2 Simple DirectMedia Layer
3 Copyright (C) 1997-2023 Sam Lantinga <slouken@libsdl.org>
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.
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:
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.
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
33 * The list of "safe" functions to use are:
39 * Seriously, here be dragons!
40 * ^^^^^^^^^^^^^^^^^^^^^^^^^^^
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
46 * There's also lots of good information here:
47 * http://www.1024cores.net/home/lock-free-algorithms
48 * http://preshing.com/
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.
56 * All of the atomic operations that modify memory are full memory barriers.
62 #include "SDL_stdinc.h"
63 #include "SDL_platform.h"
65 #include "begin_code.h"
67 /* Set up for C function definitions, even when using C++ */
73 * \name SDL AtomicLock
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.
81 * The atomic locks are not safe to lock recursively.
84 * The spin lock functions and type are required and can not be
85 * emulated because they are used in the atomic emulation code.
89 typedef int SDL_SpinLock
;
92 * Try to lock a spin lock by setting it to a non-zero value.
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!***
97 * \param lock a pointer to a lock variable
98 * \returns SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already
101 * \since This function is available since SDL 2.0.0.
104 * \sa SDL_AtomicUnlock
106 extern DECLSPEC SDL_bool SDLCALL
SDL_AtomicTryLock(SDL_SpinLock
*lock
);
109 * Lock a spin lock by setting it to a non-zero value.
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!***
114 * \param lock a pointer to a lock variable
116 * \since This function is available since SDL 2.0.0.
118 * \sa SDL_AtomicTryLock
119 * \sa SDL_AtomicUnlock
121 extern DECLSPEC
void SDLCALL
SDL_AtomicLock(SDL_SpinLock
*lock
);
124 * Unlock a spin lock by setting it to 0.
126 * Always returns immediately.
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!***
131 * \param lock a pointer to a lock variable
133 * \since This function is available since SDL 2.0.0.
136 * \sa SDL_AtomicTryLock
138 extern DECLSPEC
void SDLCALL
SDL_AtomicUnlock(SDL_SpinLock
*lock
);
140 /* @} *//* SDL AtomicLock */
144 * The compiler barrier prevents the compiler from reordering
145 * reads and writes to globally visible variables across the call.
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 [];
158 #define SDL_CompilerBarrier() \
159 { SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
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.
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
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
178 * For more information on these semantics, take a look at the blog post:
179 * http://preshing.com/20120913/acquire-and-release-semantics
181 * \since This function is available since SDL 2.0.6.
183 extern DECLSPEC
void SDLCALL
SDL_MemoryBarrierReleaseFunction(void);
184 extern DECLSPEC
void SDLCALL
SDL_MemoryBarrierAcquireFunction(void);
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__) */
195 https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
197 The Linux kernel provides a helper function which provides the right code for a memory barrier,
198 hard-coded at address 0xffff0fa0
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>
206 #define SDL_MemoryBarrierRelease() __cpu_membarrier()
207 #define SDL_MemoryBarrierAcquire() __cpu_membarrier()
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__)
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()
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__ */
223 #define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
224 #define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
225 #endif /* __LINUX__ || __ANDROID__ */
226 #endif /* __GNUC__ && __arm__ */
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()
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()
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__) && __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 /* watcom assembler rejects PAUSE if CPU < i686, and it refuses REP NOP as an invalid combination. Hardcode the bytes. */
253 extern __inline
void SDL_CPUPauseInstruction(void);
254 #pragma aux SDL_CPUPauseInstruction = "db 0f3h,90h"
256 #define SDL_CPUPauseInstruction()
261 * \brief A type representing an atomic integer value. It is a struct
262 * so people don't accidentally use numeric operations on it.
264 typedef struct { int value
; } SDL_atomic_t
;
267 * Set an atomic variable to a new value if it is currently an old value.
