SDL  2.0
SDL_atomic.h
Go to the documentation of this file.
1 /*
2  Simple DirectMedia Layer
3  Copyright (C) 1997-2019 Sam Lantinga <slouken@libsdl.org>
4 
5  This software is provided 'as-is', without any express or implied
6  warranty. In no event will the authors be held liable for any damages
7  arising from the use of this software.
8 
9  Permission is granted to anyone to use this software for any purpose,
10  including commercial applications, and to alter it and redistribute it
11  freely, subject to the following restrictions:
12 
13  1. The origin of this software must not be misrepresented; you must not
14  claim that you wrote the original software. If you use this software
15  in a product, an acknowledgment in the product documentation would be
16  appreciated but is not required.
17  2. Altered source versions must be plainly marked as such, and must not be
18  misrepresented as being the original software.
19  3. This notice may not be removed or altered from any source distribution.
20 */
21 
22 /**
23  * \file SDL_atomic.h
24  *
25  * Atomic operations.
26  *
27  * IMPORTANT:
28  * If you are not an expert in concurrent lockless programming, you should
29  * only be using the atomic lock and reference counting functions in this
30  * file. In all other cases you should be protecting your data structures
31  * with full mutexes.
32  *
33  * The list of "safe" functions to use are:
34  * SDL_AtomicLock()
35  * SDL_AtomicUnlock()
36  * SDL_AtomicIncRef()
37  * SDL_AtomicDecRef()
38  *
39  * Seriously, here be dragons!
40  * ^^^^^^^^^^^^^^^^^^^^^^^^^^^
41  *
42  * You can find out a little more about lockless programming and the
43  * subtle issues that can arise here:
44  * http://msdn.microsoft.com/en-us/library/ee418650%28v=vs.85%29.aspx
45  *
46  * There's also lots of good information here:
47  * http://www.1024cores.net/home/lock-free-algorithms
48  * http://preshing.com/
49  *
50  * These operations may or may not actually be implemented using
51  * processor specific atomic operations. When possible they are
52  * implemented as true processor specific atomic operations. When that
53  * is not possible the are implemented using locks that *do* use the
54  * available atomic operations.
55  *
56  * All of the atomic operations that modify memory are full memory barriers.
57  */
58 
59 #ifndef SDL_atomic_h_
60 #define SDL_atomic_h_
61 
62 #include "SDL_stdinc.h"
63 #include "SDL_platform.h"
64 
65 #include "begin_code.h"
66 
67 /* Set up for C function definitions, even when using C++ */
68 #ifdef __cplusplus
69 extern "C" {
70 #endif
71 
72 /**
73  * \name SDL AtomicLock
74  *
75  * The atomic locks are efficient spinlocks using CPU instructions,
76  * but are vulnerable to starvation and can spin forever if a thread
77  * holding a lock has been terminated. For this reason you should
78  * minimize the code executed inside an atomic lock and never do
79  * expensive things like API or system calls while holding them.
80  *
81  * The atomic locks are not safe to lock recursively.
82  *
83  * Porting Note:
84  * The spin lock functions and type are required and can not be
85  * emulated because they are used in the atomic emulation code.
86  */
87 /* @{ */
88 
89 typedef int SDL_SpinLock;
90 
91 /**
92  * \brief Try to lock a spin lock by setting it to a non-zero value.
93  *
94  * \param lock Points to the lock.
95  *
96  * \return SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already held.
97  */
99 
100 /**
101  * \brief Lock a spin lock by setting it to a non-zero value.
102  *
103  * \param lock Points to the lock.
104  */
106 
107 /**
108  * \brief Unlock a spin lock by setting it to 0. Always returns immediately
109  *
110  * \param lock Points to the lock.
111  */
113 
114 /* @} *//* SDL AtomicLock */
115 
116 
117 /**
118  * The compiler barrier prevents the compiler from reordering
119  * reads and writes to globally visible variables across the call.
120  */
121 #if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
122 void _ReadWriteBarrier(void);
123 #pragma intrinsic(_ReadWriteBarrier)
124 #define SDL_CompilerBarrier() _ReadWriteBarrier()
125 #elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
126 /* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
127 #define SDL_CompilerBarrier() __asm__ __volatile__ ("" : : : "memory")
128 #elif defined(__WATCOMC__)
129 extern _inline void SDL_CompilerBarrier (void);
130 #pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
131 #else
132 #define SDL_CompilerBarrier() \
133 { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
134 #endif
135 
136 /**
137  * Memory barriers are designed to prevent reads and writes from being
138  * reordered by the compiler and being seen out of order on multi-core CPUs.
