
243 *obj -= operand;
244 return r;
245}
246
247template &lt;class T&gt;
248T
249__atomic_fetch_and(T volatile* obj, T operand)
250{
251 unique_lock&lt;mutex&gt; _(some_mutex);
252 T r = *obj;
253 *obj &amp;= operand;
254 return r;
255}
256
257template &lt;class T&gt;
258T
259__atomic_fetch_or(T volatile* obj, T operand)
260{
261 unique_lock&lt;mutex&gt; _(some_mutex);
262 T r = *obj;
263 *obj |= operand;
264 return r;
265}
266
267template &lt;class T&gt;
268T
269__atomic_fetch_xor(T volatile* obj, T operand)
270{
271 unique_lock&lt;mutex&gt; _(some_mutex);
272 T r = *obj;
273 *obj ^= operand;
274 return r;
275}
276
277void*
278__atomic_fetch_add(void* volatile* obj, ptrdiff_t operand)
279{
280 unique_lock&lt;mutex&gt; _(some_mutex);
281 void* r = *obj;
282 (char*&amp;)(*obj) += operand;
283 return r;
284}
285
286void*
287__atomic_fetch_sub(void* volatile* obj, ptrdiff_t operand)
288{
289 unique_lock&lt;mutex&gt; _(some_mutex);
290 void* r = *obj;
291 (char*&amp;)(*obj) -= operand;
292 return r;
293}
294
295void __atomic_thread_fence()
296{
297 unique_lock&lt;mutex&gt; _(some_mutex);
298}
299
300void __atomic_signal_fence()
301{
302 unique_lock&lt;mutex&gt; _(some_mutex);
303}
304</pre></blockquote>
305
306
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