include/SDL_atomic.h
author Sam Lantinga <slouken@libsdl.org>
Sat, 02 Jan 2016 10:10:34 -0800
changeset 9998 f67cf37e9cd4
parent 9619 b94b6d0bff0f
child 10638 d11daa346140
permissions -rw-r--r--
Updated copyright to 2016
     1 /*
     2   Simple DirectMedia Layer
     3   Copyright (C) 1997-2016 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)
   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 #else
   129 #define SDL_CompilerBarrier()   \
   130 { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
   131 #endif
   132 
   133 /**
   134  * Memory barriers are designed to prevent reads and writes from being
   135  * reordered by the compiler and being seen out of order on multi-core CPUs.
   136  *
   137  * A typical pattern would be for thread A to write some data and a flag,
   138  * and for thread B to read the flag and get the data. In this case you
   139  * would insert a release barrier between writing the data and the flag,
   140  * guaranteeing that the data write completes no later than the flag is
   141  * written, and you would insert an acquire barrier between reading the
   142  * flag and reading the data, to ensure that all the reads associated
   143  * with the flag have completed.
   144  *
   145  * In this pattern you should always see a release barrier paired with
   146  * an acquire barrier and you should gate the data reads/writes with a
   147  * single flag variable.
   148  *
   149  * For more information on these semantics, take a look at the blog post:
   150  * http://preshing.com/20120913/acquire-and-release-semantics
   151  */
   152 #if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
   153 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("lwsync" : : : "memory")
   154 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("lwsync" : : : "memory")
   155 #elif defined(__GNUC__) && defined(__arm__)
   156 #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__)
   157 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("dmb ish" : : : "memory")
   158 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("dmb ish" : : : "memory")
   159 #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__)
   160 #ifdef __thumb__
   161 /* The mcr instruction isn't available in thumb mode, use real functions */
   162 extern DECLSPEC void SDLCALL SDL_MemoryBarrierRelease();
   163 extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquire();
   164 #else
   165 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
   166 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
   167 #endif /* __thumb__ */
   168 #else
   169 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("" : : : "memory")
   170 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("" : : : "memory")
   171 #endif /* __GNUC__ && __arm__ */
   172 #else
   173 #if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
   174 /* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
   175 #include <mbarrier.h>
   176 #define SDL_MemoryBarrierRelease()  __machine_rel_barrier()
   177 #define SDL_MemoryBarrierAcquire()  __machine_acq_barrier()
   178 #else
   179 /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
   180 #define SDL_MemoryBarrierRelease()  SDL_CompilerBarrier()
   181 #define SDL_MemoryBarrierAcquire()  SDL_CompilerBarrier()
   182 #endif
   183 #endif
   184 
   185 /**
   186  * \brief A type representing an atomic integer value.  It is a struct
   187  *        so people don't accidentally use numeric operations on it.
   188  */
   189 typedef struct { int value; } SDL_atomic_t;
   190 
   191 /**
   192  * \brief Set an atomic variable to a new value if it is currently an old value.
   193  *
   194  * \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
   195  *
   196  * \note If you don't know what this function is for, you shouldn't use it!
   197 */
   198 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
   199 
   200 /**
   201  * \brief Set an atomic variable to a value.
   202  *
   203  * \return The previous value of the atomic variable.
   204  */
   205 extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
   206 
   207 /**
   208  * \brief Get the value of an atomic variable
   209  */
   210 extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
   211 
   212 /**
   213  * \brief Add to an atomic variable.
   214  *
   215  * \return The previous value of the atomic variable.
   216  *
   217  * \note This same style can be used for any number operation
   218  */
   219 extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
   220 
   221 /**
   222  * \brief Increment an atomic variable used as a reference count.
   223  */
   224 #ifndef SDL_AtomicIncRef
   225 #define SDL_AtomicIncRef(a)    SDL_AtomicAdd(a, 1)
   226 #endif
   227 
   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 /**
   239  * \brief Set a pointer to a new value if it is currently an old value.
   240  *
   241  * \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
   242  *
   243  * \note If you don't know what this function is for, you shouldn't use it!
   244 */
   245 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
   246 
   247 /**
   248  * \brief Set a pointer to a value atomically.
   249  *
   250  * \return The previous value of the pointer.
   251  */
   252 extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
   253 
   254 /**
   255  * \brief Get the value of a pointer atomically.
   256  */
   257 extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
   258 
   259 /* Ends C function definitions when using C++ */
   260 #ifdef __cplusplus
   261 }
   262 #endif
   263 
   264 #include "close_code.h"
   265 
   266 #endif /* _SDL_atomic_h_ */
   267 
   268 /* vi: set ts=4 sw=4 expandtab: */