1/*
2 Simple DirectMedia Layer
3 Copyright (C) 1997-2021 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
69extern "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
89typedef 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 * \sa SDL_AtomicLock
102 * \sa SDL_AtomicUnlock
103 */
104extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
105
106/**
107 * Lock a spin lock by setting it to a non-zero value.
108 *
109 * ***Please note that spinlocks are dangerous if you don't know what you're
110 * doing. Please be careful using any sort of spinlock!***
111 *
112 * \param lock a pointer to a lock variable
113 *
114 * \sa SDL_AtomicTryLock
115 * \sa SDL_AtomicUnlock
116 */
117extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
118
119/**
120 * Unlock a spin lock by setting it to 0.
121 *
122 * Always returns immediately.
123 *
124 * ***Please note that spinlocks are dangerous if you don't know what you're
125 * doing. Please be careful using any sort of spinlock!***
126 *
127 * \param lock a pointer to a lock variable
128 *
129 * \since This function is available since SDL 2.0.0.
130 *
131 * \sa SDL_AtomicLock
132 * \sa SDL_AtomicTryLock
133 */
134extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
135
136/* @} *//* SDL AtomicLock */
137
138
139/**
140 * The compiler barrier prevents the compiler from reordering
141 * reads and writes to globally visible variables across the call.
142 */
143#if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
144void _ReadWriteBarrier(void);
145#pragma intrinsic(_ReadWriteBarrier)
146#define SDL_CompilerBarrier() _ReadWriteBarrier()
147#elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
148/* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
149#define SDL_CompilerBarrier() __asm__ __volatile__ ("" : : : "memory")
150#elif defined(__WATCOMC__)
151extern _inline void SDL_CompilerBarrier (void);
152#pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
153#else
154#define SDL_CompilerBarrier() \
155{ SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
156#endif
157
158/**
159 * Memory barriers are designed to prevent reads and writes from being
160 * reordered by the compiler and being seen out of order on multi-core CPUs.
161 *
162 * A typical pattern would be for thread A to write some data and a flag,
163 * and for thread B to read the flag and get the data. In this case you
164 * would insert a release barrier between writing the data and the flag,
165 * guaranteeing that the data write completes no later than the flag is
166 * written, and you would insert an acquire barrier between reading the
167 * flag and reading the data, to ensure that all the reads associated
168 * with the flag have completed.
169 *
170 * In this pattern you should always see a release barrier paired with
171 * an acquire barrier and you should gate the data reads/writes with a
172 * single flag variable.
173 *
174 * For more information on these semantics, take a look at the blog post:
175 * http://preshing.com/20120913/acquire-and-release-semantics
176 */
177extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
178extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
179
180#if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
181#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
182#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
183#elif defined(__GNUC__) && defined(__aarch64__)
184#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
185#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
186#elif defined(__GNUC__) && defined(__arm__)
187#if 0 /* defined(__LINUX__) || defined(__ANDROID__) */
188/* Information from:
189 https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
190
191 The Linux kernel provides a helper function which provides the right code for a memory barrier,
192 hard-coded at address 0xffff0fa0
193*/
194typedef void (*SDL_KernelMemoryBarrierFunc)();
195#define SDL_MemoryBarrierRelease() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
196#define SDL_MemoryBarrierAcquire() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
197#elif 0 /* defined(__QNXNTO__) */
198#include <sys/cpuinline.h>
199
200#define SDL_MemoryBarrierRelease() __cpu_membarrier()
201#define SDL_MemoryBarrierAcquire() __cpu_membarrier()
202#else
203#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__)
204#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
205#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
206#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__)
207#ifdef __thumb__
208/* The mcr instruction isn't available in thumb mode, use real functions */
209#define SDL_MEMORY_BARRIER_USES_FUNCTION
210#define SDL_MemoryBarrierRelease() SDL_MemoryBarrierReleaseFunction()
211#define SDL_MemoryBarrierAcquire() SDL_MemoryBarrierAcquireFunction()
212#else
213#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
214#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
215#endif /* __thumb__ */
216#else
217#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
218#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
219#endif /* __LINUX__ || __ANDROID__ */
220#endif /* __GNUC__ && __arm__ */
221#else
222#if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
223/* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
224#include <mbarrier.h>
225#define SDL_MemoryBarrierRelease() __machine_rel_barrier()
226#define SDL_MemoryBarrierAcquire() __machine_acq_barrier()
227#else
228/* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
229#define SDL_MemoryBarrierRelease() SDL_CompilerBarrier()
230#define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
231#endif
232#endif
233
234/**
235 * \brief A type representing an atomic integer value. It is a struct
236 * so people don't accidentally use numeric operations on it.
