1/*
2 Simple DirectMedia Layer
3 Copyright (C) 1997-2022 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 * \since This function is available since SDL 2.0.0.
102 *
103 * \sa SDL_AtomicLock
104 * \sa SDL_AtomicUnlock
105 */
106extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
107
108/**
109 * Lock a spin lock by setting it to a non-zero value.
110 *
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!***
113 *
114 * \param lock a pointer to a lock variable
115 *
116 * \since This function is available since SDL 2.0.0.
117 *
118 * \sa SDL_AtomicTryLock
119 * \sa SDL_AtomicUnlock
120 */
121extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
122
123/**
124 * Unlock a spin lock by setting it to 0.
125 *
126 * Always returns immediately.
127 *
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!***
130 *
131 * \param lock a pointer to a lock variable
132 *
133 * \since This function is available since SDL 2.0.0.
134 *
135 * \sa SDL_AtomicLock
136 * \sa SDL_AtomicTryLock
137 */
138extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
139
140/* @} *//* SDL AtomicLock */
141
142
143/**
144 * The compiler barrier prevents the compiler from reordering
145 * reads and writes to globally visible variables across the call.
146 */
147#if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
148void _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__)
155extern __inline void SDL_CompilerBarrier(void);
156#pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
157#else
158#define SDL_CompilerBarrier() \
159{ SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
160#endif
161
162/**
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.
165 *
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
172 * have completed.
173 *
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
176 * flag variable.
177 *
178 * For more information on these semantics, take a look at the blog post:
179 * http://preshing.com/20120913/acquire-and-release-semantics
180 *
181 * \since This function is available since SDL 2.0.6.
182 */
183extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
184extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
185
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__) */
194/* Information from:
195 https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
196
197 The Linux kernel provides a helper function which provides the right code for a memory barrier,
198 hard-coded at address 0xffff0fa0
199*/
200typedef 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>
205
206#define SDL_MemoryBarrierRelease() __cpu_membarrier()
207#define SDL_MemoryBarrierAcquire() __cpu_membarrier()
208#else
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__)
213#ifdef __thumb__
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()
218#else
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__ */
222#else
223#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
224#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
225#endif /* __LINUX__ || __ANDROID__ */
226#endif /* __GNUC__ && __arm__ */
227#else
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()
233#else
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()
237#endif
238#endif
239
240/**
241 * \brief A type representing an atomic integer value. It is a struct
242 * so people don't accidentally use numeric operations on it.
243 */
244typedef struct { int value; } SDL_atomic_t;
245
246/**
247 * Set an atomic variable to a new value if it is currently an old value.
248 *
249 * ***Note: If you don't know what this function is for, you shouldn't use
250 * it!***
251 *
252 * \param a a pointer to an SDL_atomic_t variable to be modified
253 * \param oldval the old value
254 * \param newval the new value
255 * \returns SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
256 *
257 * \since This function is available since SDL 2.0.0.
258 *
259 * \sa SDL_AtomicCASPtr
260 * \sa SDL_AtomicGet
261 * \sa SDL_AtomicSet
262 */
263extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
264
265/**
266 * Set an atomic variable to a value.
267 *
268 * This function also acts as a full memory barrier.
269 *
270 * ***Note: If you don't know what this function is for, you shouldn't use
271 * it!***
272 *
273 * \param a a pointer to an SDL_atomic_t variable to be modified
274 * \param v the desired value
275 * \returns the previous value of the atomic variable.
276 *
277 * \since This function is available since SDL 2.0.2.
278 *
279 * \sa SDL_AtomicGet
280 */
281extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
282
283/**
284 * Get the value of an atomic variable.
285 *
286 * ***Note: If you don't know what this function is for, you shouldn't use
287 * it!***
288 *
289 * \param a a pointer to an SDL_atomic_t variable
290 * \returns the current value of an atomic variable.
291 *
292 * \since This function is available since SDL 2.0.2.
293 *
294 * \sa SDL_AtomicSet
295 */
296extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
297
298/**
299 * Add to an atomic variable.
300 *
301 * This function also acts as a full memory barrier.
302 *
303 * ***Note: If you don't know what this function is for, you shouldn't use
304 * it!***
305 *
306 * \param a a pointer to an SDL_atomic_t variable to be modified
307 * \param v the desired value to add
308 * \returns the previous value of the atomic variable.
309 *
310 * \since This function is available since SDL 2.0.2.
311 *
312 * \sa SDL_AtomicDecRef
313 * \sa SDL_AtomicIncRef
314 */
315extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
316
317/**
318 * \brief Increment an atomic variable used as a reference count.
319 */
320#ifndef SDL_AtomicIncRef
321#define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
322#endif
323
324/**
325 * \brief Decrement an atomic variable used as a reference count.
326 *
327 * \return SDL_TRUE if the variable reached zero after decrementing,
328 * SDL_FALSE otherwise
329 */
330#ifndef SDL_AtomicDecRef
331#define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
332#endif
333
334/**
335 * Set a pointer to a new value if it is currently an old value.
336 *
337 * ***Note: If you don't know what this function is for, you shouldn't use
338 * it!***
339 *
340 * \param a a pointer to a pointer
341 * \param oldval the old pointer value
342 * \param newval the new pointer value
343 * \returns SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
344 *
345 * \since This function is available since SDL 2.0.0.
346 *
347 * \sa SDL_AtomicCAS
348 * \sa SDL_AtomicGetPtr
349 * \sa SDL_AtomicSetPtr
350 */
351extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
352
353/**
354 * Set a pointer to a value atomically.
355 *
356 * ***Note: If you don't know what this function is for, you shouldn't use
357 * it!***
358 *
359 * \param a a pointer to a pointer
360 * \param v the desired pointer value
361 * \returns the previous value of the pointer.
362 *
363 * \since This function is available since SDL 2.0.2.
364 *
365 * \sa SDL_AtomicCASPtr
366 * \sa SDL_AtomicGetPtr
367 */
368extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
369
370/**
371 * Get the value of a pointer atomically.
372 *
373 * ***Note: If you don't know what this function is for, you shouldn't use
374 * it!***
375 *
376 * \param a a pointer to a pointer
377 * \returns the current value of a pointer.
378 *
379 * \since This function is available since SDL 2.0.2.
380 *
381 * \sa SDL_AtomicCASPtr
382 * \sa SDL_AtomicSetPtr
383 */
384extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
385
386/* Ends C function definitions when using C++ */
387#ifdef __cplusplus
388}
389#endif
390
391#include "close_code.h"
392
393#endif /* SDL_atomic_h_ */
394
395/* vi: set ts=4 sw=4 expandtab: */
396