269 * ***Note: If you don't know what this function is for, you shouldn't use
272 * \param a a pointer to an SDL_atomic_t variable to be modified
273 * \param oldval the old value
274 * \param newval the new value
275 * \returns SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
277 * \since This function is available since SDL 2.0.0.
279 * \sa SDL_AtomicCASPtr
283 extern DECLSPEC SDL_bool SDLCALL
SDL_AtomicCAS(SDL_atomic_t
*a
, int oldval
, int newval
);
286 * Set an atomic variable to a value.
288 * This function also acts as a full memory barrier.
290 * ***Note: If you don't know what this function is for, you shouldn't use
293 * \param a a pointer to an SDL_atomic_t variable to be modified
294 * \param v the desired value
295 * \returns the previous value of the atomic variable.
297 * \since This function is available since SDL 2.0.2.
301 extern DECLSPEC
int SDLCALL
SDL_AtomicSet(SDL_atomic_t
*a
, int v
);
304 * Get the value of an atomic variable.
306 * ***Note: If you don't know what this function is for, you shouldn't use
309 * \param a a pointer to an SDL_atomic_t variable
310 * \returns the current value of an atomic variable.
312 * \since This function is available since SDL 2.0.2.
316 extern DECLSPEC
int SDLCALL
SDL_AtomicGet(SDL_atomic_t
*a
);
319 * Add to an atomic variable.
321 * This function also acts as a full memory barrier.
323 * ***Note: If you don't know what this function is for, you shouldn't use
326 * \param a a pointer to an SDL_atomic_t variable to be modified
327 * \param v the desired value to add
328 * \returns the previous value of the atomic variable.
330 * \since This function is available since SDL 2.0.2.
332 * \sa SDL_AtomicDecRef
333 * \sa SDL_AtomicIncRef
335 extern DECLSPEC
int SDLCALL
SDL_AtomicAdd(SDL_atomic_t
*a
, int v
);
338 * \brief Increment an atomic variable used as a reference count.
340 #ifndef SDL_AtomicIncRef
341 #define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
345 * \brief Decrement an atomic variable used as a reference count.
347 * \return SDL_TRUE if the variable reached zero after decrementing,
348 * SDL_FALSE otherwise
350 #ifndef SDL_AtomicDecRef
351 #define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
355 * Set a pointer to a new value if it is currently an old value.
357 * ***Note: If you don't know what this function is for, you shouldn't use
360 * \param a a pointer to a pointer
361 * \param oldval the old pointer value
362 * \param newval the new pointer value
363 * \returns SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
365 * \since This function is available since SDL 2.0.0.
368 * \sa SDL_AtomicGetPtr
369 * \sa SDL_AtomicSetPtr
371 extern DECLSPEC SDL_bool SDLCALL
SDL_AtomicCASPtr(void **a
, void *oldval
, void *newval
);
374 * Set a pointer to a value atomically.
376 * ***Note: If you don't know what this function is for, you shouldn't use
379 * \param a a pointer to a pointer
380 * \param v the desired pointer value
381 * \returns the previous value of the pointer.
383 * \since This function is available since SDL 2.0.2.
385 * \sa SDL_AtomicCASPtr
386 * \sa SDL_AtomicGetPtr
388 extern DECLSPEC
void* SDLCALL
SDL_AtomicSetPtr(void **a
, void* v
);
391 * Get the value of a pointer atomically.
393 * ***Note: If you don't know what this function is for, you shouldn't use
396 * \param a a pointer to a pointer
397 * \returns the current value of a pointer.
399 * \since This function is available since SDL 2.0.2.
401 * \sa SDL_AtomicCASPtr
402 * \sa SDL_AtomicSetPtr
404 extern DECLSPEC
void* SDLCALL
SDL_AtomicGetPtr(void **a
);
406 /* Ends C function definitions when using C++ */
411 #include "close_code.h"
413 #endif /* SDL_atomic_h_ */
415 /* vi: set ts=4 sw=4 expandtab: */