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  1. /*
  2.   Simple DirectMedia Layer
  3.   Copyright (C) 1997-2023 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.  * Try to lock a spin lock by setting it to a non-zero value.
  93.  *
  94.  * ***Please note that spinlocks are dangerous if you don't know what you're
  95.  * doing. Please be careful using any sort of spinlock!***
  96.  *
  97.  * \param lock a pointer to a lock variable
  98.  * \returns SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already
  99.  *          held.
  100.  *
  101.  * \since This function is available since SDL 2.0.0.
  102.  *
  103.  * \sa SDL_AtomicLock
  104.  * \sa SDL_AtomicUnlock
  105.  */
  106. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
  107.  
  108. /**
  109.  * Lock a spin lock by setting it to a non-zero value.
  110.  *
  111.  * ***Please note that spinlocks are dangerous if you don't know what you're
  112.  * doing. Please be careful using any sort of spinlock!***
  113.  *
  114.  * \param lock a pointer to a lock variable
  115.  *
  116.  * \since This function is available since SDL 2.0.0.
  117.  *
  118.  * \sa SDL_AtomicTryLock
  119.  * \sa SDL_AtomicUnlock
  120.  */
  121. extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
  122.  
  123. /**
  124.  * Unlock a spin lock by setting it to 0.
  125.  *
  126.  * Always returns immediately.
  127.  *
  128.  * ***Please note that spinlocks are dangerous if you don't know what you're
  129.  * doing. Please be careful using any sort of spinlock!***
  130.  *
  131.  * \param lock a pointer to a lock variable
  132.  *
  133.  * \since This function is available since SDL 2.0.0.
  134.  *
  135.  * \sa SDL_AtomicLock
  136.  * \sa SDL_AtomicTryLock
  137.  */
  138. extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
  139.  
  140. /* @} *//* SDL AtomicLock */
  141.  
  142.  
  143. /**
  144.  * The compiler barrier prevents the compiler from reordering
  145.  * reads and writes to globally visible variables across the call.
  146.  */
  147. #if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
  148. void _ReadWriteBarrier(void);
  149. #pragma intrinsic(_ReadWriteBarrier)
  150. #define SDL_CompilerBarrier()   _ReadWriteBarrier()
  151. #elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
  152. /* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
  153. #define SDL_CompilerBarrier()   __asm__ __volatile__ ("" : : : "memory")
  154. #elif defined(__WATCOMC__)
  155. extern __inline void SDL_CompilerBarrier(void);
  156. #pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
  157. #else
  158. #define SDL_CompilerBarrier()   \
  159. { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
  160. #endif
  161.  
  162. /**
  163.  * Memory barriers are designed to prevent reads and writes from being
  164.  * reordered by the compiler and being seen out of order on multi-core CPUs.
  165.  *
  166.  * A typical pattern would be for thread A to write some data and a flag, and
  167.  * for thread B to read the flag and get the data. In this case you would
  168.  * insert a release barrier between writing the data and the flag,
  169.  * guaranteeing that the data write completes no later than the flag is
  170.  * written, and you would insert an acquire barrier between reading the flag
  171.  * and reading the data, to ensure that all the reads associated with the flag
  172.  * have completed.
  173.  *
  174.  * In this pattern you should always see a release barrier paired with an
  175.  * acquire barrier and you should gate the data reads/writes with a single
  176.  * flag variable.
  177.  *
  178.  * For more information on these semantics, take a look at the blog post:
  179.  * http://preshing.com/20120913/acquire-and-release-semantics
  180.  *
  181.  * \since This function is available since SDL 2.0.6.
  182.  */
  183. extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
  184. extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
  185.  
  186. #if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
  187. #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("lwsync" : : : "memory")
  188. #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("lwsync" : : : "memory")
  189. #elif defined(__GNUC__) && defined(__aarch64__)
  190. #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("dmb ish" : : : "memory")
  191. #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("dmb ish" : : : "memory")
  192. #elif defined(__GNUC__) && defined(__arm__)
  193. #if 0 /* defined(__LINUX__) || defined(__ANDROID__) */
  194. /* Information from:
  195.    https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
  196.  
