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  1. /*
  2. Simple DirectMedia Layer
  3. Copyright (C) 1997-2022 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. * Try to lock a spin lock by setting it to a non-zero value.
  82. *
  83. * ***Please note that spinlocks are dangerous if you don't know what you're
  84. * doing. Please be careful using any sort of spinlock!***
  85. *
  86. * \param lock a pointer to a lock variable
  87. * \returns SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already
  88. * held.
  89. *
  90. * \since This function is available since SDL 2.0.0.
  91. *
  92. * \sa SDL_AtomicLock
  93. * \sa SDL_AtomicUnlock
  94. */
  95. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
  96. /**
  97. * Lock a spin lock by setting it to a non-zero value.
  98. *
  99. * ***Please note that spinlocks are dangerous if you don't know what you're
  100. * doing. Please be careful using any sort of spinlock!***
  101. *
  102. * \param lock a pointer to a lock variable
  103. *
  104. * \since This function is available since SDL 2.0.0.
  105. *
  106. * \sa SDL_AtomicTryLock
  107. * \sa SDL_AtomicUnlock
  108. */
  109. extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
  110. /**
  111. * Unlock a spin lock by setting it to 0.
  112. *
  113. * Always returns immediately.
  114. *
  115. * ***Please note that spinlocks are dangerous if you don't know what you're
  116. * doing. Please be careful using any sort of spinlock!***
  117. *
  118. * \param lock a pointer to a lock variable
  119. *
  120. * \since This function is available since SDL 2.0.0.
  121. *
  122. * \sa SDL_AtomicLock
  123. * \sa SDL_AtomicTryLock
  124. */
  125. extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
  126. /* @} *//* SDL AtomicLock */
  127. /**
  128. * The compiler barrier prevents the compiler from reordering
  129. * reads and writes to globally visible variables across the call.
  130. */
  131. #if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
  132. void _ReadWriteBarrier(void);
  133. #pragma intrinsic(_ReadWriteBarrier)
  134. #define SDL_CompilerBarrier() _ReadWriteBarrier()
  135. #elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
  136. /* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
  137. #define SDL_CompilerBarrier() __asm__ __volatile__ ("" : : : "memory")
  138. #elif defined(__WATCOMC__)
  139. extern __inline void SDL_CompilerBarrier(void);
  140. #pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
  141. #else
  142. #define SDL_CompilerBarrier() \
  143. { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
  144. #endif
  145. /**
  146. * Memory barriers are designed to prevent reads and writes from being
  147. * reordered by the compiler and being seen out of order on multi-core CPUs.
  148. *
  149. * A typical pattern would be for thread A to write some data and a flag, and
  150. * for thread B to read the flag and get the data. In this case you would
  151. * insert a release barrier between writing the data and the flag,
  152. * guaranteeing that the data write completes no later than the flag is
  153. * written, and you would insert an acquire barrier between reading the flag
  154. * and reading the data, to ensure that all the reads associated with the flag
  155. * have completed.
  156. *
  157. * In this pattern you should always see a release barrier paired with an
  158. * acquire barrier and you should gate the data reads/writes with a single
  159. * flag variable.
  160. *
  161. * For more information on these semantics, take a look at the blog post:
  162. * http://preshing.com/20120913/acquire-and-release-semantics
  163. *
  164. * \since This function is available since SDL 2.0.6.
