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  1. /*
  2.   Simple DirectMedia Layer
  3.   Copyright (C) 1997-2017 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.  */
  98. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
  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.  */
  105. extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
  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.  */
  112. extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
  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.  */
  155. extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
  156. extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
  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(__arm__)
  162. #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__)
  163. #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("dmb ish" : : : "memory")
  164. #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("dmb ish" : : : "memory")
  165. #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__)
  166. #ifdef __thumb__
  167. /* The mcr instruction isn't available in thumb mode, use real functions */
  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 /* __GNUC__ && __arm__ */
  178. #else
  179. #if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
  180. /* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
  181. #include <mbarrier.h>
  182. #define SDL_MemoryBarrierRelease()  __machine_rel_barrier()
  183. #define SDL_MemoryBarrierAcquire()  __machine_acq_barrier()
  184. #else
  185. /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
  186. #define SDL_MemoryBarrierRelease()  SDL_CompilerBarrier()
  187. #define SDL_MemoryBarrierAcquire()  SDL_CompilerBarrier()
  188. #endif
  189. #endif
  190.  
  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. /**
  198.  * \brief Set an atomic variable to a new value if it is currently an old value.
  199.  *
  200.  * \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
  201.  *
  202.  * \note If you don't know what this function is for, you shouldn't use it!
  203. */
  204. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
  205.  
  206. /**
  207.  * \brief Set an atomic variable to a value.
  208.  *
  209.  * \return The previous value of the atomic variable.
  210.  */
  211. extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
  212.  
  213. /**
  214.  * \brief Get the value of an atomic variable
  215.  */
  216. extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
  217.  
  218. /**
  219.  * \brief Add to an atomic variable.
  220.  *
  221.  * \return The previous value of the atomic variable.
  222.  *
  223.  * \note This same style can be used for any number operation
  224.  */
  225. extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
  226.  
  227. /**
  228.  * \brief Increment an atomic variable used as a reference count.
  229.  */
  230. #ifndef SDL_AtomicIncRef
  231. #define SDL_AtomicIncRef(a)    SDL_AtomicAdd(a, 1)
  232. #endif
  233.  
  234. /**
  235.  * \brief Decrement an atomic variable used as a reference count.
  236.  *
  237.  * \return SDL_TRUE if the variable reached zero after decrementing,
  238.  *         SDL_FALSE otherwise
  239.  */
  240. #ifndef SDL_AtomicDecRef
  241. #define SDL_AtomicDecRef(a)    (SDL_AtomicAdd(a, -1) == 1)
  242. #endif
  243.  
  244. /**
  245.  * \brief Set a pointer to a new value if it is currently an old value.
  246.  *
  247.  * \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
  248.  *
  249.  * \note If you don't know what this function is for, you shouldn't use it!
  250. */
  251. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
  252.  
  253. /**
  254.  * \brief Set a pointer to a value atomically.
  255.  *
  256.  * \return The previous value of the pointer.
  257.  */
  258. extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
  259.  
  260. /**
  261.  * \brief Get the value of a pointer atomically.
  262.  */
  263. extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
  264.  
  265. /* Ends C function definitions when using C++ */
  266. #ifdef __cplusplus
  267. }
  268. #endif
  269.  
  270. #include "close_code.h"
  271.  
  272. #endif /* SDL_atomic_h_ */
  273.  
  274. /* vi: set ts=4 sw=4 expandtab: */
  275.