  197.    The Linux kernel provides a helper function which provides the right code for a memory barrier,
  198.    hard-coded at address 0xffff0fa0
  199. */
  200. typedef void (*SDL_KernelMemoryBarrierFunc)();
  201. #define SDL_MemoryBarrierRelease()      ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
  202. #define SDL_MemoryBarrierAcquire()      ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
  203. #elif 0 /* defined(__QNXNTO__) */
  204. #include <sys/cpuinline.h>
  205.  
  206. #define SDL_MemoryBarrierRelease()   __cpu_membarrier()
  207. #define SDL_MemoryBarrierAcquire()   __cpu_membarrier()
  208. #else
  209. #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__)
  210. #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("dmb ish" : : : "memory")
  211. #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("dmb ish" : : : "memory")
  212. #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__)
  213. #ifdef __thumb__
  214. /* The mcr instruction isn't available in thumb mode, use real functions */
  215. #define SDL_MEMORY_BARRIER_USES_FUNCTION
  216. #define SDL_MemoryBarrierRelease()   SDL_MemoryBarrierReleaseFunction()
  217. #define SDL_MemoryBarrierAcquire()   SDL_MemoryBarrierAcquireFunction()
  218. #else
  219. #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
  220. #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
  221. #endif /* __thumb__ */
  222. #else
  223. #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("" : : : "memory")
  224. #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("" : : : "memory")
  225. #endif /* __LINUX__ || __ANDROID__ */
  226. #endif /* __GNUC__ && __arm__ */
  227. #else
  228. #if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
  229. /* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
  230. #include <mbarrier.h>
  231. #define SDL_MemoryBarrierRelease()  __machine_rel_barrier()
  232. #define SDL_MemoryBarrierAcquire()  __machine_acq_barrier()
  233. #else
  234. /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
  235. #define SDL_MemoryBarrierRelease()  SDL_CompilerBarrier()
  236. #define SDL_MemoryBarrierAcquire()  SDL_CompilerBarrier()
  237. #endif
  238. #endif
  239.  
  240. /* "REP NOP" is PAUSE, coded for tools that don't know it by that name. */
  241. #if (defined(__GNUC__) || defined(__clang__)) && (defined(__i386__) || defined(__x86_64__))
  242.     #define SDL_CPUPauseInstruction() __asm__ __volatile__("pause\n")  /* Some assemblers can't do REP NOP, so go with PAUSE. */
  243. #elif (defined(__arm__) && defined(__ARM_ARCH) && __ARM_ARCH >= 7) || defined(__aarch64__)
  244.     #define SDL_CPUPauseInstruction() __asm__ __volatile__("yield" ::: "memory")
  245. #elif (defined(__powerpc__) || defined(__powerpc64__))
  246.     #define SDL_CPUPauseInstruction() __asm__ __volatile__("or 27,27,27");
  247. #elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
  248.     #define SDL_CPUPauseInstruction() _mm_pause()  /* this is actually "rep nop" and not a SIMD instruction. No inline asm in MSVC x86-64! */
  249. #elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
  250.     #define SDL_CPUPauseInstruction() __yield()
  251. #elif defined(__WATCOMC__) && defined(__386__)
  252.     extern __inline void SDL_CPUPauseInstruction(void);
  253.     #pragma aux SDL_CPUPauseInstruction = ".686p" ".xmm2" "pause"
  254. #else
  255.     #define SDL_CPUPauseInstruction()
  256. #endif
  257.  
  258.  
  259. /**
  260.  * \brief A type representing an atomic integer value.  It is a struct
  261.  *        so people don't accidentally use numeric operations on it.
  262.  */
  263. typedef struct { int value; } SDL_atomic_t;
  264.  
  265. /**
  266.  * Set an atomic variable to a new value if it is currently an old value.
  267.  *
  268.  * ***Note: If you don't know what this function is for, you shouldn't use
  269.  * it!***
  270.  *
  271.  * \param a a pointer to an SDL_atomic_t variable to be modified
  272.  * \param oldval the old value
  273.  * \param newval the new value
  274.  * \returns SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
  275.  *
  276.  * \since This function is available since SDL 2.0.0.
  277.  *
  278.  * \sa SDL_AtomicCASPtr
  279.  * \sa SDL_AtomicGet
  280.  * \sa SDL_AtomicSet
  281.  */
  282. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
  283.  
  284. /**
  285.  * Set an atomic variable to a value.
  286.  *
  287.  * This function also acts as a full memory barrier.