  165. */
  166. extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
  167. extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
  168. #if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
  169. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
  170. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
  171. #elif defined(__GNUC__) && defined(__aarch64__)
  172. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
  173. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
  174. #elif defined(__GNUC__) && defined(__arm__)
  175. #if 0 /* defined(__LINUX__) || defined(__ANDROID__) */
  176. /* Information from:
  177. https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
  178. The Linux kernel provides a helper function which provides the right code for a memory barrier,
  179. hard-coded at address 0xffff0fa0
  180. */
  181. typedef void (*SDL_KernelMemoryBarrierFunc)();
  182. #define SDL_MemoryBarrierRelease() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
  183. #define SDL_MemoryBarrierAcquire() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
  184. #elif 0 /* defined(__QNXNTO__) */
  185. #include <sys/cpuinline.h>
  186. #define SDL_MemoryBarrierRelease() __cpu_membarrier()
  187. #define SDL_MemoryBarrierAcquire() __cpu_membarrier()
  188. #else
  189. #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__)
  190. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
  191. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
  192. #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__)
  193. #ifdef __thumb__
  194. /* The mcr instruction isn't available in thumb mode, use real functions */
  195. #define SDL_MEMORY_BARRIER_USES_FUNCTION
  196. #define SDL_MemoryBarrierRelease() SDL_MemoryBarrierReleaseFunction()
  197. #define SDL_MemoryBarrierAcquire() SDL_MemoryBarrierAcquireFunction()
  198. #else
  199. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
  200. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
  201. #endif /* __thumb__ */
  202. #else
  203. #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
  204. #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
  205. #endif /* __LINUX__ || __ANDROID__ */
  206. #endif /* __GNUC__ && __arm__ */
  207. #else
  208. #if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
  209. /* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
  210. #include <mbarrier.h>
  211. #define SDL_MemoryBarrierRelease() __machine_rel_barrier()
  212. #define SDL_MemoryBarrierAcquire() __machine_acq_barrier()
  213. #else
  214. /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
  215. #define SDL_MemoryBarrierRelease() SDL_CompilerBarrier()
  216. #define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
  217. #endif
  218. #endif
  219. /* "REP NOP" is PAUSE, coded for tools that don't know it by that name. */
  220. #if (defined(__GNUC__) || defined(__clang__)) && (defined(__i386__) || defined(__x86_64__))
  221. #define SDL_CPUPauseInstruction() __asm__ __volatile__("pause\n") /* Some assemblers can't do REP NOP, so go with PAUSE. */
  222. #elif (defined(__arm__) && __ARM_ARCH >= 7) || defined(__aarch64__)
  223. #define SDL_CPUPauseInstruction() __asm__ __volatile__("yield" ::: "memory")
  224. #elif (defined(__powerpc__) || defined(__powerpc64__))
  225. #define SDL_CPUPauseInstruction() __asm__ __volatile__("or 27,27,27");
  226. #elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
  227. #define SDL_CPUPauseInstruction() _mm_pause() /* this is actually "rep nop" and not a SIMD instruction. No inline asm in MSVC x86-64! */
  228. #elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
  229. #define SDL_CPUPauseInstruction() __yield()
  230. #elif defined(__WATCOMC__) && defined(__386__)
  231. /* watcom assembler rejects PAUSE if CPU < i686, and it refuses REP NOP as an invalid combination. Hardcode the bytes. */
  232. extern __inline void SDL_CPUPauseInstruction(void);
  233. #pragma aux SDL_CPUPauseInstruction = "db 0f3h,90h"
  234. #else
  235. #define SDL_CPUPauseInstruction()
  236. #endif
  237. /**
  238. * \brief A type representing an atomic integer value. It is a struct
  239. * so people don't accidentally use numeric operations on it.
  240. */
  241. typedef struct { int value; } SDL_atomic_t;
  242. /**
  243. * Set an atomic variable to a new value if it is currently an old value.
  244. *
  245. * ***Note: If you don't know what this function is for, you shouldn't use
  246. * it!***
  247. *
  248. * \param a a pointer to an SDL_atomic_t variable to be modified
  249. * \param oldval the old value
  250. * \param newval the new value
  251. * \returns SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
  252. *
  253. * \since This function is available since SDL 2.0.0.
  254. *
  255. * \sa SDL_AtomicCASPtr
  256. * \sa SDL_AtomicGet
  257. * \sa SDL_AtomicSet
  258. */
  259. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
  260. /**
  261. * Set an atomic variable to a value.
