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  1. /*
  2. Simple DirectMedia Layer
  3. Copyright (C) 1997-2020 Sam Lantinga <slouken@libsdl.org>
  4. This software is provided 'as-is', without any express or implied
  5. warranty. In no event will the authors be held liable for any damages
  6. arising from the use of this software.
  7. Permission is granted to anyone to use this software for any purpose,
  8. including commercial applications, and to alter it and redistribute it
  9. freely, subject to the following restrictions:
  10. 1. The origin of this software must not be misrepresented; you must not
  11. claim that you wrote the original software. If you use this software
  12. in a product, an acknowledgment in the product documentation would be
  13. appreciated but is not required.
  14. 2. Altered source versions must be plainly marked as such, and must not be
  15. misrepresented as being the original software.
  16. 3. This notice may not be removed or altered from any source distribution.
  17. */
  18. /**
  19. * \file SDL_atomic.h
  20. *
  21. * Atomic operations.
  22. *
  23. * IMPORTANT:
  24. * If you are not an expert in concurrent lockless programming, you should
  25. * only be using the atomic lock and reference counting functions in this
  26. * file. In all other cases you should be protecting your data structures
  27. * with full mutexes.
  28. *
  29. * The list of "safe" functions to use are:
  30. * SDL_AtomicLock()
  31. * SDL_AtomicUnlock()
  32. * SDL_AtomicIncRef()
  33. * SDL_AtomicDecRef()
  34. *
  35. * Seriously, here be dragons!
  36. * ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  37. *
  38. * You can find out a little more about lockless programming and the
  39. * subtle issues that can arise here:
  40. * http://msdn.microsoft.com/en-us/library/ee418650%28v=vs.85%29.aspx
  41. *
  42. * There's also lots of good information here:
  43. * http://www.1024cores.net/home/lock-free-algorithms
  44. * http://preshing.com/
  45. *
  46. * These operations may or may not actually be implemented using
  47. * processor specific atomic operations. When possible they are
  48. * implemented as true processor specific atomic operations. When that
  49. * is not possible the are implemented using locks that *do* use the
  50. * available atomic operations.
  51. *
  52. * All of the atomic operations that modify memory are full memory barriers.
  53. */
  54. #ifndef SDL_atomic_h_
  55. #define SDL_atomic_h_
  56. #include "SDL_stdinc.h"
  57. #include "SDL_platform.h"
  58. #include "begin_code.h"
  59. /* Set up for C function definitions, even when using C++ */
  60. #ifdef __cplusplus
  61. extern "C" {
  62. #endif
  63. /**
  64. * \name SDL AtomicLock
  65. *
  66. * The atomic locks are efficient spinlocks using CPU instructions,
  67. * but are vulnerable to starvation and can spin forever if a thread
  68. * holding a lock has been terminated. For this reason you should
  69. * minimize the code executed inside an atomic lock and never do
  70. * expensive things like API or system calls while holding them.
  71. *
  72. * The atomic locks are not safe to lock recursively.
  73. *
  74. * Porting Note:
  75. * The spin lock functions and type are required and can not be
  76. * emulated because they are used in the atomic emulation code.
  77. */
  78. /* @{ */
  79. typedef int SDL_SpinLock;
  80. /**
  81. * \brief Try to lock a spin lock by setting it to a non-zero value.
  82. *
  83. * \param lock Points to the lock.
  84. *
  85. * \return SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already held.
  86. */
  87. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
  88. /**
  89. * \brief Lock a spin lock by setting it to a non-zero value.
  90. *
  91. * \param lock Points to the lock.
  92. */
  93. extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
  94. /**
  95. * \brief Unlock a spin lock by setting it to 0. Always returns immediately
  96. *
  97. * \param lock Points to the lock.
  98. */
  99. extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
  100. /* @} *//* SDL AtomicLock */
  101. /**
  102. * The compiler barrier prevents the compiler from reordering
  103. * reads and writes to globally visible variables across the call.
  104. */
  105. #if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
  106. void _ReadWriteBarrier(void);
  107. #pragma intrinsic(_ReadWriteBarrier)
  108. #define SDL_CompilerBarrier() _ReadWriteBarrier()
  109. #elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
  110. /* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
  111. #define SDL_CompilerBarrier() __asm__ __volatile__ ("" : : : "memory")
  112. #elif defined(__WATCOMC__)
  113. extern _inline void SDL_CompilerBarrier (void);
  114. #pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
  115. #else
  116. #define SDL_CompilerBarrier() \
  117. { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
  118. #endif
  119. /**
  120. * Memory barriers are designed to prevent reads and writes from being
  121. * reordered by the compiler and being seen out of order on multi-core CPUs.
