1 /*===-- atomic.c - Implement support functions for atomic operations.------===
3 * The LLVM Compiler Infrastructure
5 * This file is dual licensed under the MIT and the University of Illinois Open
6 * Source Licenses. See LICENSE.TXT for details.
8 *===----------------------------------------------------------------------===
10 * atomic.c defines a set of functions for performing atomic accesses on
11 * arbitrary-sized memory locations. This design uses locks that should
12 * be fast in the uncontended case, for two reasons:
14 * 1) This code must work with C programs that do not link to anything
15 * (including pthreads) and so it should not depend on any pthread
17 * 2) Atomic operations, rather than explicit mutexes, are most commonly used
18 * on code where contended operations are rate.
20 * To avoid needing a per-object lock, this code allocates an array of
21 * locks and hashes the object pointers to find the one that it should use.
22 * For operations that must be atomic on two locations, the lower lock is
23 * always acquired first, to avoid deadlock.
25 *===----------------------------------------------------------------------===
31 // Clang objects if you redefine a builtin. This little hack allows us to
32 // define a function with the same name as an intrinsic.
34 // mach-o has extra leading underscore
35 #pragma redefine_extname __atomic_load_c ___atomic_load
36 #pragma redefine_extname __atomic_store_c ___atomic_store
37 #pragma redefine_extname __atomic_exchange_c ___atomic_exchange
38 #pragma redefine_extname __atomic_compare_exchange_c ___atomic_compare_exchange
40 #pragma redefine_extname __atomic_load_c __atomic_load
41 #pragma redefine_extname __atomic_store_c __atomic_store
42 #pragma redefine_extname __atomic_exchange_c __atomic_exchange
43 #pragma redefine_extname __atomic_compare_exchange_c __atomic_compare_exchange
46 /// Number of locks. This allocates one page on 32-bit platforms, two on
47 /// 64-bit. This can be specified externally if a different trade between
48 /// memory usage and contention probability is required for a given platform.
49 #ifndef SPINLOCK_COUNT
50 #define SPINLOCK_COUNT (1<<10)
52 static const long SPINLOCK_MASK = SPINLOCK_COUNT - 1;
54 ////////////////////////////////////////////////////////////////////////////////
55 // Platform-specific lock implementation. Falls back to spinlocks if none is
56 // defined. Each platform should define the Lock type, and corresponding
57 // lock() and unlock() functions.
58 ////////////////////////////////////////////////////////////////////////////////
61 #include <sys/types.h>
62 #include <machine/atomic.h>
64 typedef struct _usem Lock;
65 inline static void unlock(Lock *l) {
66 __c11_atomic_store((_Atomic(uint32_t)*)&l->_count, 1, __ATOMIC_RELEASE);
67 __c11_atomic_thread_fence(__ATOMIC_SEQ_CST);
69 _umtx_op(l, UMTX_OP_SEM_WAKE, 1, 0, 0);
71 inline static void lock(Lock *l) {
73 while (!__c11_atomic_compare_exchange_weak((_Atomic(uint32_t)*)&l->_count, &old,
74 0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)) {
75 _umtx_op(l, UMTX_OP_SEM_WAIT, 0, 0, 0);
79 /// locks for atomic operations
80 static Lock locks[SPINLOCK_COUNT] = { [0 ... SPINLOCK_COUNT-1] = {0,1,0} };
82 #elif defined(__APPLE__)
83 #include <libkern/OSAtomic.h>
84 typedef OSSpinLock Lock;
85 inline static void unlock(Lock *l) {
88 /// Locks a lock. In the current implementation, this is potentially
89 /// unbounded in the contended case.
90 inline static void lock(Lock *l) {
93 static Lock locks[SPINLOCK_COUNT]; // initialized to OS_SPINLOCK_INIT which is 0
96 typedef _Atomic(uintptr_t) Lock;
97 /// Unlock a lock. This is a release operation.
98 inline static void unlock(Lock *l) {
99 __c11_atomic_store(l, 0, __ATOMIC_RELEASE);
101 /// Locks a lock. In the current implementation, this is potentially
102 /// unbounded in the contended case.
103 inline static void lock(Lock *l) {
105 while (!__c11_atomic_compare_exchange_weak(l, &old, 1, __ATOMIC_ACQUIRE,
109 /// locks for atomic operations
110 static Lock locks[SPINLOCK_COUNT];
114 /// Returns a lock to use for a given pointer.
115 static inline Lock *lock_for_pointer(void *ptr) {
116 intptr_t hash = (intptr_t)ptr;
117 // Disregard the lowest 4 bits. We want all values that may be part of the
118 // same memory operation to hash to the same value and therefore use the same
121 // Use the next bits as the basis for the hash
122 intptr_t low = hash & SPINLOCK_MASK;
123 // Now use the high(er) set of bits to perturb the hash, so that we don't
124 // get collisions from atomic fields in a single object
127 // Return a pointer to the word to use
128 return locks + (hash & SPINLOCK_MASK);
131 /// Macros for determining whether a size is lock free. Clang can not yet
132 /// codegen __atomic_is_lock_free(16), so for now we assume 16-byte values are
134 #define IS_LOCK_FREE_1 __c11_atomic_is_lock_free(1)
135 #define IS_LOCK_FREE_2 __c11_atomic_is_lock_free(2)
136 #define IS_LOCK_FREE_4 __c11_atomic_is_lock_free(4)
137 #define IS_LOCK_FREE_8 __c11_atomic_is_lock_free(8)
138 #define IS_LOCK_FREE_16 0
140 /// Macro that calls the compiler-generated lock-free versions of functions
142 #define LOCK_FREE_CASES() \
146 if (IS_LOCK_FREE_2) {\
147 LOCK_FREE_ACTION(uint16_t);\
150 if (IS_LOCK_FREE_4) {\
151 LOCK_FREE_ACTION(uint32_t);\
154 if (IS_LOCK_FREE_8) {\
155 LOCK_FREE_ACTION(uint64_t);\
158 if (IS_LOCK_FREE_16) {\
159 /* FIXME: __uint128_t isn't available on 32 bit platforms.