139  *
140  * A typical pattern would be for thread A to write some data and a flag,
141  * and for thread B to read the flag and get the data. In this case you
142  * would insert a release barrier between writing the data and the flag,
143  * guaranteeing that the data write completes no later than the flag is
144  * written, and you would insert an acquire barrier between reading the
145  * flag and reading the data, to ensure that all the reads associated
146  * with the flag have completed.
147  *
148  * In this pattern you should always see a release barrier paired with
149  * an acquire barrier and you should gate the data reads/writes with a
150  * single flag variable.
151  *
152  * For more information on these semantics, take a look at the blog post:
153  * http://preshing.com/20120913/acquire-and-release-semantics
154  */
157 
158 #if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
159 #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
160 #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
161 #elif defined(__GNUC__) && defined(__aarch64__)
162 #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
163 #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
164 #elif defined(__GNUC__) && defined(__arm__)
165 #if 0 /* defined(__LINUX__) || defined(__ANDROID__) */
166 /* Information from:
167  https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
168 
169  The Linux kernel provides a helper function which provides the right code for a memory barrier,
170  hard-coded at address 0xffff0fa0
171 */
172 typedef void (*SDL_KernelMemoryBarrierFunc)();
173 #define SDL_MemoryBarrierRelease() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
174 #define SDL_MemoryBarrierAcquire() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
175 #elif 0 /* defined(__QNXNTO__) */
176 #include <sys/cpuinline.h>
177 
178 #define SDL_MemoryBarrierRelease() __cpu_membarrier()
179 #define SDL_MemoryBarrierAcquire() __cpu_membarrier()
180 #else
181 #if defined(__ARM_ARCH_7__) || defined(__ARM_ARCH_7A__) || defined(__ARM_ARCH_7EM__) || defined(__ARM_ARCH_7R__) || defined(__ARM_ARCH_7M__) || defined(__ARM_ARCH_7S__) || defined(__ARM_ARCH_8A__)
182 #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
183 #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
184 #elif defined(__ARM_ARCH_6__) || defined(__ARM_ARCH_6J__) || defined(__ARM_ARCH_6K__) || defined(__ARM_ARCH_6T2__) || defined(__ARM_ARCH_6Z__) || defined(__ARM_ARCH_6ZK__) || defined(__ARM_ARCH_5TE__)
185 #ifdef __thumb__
186 /* The mcr instruction isn't available in thumb mode, use real functions */
187 #define SDL_MEMORY_BARRIER_USES_FUNCTION
188 #define SDL_MemoryBarrierRelease() SDL_MemoryBarrierReleaseFunction()
189 #define SDL_MemoryBarrierAcquire() SDL_MemoryBarrierAcquireFunction()
190 #else
191 #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
192 #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
193 #endif /* __thumb__ */
194 #else
195 #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
196 #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
197 #endif /* __LINUX__ || __ANDROID__ */
198 #endif /* __GNUC__ && __arm__ */
199 #else
200 #if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
201 /* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
202 #include <mbarrier.h>
203 #define SDL_MemoryBarrierRelease() __machine_rel_barrier()
204 #define SDL_MemoryBarrierAcquire() __machine_acq_barrier()
205 #else
206 /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
207 #define SDL_MemoryBarrierRelease() SDL_CompilerBarrier()
208 #define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
209 #endif
210 #endif
211 
212 /**
213  * \brief A type representing an atomic integer value. It is a struct
214  * so people don't accidentally use numeric operations on it.
215  */
216 typedef struct { int value; } SDL_atomic_t;
217 
218 /**
219  * \brief Set an atomic variable to a new value if it is currently an old value.
220  *
221  * \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
222  *
223  * \note If you don't know what this function is for, you shouldn't use it!
224 */
225 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
226 
227 /**
228  * \brief Set an atomic variable to a value.
229  *
230  * \return The previous value of the atomic variable.
231  */
232 extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
233 
234 /**
235  * \brief Get the value of an atomic variable
236  */
238 
239 /**
240  * \brief Add to an atomic variable.
241  *
242  * \return The previous value of the atomic variable.
243  *
244  * \note This same style can be used for any number operation
245  */
246 extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
247 
248 /**
249  * \brief Increment an atomic variable used as a reference count.
250  */
251 #ifndef SDL_AtomicIncRef
252 #define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
253 #endif
254 
255 /**
256  * \brief Decrement an atomic variable used as a reference count.
