SDL_atomic.h

00001 /*
00002   Simple DirectMedia Layer
00003   Copyright (C) 1997-2014 Sam Lantinga <slouken@libsdl.org>
00004 
00005   This software is provided 'as-is', without any express or implied
00006   warranty.  In no event will the authors be held liable for any damages
00007   arising from the use of this software.
00008 
00009   Permission is granted to anyone to use this software for any purpose,
00010   including commercial applications, and to alter it and redistribute it
00011   freely, subject to the following restrictions:
00012 
00013   1. The origin of this software must not be misrepresented; you must not
00014      claim that you wrote the original software. If you use this software
00015      in a product, an acknowledgment in the product documentation would be
00016      appreciated but is not required.
00017   2. Altered source versions must be plainly marked as such, and must not be
00018      misrepresented as being the original software.
00019   3. This notice may not be removed or altered from any source distribution.
00020 */
00021 
00022 /**
00023  * \file SDL_atomic.h
00024  *
00025  * Atomic operations.
00026  *
00027  * IMPORTANT:
00028  * If you are not an expert in concurrent lockless programming, you should
00029  * only be using the atomic lock and reference counting functions in this
00030  * file.  In all other cases you should be protecting your data structures
00031  * with full mutexes.
00032  *
00033  * The list of "safe" functions to use are:
00034  *  SDL_AtomicLock()
00035  *  SDL_AtomicUnlock()
00036  *  SDL_AtomicIncRef()
00037  *  SDL_AtomicDecRef()
00038  *
00039  * Seriously, here be dragons!
00040  * ^^^^^^^^^^^^^^^^^^^^^^^^^^^
00041  *
00042  * You can find out a little more about lockless programming and the
00043  * subtle issues that can arise here:
00044  * http://msdn.microsoft.com/en-us/library/ee418650%28v=vs.85%29.aspx
00045  *
00046  * There's also lots of good information here:
00047  * http://www.1024cores.net/home/lock-free-algorithms
00048  * http://preshing.com/
00049  *
00050  * These operations may or may not actually be implemented using
00051  * processor specific atomic operations. When possible they are
00052  * implemented as true processor specific atomic operations. When that
00053  * is not possible the are implemented using locks that *do* use the
00054  * available atomic operations.
00055  *
00056  * All of the atomic operations that modify memory are full memory barriers.
00057  */
00058 
00059 #ifndef _SDL_atomic_h_
00060 #define _SDL_atomic_h_
00061 
00062 #include "SDL_stdinc.h"
00063 #include "SDL_platform.h"
00064 
00065 #include "begin_code.h"
00066 
00067 /* Set up for C function definitions, even when using C++ */
00068 #ifdef __cplusplus
00069 extern "C" {
00070 #endif
00071 
00072 /**
00073  * \name SDL AtomicLock
00074  *
00075  * The atomic locks are efficient spinlocks using CPU instructions,
00076  * but are vulnerable to starvation and can spin forever if a thread
00077  * holding a lock has been terminated.  For this reason you should
00078  * minimize the code executed inside an atomic lock and never do
00079  * expensive things like API or system calls while holding them.
00080  *
00081  * The atomic locks are not safe to lock recursively.
00082  *
00083  * Porting Note:
00084  * The spin lock functions and type are required and can not be
00085  * emulated because they are used in the atomic emulation code.
00086  */
00087 /* @{ */
00088 
00089 typedef int SDL_SpinLock;
00090 
00091 /**
00092  * \brief Try to lock a spin lock by setting it to a non-zero value.
00093  *
00094  * \param lock Points to the lock.
00095  *
00096  * \return SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already held.
00097  */
00098 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
00099 
00100 /**
00101  * \brief Lock a spin lock by setting it to a non-zero value.
00102  *
00103  * \param lock Points to the lock.
00104  */
00105 extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
00106 
00107 /**
00108  * \brief Unlock a spin lock by setting it to 0. Always returns immediately
00109  *
00110  * \param lock Points to the lock.
00111  */
00112 extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
00113 
00114 /* @} *//* SDL AtomicLock */
00115 
00116 
00117 /**
00118  * The compiler barrier prevents the compiler from reordering
00119  * reads and writes to globally visible variables across the call.
00120  */
00121 #if defined(_MSC_VER) && (_MSC_VER > 1200)
00122 void _ReadWriteBarrier(void);
00123 #pragma intrinsic(_ReadWriteBarrier)
00124 #define SDL_CompilerBarrier()   _ReadWriteBarrier()
00125 #elif defined(__GNUC__)
00126 #define SDL_CompilerBarrier()   __asm__ __volatile__ ("" : : : "memory")
00127 #else
00128 #define SDL_CompilerBarrier()   \
00129 { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
00130 #endif
00131 
00132 /**
00133  * Memory barriers are designed to prevent reads and writes from being
00134  * reordered by the compiler and being seen out of order on multi-core CPUs.