  288.  *
  289.  * ***Note: If you don't know what this function is for, you shouldn't use
  290.  * it!***
  291.  *
  292.  * \param a a pointer to an SDL_atomic_t variable to be modified
  293.  * \param v the desired value
  294.  * \returns the previous value of the atomic variable.
  295.  *
  296.  * \since This function is available since SDL 2.0.2.
  297.  *
  298.  * \sa SDL_AtomicGet
  299.  */
  300. extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
  301.  
  302. /**
  303.  * Get the value of an atomic variable.
  304.  *
  305.  * ***Note: If you don't know what this function is for, you shouldn't use
  306.  * it!***
  307.  *
  308.  * \param a a pointer to an SDL_atomic_t variable
  309.  * \returns the current value of an atomic variable.
  310.  *
  311.  * \since This function is available since SDL 2.0.2.
  312.  *
  313.  * \sa SDL_AtomicSet
  314.  */
  315. extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
  316.  
  317. /**
  318.  * Add to an atomic variable.
  319.  *
  320.  * This function also acts as a full memory barrier.
  321.  *
  322.  * ***Note: If you don't know what this function is for, you shouldn't use
  323.  * it!***
  324.  *
  325.  * \param a a pointer to an SDL_atomic_t variable to be modified
  326.  * \param v the desired value to add
  327.  * \returns the previous value of the atomic variable.
  328.  *
  329.  * \since This function is available since SDL 2.0.2.
  330.  *
  331.  * \sa SDL_AtomicDecRef
  332.  * \sa SDL_AtomicIncRef
  333.  */
  334. extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
  335.  
  336. /**
  337.  * \brief Increment an atomic variable used as a reference count.
  338.  */
  339. #ifndef SDL_AtomicIncRef
  340. #define SDL_AtomicIncRef(a)    SDL_AtomicAdd(a, 1)
  341. #endif
  342.  
  343. /**
  344.  * \brief Decrement an atomic variable used as a reference count.
  345.  *
  346.  * \return SDL_TRUE if the variable reached zero after decrementing,
  347.  *         SDL_FALSE otherwise
  348.  */
  349. #ifndef SDL_AtomicDecRef
  350. #define SDL_AtomicDecRef(a)    (SDL_AtomicAdd(a, -1) == 1)
  351. #endif
  352.  
  353. /**
  354.  * Set a pointer to a new value if it is currently an old value.
  355.  *
  356.  * ***Note: If you don't know what this function is for, you shouldn't use
  357.  * it!***
  358.  *
  359.  * \param a a pointer to a pointer
  360.  * \param oldval the old pointer value
  361.  * \param newval the new pointer value
  362.  * \returns SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
  363.  *
  364.  * \since This function is available since SDL 2.0.0.
  365.  *
  366.  * \sa SDL_AtomicCAS
  367.  * \sa SDL_AtomicGetPtr
  368.  * \sa SDL_AtomicSetPtr
  369.  */
  370. extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
  371.  
  372. /**
  373.  * Set a pointer to a value atomically.
  374.  *
  375.  * ***Note: If you don't know what this function is for, you shouldn't use
  376.  * it!***
  377.  *
  378.  * \param a a pointer to a pointer
  379.  * \param v the desired pointer value
  380.  * \returns the previous value of the pointer.
  381.  *
  382.  * \since This function is available since SDL 2.0.2.
  383.  *
  384.  * \sa SDL_AtomicCASPtr
  385.  * \sa SDL_AtomicGetPtr
  386.  */
  387. extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
  388.  
  389. /**
  390.  * Get the value of a pointer atomically.
  391.  *
  392.  * ***Note: If you don't know what this function is for, you shouldn't use
  393.  * it!***
  394.  *
  395.  * \param a a pointer to a pointer
  396.  * \returns the current value of a pointer.
  397.  *
  398.  * \since This function is available since SDL 2.0.2.
  399.  *
  400.  * \sa SDL_AtomicCASPtr
  401.  * \sa SDL_AtomicSetPtr
  402.  */
  403. extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
  404.  
  405. /* Ends C function definitions when using C++ */
  406. #ifdef __cplusplus
  407. }
  408. #endif
  409.  
  410. #include "close_code.h"
  411.  
  412. #endif /* SDL_atomic_h_ */
  413.  
  414. /* vi: set ts=4 sw=4 expandtab: */
  415.