237 */
238typedef struct { int value; } SDL_atomic_t;
239
240/**
241 * Set an atomic variable to a new value if it is
242 * currently an old value.
243 *
244 * ***Note: If you don't know what this function is for, you shouldn't use
245 * it!***
246 *
247 * \param a a pointer to an SDL_atomic_t variable to be modified
248 * \param oldval the old value
249 * \param newval the new value
250 * \returns SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
251 *
252 * \since This function is available since SDL 2.0.0.
253 *
254 * \sa SDL_AtomicCASPtr
255 * \sa SDL_AtomicGet
256 * \sa SDL_AtomicSet
257 */
258extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
259
260/**
261 * Set an atomic variable to a value.
262 *
263 * This function also acts as a full memory barrier.
264 *
265 * ***Note: If you don't know what this function is for, you shouldn't use
266 * it!***
267 *
268 * \param a a pointer to an SDL_atomic_t variable to be modified
269 * \param v the desired value
270 * \returns the previous value of the atomic variable.
271 *
272 * \sa SDL_AtomicGet
273 */
274extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
275
276/**
277 * Get the value of an atomic variable.
278 *
279 * ***Note: If you don't know what this function is for, you shouldn't use
280 * it!***
281 *
282 * \param a a pointer to an SDL_atomic_t variable
283 * \returns the current value of an atomic variable.
284 *
285 * \sa SDL_AtomicSet
286 */
287extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
288
289/**
290 * Add to an atomic variable.
291 *
292 * This function also acts as a full memory barrier.
293 *
294 * ***Note: If you don't know what this function is for, you shouldn't use
295 * it!***
296 *
297 * \param a a pointer to an SDL_atomic_t variable to be modified
298 * \param v the desired value to add
299 * \returns the previous value of the atomic variable.
300 *
301 * \sa SDL_AtomicDecRef
302 * \sa SDL_AtomicIncRef
303 */
304extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
305
306/**
307 * \brief Increment an atomic variable used as a reference count.
308 */
309#ifndef SDL_AtomicIncRef
310#define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
311#endif
312
313/**
314 * \brief Decrement an atomic variable used as a reference count.
315 *
316 * \return SDL_TRUE if the variable reached zero after decrementing,
317 * SDL_FALSE otherwise
318 */
319#ifndef SDL_AtomicDecRef
320#define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
321#endif
322
323/**
324 * Set a pointer to a new value if it is currently an old
325 * value.
326 *
327 * ***Note: If you don't know what this function is for, you shouldn't use
328 * it!***
329 *
330 * \param a a pointer to a pointer
331 * \param oldval the old pointer value
332 * \param newval the new pointer value
333 * \returns SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
334 *
335 * \since This function is available since SDL 2.0.0.
336 *
337 * \sa SDL_AtomicCAS
338 * \sa SDL_AtomicGetPtr
339 * \sa SDL_AtomicSetPtr
340 */
341extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
342
343/**
344 * Set a pointer to a value atomically.
345 *
346 * ***Note: If you don't know what this function is for, you shouldn't use
347 * it!***
348 *
349 * \param a a pointer to a pointer
350 * \param v the desired pointer value
351 * \returns the previous value of the pointer.
352 *
353 * \sa SDL_AtomicCASPtr
354 * \sa SDL_AtomicGetPtr
355 */
356extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
357
358/**
359 * Get the value of a pointer atomically.
360 *
361 * ***Note: If you don't know what this function is for, you shouldn't use
362 * it!***
363 *
364 * \param a a pointer to a pointer
365 * \returns the current value of a pointer.
366 *
367 * \sa SDL_AtomicCASPtr
368 * \sa SDL_AtomicSetPtr
369 */
370extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
371
372/* Ends C function definitions when using C++ */
373#ifdef __cplusplus
374}
375#endif
376
377#include "close_code.h"
378
379#endif /* SDL_atomic_h_ */
380
381/* vi: set ts=4 sw=4 expandtab: */
382