00135  *
00136  * A typical pattern would be for thread A to write some data and a flag,
00137  * and for thread B to read the flag and get the data. In this case you
00138  * would insert a release barrier between writing the data and the flag,
00139  * guaranteeing that the data write completes no later than the flag is
00140  * written, and you would insert an acquire barrier between reading the
00141  * flag and reading the data, to ensure that all the reads associated
00142  * with the flag have completed.
00143  *
00144  * In this pattern you should always see a release barrier paired with
00145  * an acquire barrier and you should gate the data reads/writes with a
00146  * single flag variable.
00147  *
00148  * For more information on these semantics, take a look at the blog post:
00149  * http://preshing.com/20120913/acquire-and-release-semantics
00150  */
00151 #if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
00152 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("lwsync" : : : "memory")
00153 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("lwsync" : : : "memory")
00154 #elif defined(__GNUC__) && defined(__arm__)
00155 #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__)
00156 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("dmb ish" : : : "memory")
00157 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("dmb ish" : : : "memory")
00158 #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__)
00159 #ifdef __thumb__
00160 /* The mcr instruction isn't available in thumb mode, use real functions */
00161 extern DECLSPEC void SDLCALL SDL_MemoryBarrierRelease();
00162 extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquire();
00163 #else
00164 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
00165 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
00166 #endif /* __thumb__ */
00167 #else
00168 #define SDL_MemoryBarrierRelease()   __asm__ __volatile__ ("" : : : "memory")
00169 #define SDL_MemoryBarrierAcquire()   __asm__ __volatile__ ("" : : : "memory")
00170 #endif /* __GNUC__ && __arm__ */
00171 #else
00172 /* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
00173 #define SDL_MemoryBarrierRelease()  SDL_CompilerBarrier()
00174 #define SDL_MemoryBarrierAcquire()  SDL_CompilerBarrier()
00175 #endif
00176 
00177 /**
00178  * \brief A type representing an atomic integer value.  It is a struct
00179  *        so people don't accidentally use numeric operations on it.
00180  */
00181 typedef struct { int value; } SDL_atomic_t;
00182 
00183 /**
00184  * \brief Set an atomic variable to a new value if it is currently an old value.
00185  *
00186  * \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
00187  *
00188  * \note If you don't know what this function is for, you shouldn't use it!
00189 */
00190 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
00191 
00192 /**
00193  * \brief Set an atomic variable to a value.
00194  *
00195  * \return The previous value of the atomic variable.
00196  */
00197 extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
00198 
00199 /**
00200  * \brief Get the value of an atomic variable
00201  */
00202 extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
00203 
00204 /**
00205  * \brief Add to an atomic variable.
00206  *
00207  * \return The previous value of the atomic variable.
00208  *
00209  * \note This same style can be used for any number operation
00210  */
00211 extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
00212 
00213 /**
00214  * \brief Increment an atomic variable used as a reference count.
00215  */
00216 #ifndef SDL_AtomicIncRef
00217 #define SDL_AtomicIncRef(a)    SDL_AtomicAdd(a, 1)
00218 #endif
00219 
00220 /**
00221  * \brief Decrement an atomic variable used as a reference count.
00222  *
00223  * \return SDL_TRUE if the variable reached zero after decrementing,
00224  *         SDL_FALSE otherwise
00225  */
00226 #ifndef SDL_AtomicDecRef
00227 #define SDL_AtomicDecRef(a)    (SDL_AtomicAdd(a, -1) == 1)
00228 #endif
00229 
00230 /**
00231  * \brief Set a pointer to a new value if it is currently an old value.
00232  *
00233  * \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
00234  *
00235  * \note If you don't know what this function is for, you shouldn't use it!
00236 */
00237 extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
00238 
00239 /**
00240  * \brief Set a pointer to a value atomically.
00241  *
00242  * \return The previous value of the pointer.
00243  */
00244 extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
00245 
00246 /**
00247  * \brief Get the value of a pointer atomically.
00248  */
00249 extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
00250 
00251 /* Ends C function definitions when using C++ */
00252 #ifdef __cplusplus
00253 }
00254 #endif
00255 
00256 #include "close_code.h"
00257 
00258 #endif /* _SDL_atomic_h_ */
00259 
00260 /* vi: set ts=4 sw=4 expandtab: */