257  *
258  * \return SDL_TRUE if the variable reached zero after decrementing,
259  * SDL_FALSE otherwise
260  */
261 #ifndef SDL_AtomicDecRef
262 #define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
263 #endif
264 
265 /**
266  * \brief Set a pointer to a new value if it is currently an old value.
267  *
268  * \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
269  *
270  * \note If you don't know what this function is for, you shouldn't use it!
271 */
272 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
273 
274 /**
275  * \brief Set a pointer to a value atomically.
276  *
277  * \return The previous value of the pointer.
278  */
279 extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
280 
281 /**
282  * \brief Get the value of a pointer atomically.
283  */
284 extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
285 
286 /* Ends C function definitions when using C++ */
287 #ifdef __cplusplus
288 }
289 #endif
290 
291 #include "close_code.h"
292 
293 #endif /* SDL_atomic_h_ */
294 
295 /* vi: set ts=4 sw=4 expandtab: */
DECLSPEC
#define DECLSPEC
Definition: SDL_internal.h:48
SDL_MemoryBarrierAcquireFunction
void SDL_MemoryBarrierAcquireFunction(void)
Definition: SDL_atomic.c:303
SDL_AtomicLock
void SDL_AtomicLock(SDL_SpinLock *lock)
Lock a spin lock by setting it to a non-zero value.
Definition: SDL_spinlock.c:137
SDLCALL
#define SDLCALL
Definition: SDL_internal.h:49
SDL_AtomicCASPtr
SDL_bool SDL_AtomicCASPtr(void **a, void *oldval, void *newval)
Set a pointer to a new value if it is currently an old value.
Definition: SDL_atomic.c:161
a
GLboolean GLboolean GLboolean GLboolean a
Definition: SDL_opengl_glext.h:1109
v
const GLdouble * v
Definition: SDL_opengl.h:2063
SDL_AtomicAdd
int SDL_AtomicAdd(SDL_atomic_t *a, int v)
Add to an atomic variable.
Definition: SDL_atomic.c:238
close_code.h
SDL_atomic_t::value
int value
Definition: SDL_atomic.h:216
begin_code.h
SDL_AtomicSetPtr
void * SDL_AtomicSetPtr(void **a, void *v)
Set a pointer to a value atomically.
Definition: SDL_atomic.c:216
SDL_AtomicCAS
SDL_bool SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval)
Set an atomic variable to a new value if it is currently an old value.
Definition: SDL_atomic.c:130
SDL_AtomicGetPtr
void * SDL_AtomicGetPtr(void **a)
Get the value of a pointer atomically.
Definition: SDL_atomic.c:279
SDL_platform.h
SDL_AtomicSet
int SDL_AtomicSet(SDL_atomic_t *a, int v)
Set an atomic variable to a value.
Definition: SDL_atomic.c:194
SDL_CompilerBarrier
#define SDL_CompilerBarrier()
Definition: SDL_atomic.h:132
SDL_MemoryBarrierReleaseFunction
void SDL_MemoryBarrierReleaseFunction(void)
Definition: SDL_atomic.c:297
SDL_SpinLock
int SDL_SpinLock
Definition: SDL_atomic.h:89
SDL_bool
SDL_bool
Definition: SDL_stdinc.h:161
SDL_AtomicTryLock
SDL_bool SDL_AtomicTryLock(SDL_SpinLock *lock)
Try to lock a spin lock by setting it to a non-zero value.
Definition: SDL_spinlock.c:51
SDL_stdinc.h
SDL_atomic_t
A type representing an atomic integer value. It is a struct so people don't accidentally use numeric ...
Definition: SDL_atomic.h:216
SDL_AtomicUnlock
void SDL_AtomicUnlock(SDL_SpinLock *lock)
Unlock a spin lock by setting it to 0. Always returns immediately.
Definition: SDL_spinlock.c:153
SDL_AtomicGet
int SDL_AtomicGet(SDL_atomic_t *a)
Get the value of an atomic variable.
Definition: SDL_atomic.c:265
void
const SDL_PRINTF_FORMAT_STRING char int const SDL_PRINTF_FORMAT_STRING char int const SDL_PRINTF_FORMAT_STRING char int const SDL_PRINTF_FORMAT_STRING char const char const SDL_SCANF_FORMAT_STRING char return SDL_ThreadFunction const char void return Uint32 return Uint32 void
Definition: SDL_dynapi_procs.h:89
lock
SDL_mutex * lock
Definition: SDL_events.c:76