SDL 3.0
SDL_atomic.h
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1/*
2 Simple DirectMedia Layer
3 Copyright (C) 1997-2023 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 * https://learn.microsoft.com/en-us/windows/win32/dxtecharts/lockless-programming
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 <SDL3/SDL_stdinc.h>
64
65#include <SDL3/SDL_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 * They are also vulnerable to starvation if the thread holding
82 * the lock is lower priority than other threads and doesn't get
83 * scheduled. In general you should use mutexes instead, since
84 * they have better performance and contention behavior.
85 *
86 * The atomic locks are not safe to lock recursively.
87 *
88 * Porting Note:
89 * The spin lock functions and type are required and can not be
90 * emulated because they are used in the atomic emulation code.
91 */
92/* @{ */
93
94typedef int SDL_SpinLock;
95
96/**
97 * Try to lock a spin lock by setting it to a non-zero value.
98 *
99 * ***Please note that spinlocks are dangerous if you don't know what you're
100 * doing. Please be careful using any sort of spinlock!***
101 *
102 * \param lock a pointer to a lock variable
103 * \returns SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already
104 * held.
105 *
106 * \since This function is available since SDL 3.0.0.
107 *
108 * \sa SDL_AtomicLock
109 * \sa SDL_AtomicUnlock
110 */
111extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
112
113/**
114 * Lock a spin lock by setting it to a non-zero value.
115 *
116 * ***Please note that spinlocks are dangerous if you don't know what you're
117 * doing. Please be careful using any sort of spinlock!***
118 *
119 * \param lock a pointer to a lock variable
120 *
121 * \since This function is available since SDL 3.0.0.
122 *
123 * \sa SDL_AtomicTryLock
124 * \sa SDL_AtomicUnlock
125 */
126extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
127
128/**
129 * Unlock a spin lock by setting it to 0.
130 *
131 * Always returns immediately.
132 *
133 * ***Please note that spinlocks are dangerous if you don't know what you're
134 * doing. Please be careful using any sort of spinlock!***
135 *
136 * \param lock a pointer to a lock variable
137 *
138 * \since This function is available since SDL 3.0.0.
139 *
140 * \sa SDL_AtomicLock
141 * \sa SDL_AtomicTryLock
142 */
143extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
144
145/* @} *//* SDL AtomicLock */
146
147
148/**
149 * The compiler barrier prevents the compiler from reordering
150 * reads and writes to globally visible variables across the call.
151 */
152#if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
153void _ReadWriteBarrier(void);
154#pragma intrinsic(_ReadWriteBarrier)
155#define SDL_CompilerBarrier() _ReadWriteBarrier()
156#elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
157/* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
158#define SDL_CompilerBarrier() __asm__ __volatile__ ("" : : : "memory")
159#elif defined(__WATCOMC__)
160extern __inline void SDL_CompilerBarrier(void);
161#pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
162#else
163#define SDL_CompilerBarrier() \
164{ SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
165#endif
166
167/**
168 * Memory barriers are designed to prevent reads and writes from being
169 * reordered by the compiler and being seen out of order on multi-core CPUs.
170 *
171 * A typical pattern would be for thread A to write some data and a flag, and
172 * for thread B to read the flag and get the data. In this case you would
173 * insert a release barrier between writing the data and the flag,
174 * guaranteeing that the data write completes no later than the flag is
175 * written, and you would insert an acquire barrier between reading the flag
176 * and reading the data, to ensure that all the reads associated with the flag
177 * have completed.
178 *
179 * In this pattern you should always see a release barrier paired with an
180 * acquire barrier and you should gate the data reads/writes with a single
181 * flag variable.
182 *
183 * For more information on these semantics, take a look at the blog post:
184 * http://preshing.com/20120913/acquire-and-release-semantics
185 *
186 * \since This function is available since SDL 3.0.0.
187 */
188extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
189
190/*
191 * \since This function is available since SDL 3.0.0.