  262. *
  263. * This function also acts as a full memory barrier.
  264. *
  265. * ***Note: If you don't know what this function is for, you shouldn't use
  266. * it!***
  267. *
  268. * \param a a pointer to an SDL_atomic_t variable to be modified
  269. * \param v the desired value
  270. * \returns the previous value of the atomic variable.
  271. *
  272. * \since This function is available since SDL 2.0.2.
  273. *
  274. * \sa SDL_AtomicGet
  275. */
  276. extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
  277. /**
  278. * Get the value of an atomic variable.
  279. *
  280. * ***Note: If you don't know what this function is for, you shouldn't use
  281. * it!***
  282. *
  283. * \param a a pointer to an SDL_atomic_t variable
  284. * \returns the current value of an atomic variable.
  285. *
  286. * \since This function is available since SDL 2.0.2.
  287. *
  288. * \sa SDL_AtomicSet
  289. */
  290. extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
  291. /**
  292. * Add to an atomic variable.
  293. *
  294. * This function also acts as a full memory barrier.
  295. *
  296. * ***Note: If you don't know what this function is for, you shouldn't use
  297. * it!***
  298. *
  299. * \param a a pointer to an SDL_atomic_t variable to be modified
  300. * \param v the desired value to add
  301. * \returns the previous value of the atomic variable.
  302. *
  303. * \since This function is available since SDL 2.0.2.
  304. *
  305. * \sa SDL_AtomicDecRef
  306. * \sa SDL_AtomicIncRef
  307. */
  308. extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
  309. /**
  310. * \brief Increment an atomic variable used as a reference count.
  311. */
  312. #ifndef SDL_AtomicIncRef
  313. #define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
  314. #endif
  315. /**
  316. * \brief Decrement an atomic variable used as a reference count.
  317. *
  318. * \return SDL_TRUE if the variable reached zero after decrementing,
  319. * SDL_FALSE otherwise
  320. */
  321. #ifndef SDL_AtomicDecRef
  322. #define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
  323. #endif
  324. /**
  325. * Set a pointer to a new value if it is currently an old value.
  326. *
  327. * ***Note: If you don't know what this function is for, you shouldn't use
  328. * it!***
  329. *
  330. * \param a a pointer to a pointer
  331. * \param oldval the old pointer value
  332. * \param newval the new pointer value
  333. * \returns SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
  334. *
  335. * \since This function is available since SDL 2.0.0.
  336. *
  337. * \sa SDL_AtomicCAS
  338. * \sa SDL_AtomicGetPtr
  339. * \sa SDL_AtomicSetPtr
  340. */
  341. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
  342. /**
  343. * Set a pointer to a value atomically.
  344. *
  345. * ***Note: If you don't know what this function is for, you shouldn't use
  346. * it!***
  347. *
  348. * \param a a pointer to a pointer
  349. * \param v the desired pointer value
  350. * \returns the previous value of the pointer.
  351. *
  352. * \since This function is available since SDL 2.0.2.
  353. *
  354. * \sa SDL_AtomicCASPtr
  355. * \sa SDL_AtomicGetPtr
  356. */
  357. extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
  358. /**
  359. * Get the value of a pointer atomically.
  360. *
  361. * ***Note: If you don't know what this function is for, you shouldn't use
  362. * it!***
  363. *
  364. * \param a a pointer to a pointer
  365. * \returns the current value of a pointer.
  366. *
  367. * \since This function is available since SDL 2.0.2.
  368. *
  369. * \sa SDL_AtomicCASPtr
  370. * \sa SDL_AtomicSetPtr
  371. */
  372. extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
  373. /* Ends C function definitions when using C++ */
  374. #ifdef __cplusplus
  375. }
  376. #endif
  377. #include "close_code.h"
  378. #endif /* SDL_atomic_h_ */
  379. /* vi: set ts=4 sw=4 expandtab: */