include/SDL_atomic.h
author Ryan C. Gordon <icculus@icculus.org>
Sun, 24 Nov 2013 21:04:51 -0500
changeset 8088 e5d77d5e0fe2
parent 7678 286c42d7c5ed
child 8149 681eb46b8ac4
permissions -rw-r--r--
Moved atomic API implementation out of headers.
     1 /*
     2   Simple DirectMedia Layer
     3   Copyright (C) 1997-2013 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__)
   126 #define SDL_CompilerBarrier()   __asm__ __volatile__ ("" : : : "memory")
   127 #else
   128 #define SDL_CompilerBarrier()   \
   129 { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
   130 #endif
   131 
   132 /**
   133  * Memory barriers are designed to prevent reads and writes from being
   134  * reordered by the compiler and being seen out of order on multi-core CPUs.
   135  *
   136  * A typical pattern would be for thread A to write some data and a flag,
   137  * and for thread B to read the flag and get the data. In this case you
   138  * would insert a release barrier between writing the data and the flag,
   139  * guaranteeing that the data write completes no later than the flag is
   140  * written, and you would insert an acquire barrier between reading the
   141  * flag and reading the data, to ensure that all the reads associated
   142  * with the flag have completed.
   143  *
   144  * In this pattern you should always see a release barrier paired with
   145  * an acquire barrier and you should gate the data reads/writes with a
   146  * single flag variable.
   147  *
   148  * For more information on these semantics, take a look at the blog post:
   149  * http://preshing.com/20120913/acquire-and-release-semantics
   150  */
   151 #if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
   152 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("lwsync" : : : "memory")
   153 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("lwsync" : : : "memory")
   154 #elif defined(__GNUC__) && defined(__arm__)
   155 #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__)
   156 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("dmb ish" : : : "memory")
   157 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("dmb ish" : : : "memory")
   158 #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__)
   159 #ifdef __thumb__
   160 /* The mcr instruction isn't available in thumb mode, use real functions */
   161 extern DECLSPEC void SDLCALL SDL_MemoryBarrierRelease();
   162 extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquire();
   163 #else
   164 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
   165 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
   166 #endif /* __thumb__ */
   167 #else
   168 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("" : : : "memory")
   169 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("" : : : "memory")
   170 #endif /* __GNUC__ && __arm__ */
   171 #else
   172 /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
   173 #define SDL_MemoryBarrierRelease()  SDL_CompilerBarrier()
   174 #define SDL_MemoryBarrierAcquire()  SDL_CompilerBarrier()
   175 #endif
   176 
   177 /**
   178  * \brief A type representing an atomic integer value.  It is a struct
   179  *        so people don't accidentally use numeric operations on it.
   180  */
   181 typedef struct { int value; } SDL_atomic_t;
   182 
   183 /**
   184  * \brief Set an atomic variable to a new value if it is currently an old value.
   185  *
   186  * \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
   187  *
   188  * \note If you don't know what this function is for, you shouldn't use it!
   189 */
   190 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
   191 
   192 /**
   193  * \brief Set an atomic variable to a value.
   194  *
   195  * \return The previous value of the atomic variable.
   196  */
   197 extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
   198 
   199 /**
   200  * \brief Get the value of an atomic variable
   201  */
   202 extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
   203 
   204 /**
   205  * \brief Add to an atomic variable.
   206  *
   207  * \return The previous value of the atomic variable.
   208  *
   209  * \note This same style can be used for any number operation
   210  */
   211 extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
   212 
   213 /**
   214  * \brief Increment an atomic variable used as a reference count.
   215  */
   216 #ifndef SDL_AtomicIncRef
   217 #define SDL_AtomicIncRef(a)    SDL_AtomicAdd(a, 1)
   218 #endif
   219 
   220 /**
   221  * \brief Decrement an atomic variable used as a reference count.
   222  *
   223  * \return SDL_TRUE if the variable reached zero after decrementing,
   224  *         SDL_FALSE otherwise
   225  */
   226 #ifndef SDL_AtomicDecRef
   227 #define SDL_AtomicDecRef(a)    (SDL_AtomicAdd(a, -1) == 1)
   228 #endif
   229 
   230 /**
   231  * \brief Set a pointer to a new value if it is currently an old value.
   232  *
   233  * \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
   234  *
   235  * \note If you don't know what this function is for, you shouldn't use it!
   236 */
   237 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
   238 
   239 /**
   240  * \brief Set a pointer to a value atomically.
   241  *
   242  * \return The previous value of the pointer.
   243  */
   244 extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
   245 
   246 /**
   247  * \brief Get the value of a pointer atomically.
   248  */
   249 extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
   250 
   251 /* Ends C function definitions when using C++ */
   252 #ifdef __cplusplus
   253 }
   254 #endif
   255 
   256 #include "close_code.h"
   257 
   258 #endif /* _SDL_atomic_h_ */
   259 
   260 /* vi: set ts=4 sw=4 expandtab: */