160 LOCK_FREE_ACTION(__uint128_t);*/\
166 /// An atomic load operation. This is atomic with respect to the source
168 void __atomic_load_c(int size, void *src, void *dest, int model) {
169 #define LOCK_FREE_ACTION(type) \
170 *((type*)dest) = __c11_atomic_load((_Atomic(type)*)src, model);\
173 #undef LOCK_FREE_ACTION
174 Lock *l = lock_for_pointer(src);
176 memcpy(dest, src, size);
180 /// An atomic store operation. This is atomic with respect to the destination
182 void __atomic_store_c(int size, void *dest, void *src, int model) {
183 #define LOCK_FREE_ACTION(type) \
184 __c11_atomic_store((_Atomic(type)*)dest, *(type*)dest, model);\
187 #undef LOCK_FREE_ACTION
188 Lock *l = lock_for_pointer(dest);
190 memcpy(dest, src, size);
194 /// Atomic compare and exchange operation. If the value at *ptr is identical
195 /// to the value at *expected, then this copies value at *desired to *ptr. If
196 /// they are not, then this stores the current value from *ptr in *expected.
198 /// This function returns 1 if the exchange takes place or 0 if it fails.
199 int __atomic_compare_exchange_c(int size, void *ptr, void *expected,
200 void *desired, int success, int failure) {
201 #define LOCK_FREE_ACTION(type) \
202 return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, (type*)expected,\
203 *(type*)desired, success, failure)
205 #undef LOCK_FREE_ACTION
206 Lock *l = lock_for_pointer(ptr);
208 if (memcmp(ptr, expected, size) == 0) {
209 memcpy(ptr, desired, size);
213 memcpy(expected, ptr, size);
218 /// Performs an atomic exchange operation between two pointers. This is atomic
219 /// with respect to the target address.
220 void __atomic_exchange_c(int size, void *ptr, void *val, void *old, int model) {
221 #define LOCK_FREE_ACTION(type) \
222 *(type*)old = __c11_atomic_exchange((_Atomic(type)*)ptr, *(type*)val,\
226 #undef LOCK_FREE_ACTION
227 Lock *l = lock_for_pointer(ptr);
229 memcpy(old, ptr, size);
230 memcpy(ptr, val, size);
234 ////////////////////////////////////////////////////////////////////////////////
235 // Where the size is known at compile time, the compiler may emit calls to
236 // specialised versions of the above functions.
237 ////////////////////////////////////////////////////////////////////////////////
238 #define OPTIMISED_CASES\
239 OPTIMISED_CASE(1, IS_LOCK_FREE_1, uint8_t)\
240 OPTIMISED_CASE(2, IS_LOCK_FREE_2, uint16_t)\
241 OPTIMISED_CASE(4, IS_LOCK_FREE_4, uint32_t)\
242 OPTIMISED_CASE(8, IS_LOCK_FREE_8, uint64_t)\
243 /* FIXME: __uint128_t isn't available on 32 bit platforms.
244 OPTIMISED_CASE(16, IS_LOCK_FREE_16, __uint128_t)*/\
246 #define OPTIMISED_CASE(n, lockfree, type)\
247 type __atomic_load_##n(type *src, int model) {\
249 return __c11_atomic_load((_Atomic(type)*)src, model);\
250 Lock *l = lock_for_pointer(src);\
257 #undef OPTIMISED_CASE
259 #define OPTIMISED_CASE(n, lockfree, type)\
260 void __atomic_store_##n(type *dest, type val, int model) {\
262 __c11_atomic_store((_Atomic(type)*)dest, val, model);\
265 Lock *l = lock_for_pointer(dest);\
272 #undef OPTIMISED_CASE
274 #define OPTIMISED_CASE(n, lockfree, type)\
275 type __atomic_exchange_##n(type *dest, type val, int model) {\
277 return __c11_atomic_exchange((_Atomic(type)*)dest, val, model);\
278 Lock *l = lock_for_pointer(dest);\
286 #undef OPTIMISED_CASE
288 #define OPTIMISED_CASE(n, lockfree, type)\
289 int __atomic_compare_exchange_##n(type *ptr, type *expected, type desired,\
290 int success, int failure) {\
292 return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, expected, desired,\
294 Lock *l = lock_for_pointer(ptr);\
296 if (*ptr == *expected) {\
306 #undef OPTIMISED_CASE
308 ////////////////////////////////////////////////////////////////////////////////
309 // Atomic read-modify-write operations for integers of various sizes.
310 ////////////////////////////////////////////////////////////////////////////////
311 #define ATOMIC_RMW(n, lockfree, type, opname, op) \
312 type __atomic_fetch_##opname##_##n(type *ptr, type val, int model) {\
314 return __c11_atomic_fetch_##opname((_Atomic(type)*)ptr, val, model);\
315 Lock *l = lock_for_pointer(ptr);\
323 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, add, +)
325 #undef OPTIMISED_CASE
326 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, sub, -)
328 #undef OPTIMISED_CASE
329 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, and, &)
331 #undef OPTIMISED_CASE
332 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, or, |)
334 #undef OPTIMISED_CASE
335 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, xor, ^)
337 #undef OPTIMISED_CASE