  122. *
  123. * A typical pattern would be for thread A to write some data and a flag,
  124. * and for thread B to read the flag and get the data. In this case you
  125. * would insert a release barrier between writing the data and the flag,
  126. * guaranteeing that the data write completes no later than the flag is
  127. * written, and you would insert an acquire barrier between reading the
  128. * flag and reading the data, to ensure that all the reads associated
  129. * with the flag have completed.
  130. *
  131. * In this pattern you should always see a release barrier paired with
  132. * an acquire barrier and you should gate the data reads/writes with a
  133. * single flag variable.
  134. *
  135. * For more information on these semantics, take a look at the blog post:
  136. * http://preshing.com/20120913/acquire-and-release-semantics
  137. */
  138. extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
  139. extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
  140. #if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
  141. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
  142. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
  143. #elif defined(__GNUC__) && defined(__aarch64__)
  144. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
  145. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
  146. #elif defined(__GNUC__) && defined(__arm__)
  147. #if 0 /* defined(__LINUX__) || defined(__ANDROID__) */
  148. /* Information from:
  149. https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
  150. The Linux kernel provides a helper function which provides the right code for a memory barrier,
  151. hard-coded at address 0xffff0fa0
  152. */
  153. typedef void (*SDL_KernelMemoryBarrierFunc)();
  154. #define SDL_MemoryBarrierRelease() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
  155. #define SDL_MemoryBarrierAcquire() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
  156. #elif 0 /* defined(__QNXNTO__) */
  157. #include <sys/cpuinline.h>
  158. #define SDL_MemoryBarrierRelease() __cpu_membarrier()
  159. #define SDL_MemoryBarrierAcquire() __cpu_membarrier()
  160. #else
  161. #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__)
  162. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
  163. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
  164. #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__)
  165. #ifdef __thumb__
  166. /* The mcr instruction isn't available in thumb mode, use real functions */
  167. #define SDL_MEMORY_BARRIER_USES_FUNCTION
  168. #define SDL_MemoryBarrierRelease() SDL_MemoryBarrierReleaseFunction()
  169. #define SDL_MemoryBarrierAcquire() SDL_MemoryBarrierAcquireFunction()
  170. #else
  171. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
  172. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
  173. #endif /* __thumb__ */
  174. #else
  175. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
  176. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
  177. #endif /* __LINUX__ || __ANDROID__ */
  178. #endif /* __GNUC__ && __arm__ */
  179. #else
  180. #if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
  181. /* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
  182. #include <mbarrier.h>
  183. #define SDL_MemoryBarrierRelease() __machine_rel_barrier()
  184. #define SDL_MemoryBarrierAcquire() __machine_acq_barrier()
  185. #else
  186. /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
  187. #define SDL_MemoryBarrierRelease() SDL_CompilerBarrier()
  188. #define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
  189. #endif
  190. #endif
  191. /**
  192. * \brief A type representing an atomic integer value. It is a struct
  193. * so people don't accidentally use numeric operations on it.
  194. */
  195. typedef struct { int value; } SDL_atomic_t;
  196. /**
  197. * \brief Set an atomic variable to a new value if it is currently an old value.
  198. *
  199. * \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
  200. *
  201. * \note If you don't know what this function is for, you shouldn't use it!
  202. */
  203. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
  204. /**
  205. * \brief Set an atomic variable to a value.
  206. *
  207. * \return The previous value of the atomic variable.
  208. */
  209. extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
  210. /**
  211. * \brief Get the value of an atomic variable
  212. */
  213. extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
  214. /**
  215. * \brief Add to an atomic variable.
  216. *
  217. * \return The previous value of the atomic variable.
  218. *
  219. * \note This same style can be used for any number operation
  220. */
  221. extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
  222. /**
  223. * \brief Increment an atomic variable used as a reference count.
  224. */
  225. #ifndef SDL_AtomicIncRef
  226. #define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
  227. #endif
  228. /**
  229. * \brief Decrement an atomic variable used as a reference count.
  230. *
  231. * \return SDL_TRUE if the variable reached zero after decrementing,
  232. * SDL_FALSE otherwise
  233. */
  234. #ifndef SDL_AtomicDecRef
  235. #define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
  236. #endif
  237. /**
  238. * \brief Set a pointer to a new value if it is currently an old value.
  239. *
  240. * \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
  241. *
  242. * \note If you don't know what this function is for, you shouldn't use it!
  243. */
  244. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
  245. /**
  246. * \brief Set a pointer to a value atomically.
  247. *
  248. * \return The previous value of the pointer.
  249. */
  250. extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
  251. /**
  252. * \brief Get the value of a pointer atomically.
  253. */
  254. extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
  255. /* Ends C function definitions when using C++ */
  256. #ifdef __cplusplus
  257. }
  258. #endif
  259. #include "close_code.h"
  260. #endif /* SDL_atomic_h_ */
  261. /* vi: set ts=4 sw=4 expandtab: */