192 */
193extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
194
195#if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
196#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
197#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
198#elif defined(__GNUC__) && defined(__aarch64__)
199#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
200#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
201#elif defined(__GNUC__) && defined(__arm__)
202#if 0 /* defined(__LINUX__) || defined(__ANDROID__) */
203/* Information from:
204 https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
205
206 The Linux kernel provides a helper function which provides the right code for a memory barrier,
207 hard-coded at address 0xffff0fa0
208*/
209typedef void (*SDL_KernelMemoryBarrierFunc)();
210#define SDL_MemoryBarrierRelease() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
211#define SDL_MemoryBarrierAcquire() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
212#else
213#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__)
214#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
215#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
216#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__)
217#ifdef __thumb__
218/* The mcr instruction isn't available in thumb mode, use real functions */
219#define SDL_MEMORY_BARRIER_USES_FUNCTION
220#define SDL_MemoryBarrierRelease() SDL_MemoryBarrierReleaseFunction()
221#define SDL_MemoryBarrierAcquire() SDL_MemoryBarrierAcquireFunction()
222#else
223#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
224#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
225#endif /* __thumb__ */
226#else
227#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
228#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
229#endif /* __LINUX__ || __ANDROID__ */
230#endif /* __GNUC__ && __arm__ */
231#else
232#if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
233/* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
234#include <mbarrier.h>
235#define SDL_MemoryBarrierRelease() __machine_rel_barrier()
236#define SDL_MemoryBarrierAcquire() __machine_acq_barrier()
237#else
238/* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
239#define SDL_MemoryBarrierRelease() SDL_CompilerBarrier()
240#define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
241#endif
242#endif
243
244/* "REP NOP" is PAUSE, coded for tools that don't know it by that name. */
245#if (defined(__GNUC__) || defined(__clang__)) && (defined(__i386__) || defined(__x86_64__))
246 #define SDL_CPUPauseInstruction() __asm__ __volatile__("pause\n") /* Some assemblers can't do REP NOP, so go with PAUSE. */
247#elif (defined(__arm__) && defined(__ARM_ARCH) && __ARM_ARCH >= 7) || defined(__aarch64__)
248 #define SDL_CPUPauseInstruction() __asm__ __volatile__("yield" ::: "memory")
249#elif (defined(__powerpc__) || defined(__powerpc64__))
250 #define SDL_CPUPauseInstruction() __asm__ __volatile__("or 27,27,27");
251#elif (defined(__riscv) && __riscv_xlen == 64)
252 #define SDL_CPUPauseInstruction() __asm__ __volatile__(".insn i 0x0F, 0, x0, x0, 0x010");
253#elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
254 #define SDL_CPUPauseInstruction() _mm_pause() /* this is actually "rep nop" and not a SIMD instruction. No inline asm in MSVC x86-64! */
255#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
256 #define SDL_CPUPauseInstruction() __yield()
257#elif defined(__WATCOMC__) && defined(__386__)
258 extern __inline void SDL_CPUPauseInstruction(void);
259 #pragma aux SDL_CPUPauseInstruction = ".686p" ".xmm2" "pause"
260#else
261 #define SDL_CPUPauseInstruction()
262#endif
263
264
265/**
266 * A type representing an atomic integer value.
267 *
268 * It is a struct so people don't accidentally use numeric operations on it.
269 */
270typedef struct { int value; } SDL_AtomicInt;
271
272/**
273 * Set an atomic variable to a new value if it is currently an old value.
274 *
275 * ***Note: If you don't know what this function is for, you shouldn't use
276 * it!***
277 *
278 * \param a a pointer to an SDL_AtomicInt variable to be modified
279 * \param oldval the old value
280 * \param newval the new value
281 * \returns SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
282 *
283 * \since This function is available since SDL 3.0.0.
284 *
285 * \sa SDL_AtomicCASPtr
286 * \sa SDL_AtomicGet
287 * \sa SDL_AtomicSet
288 */
289extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_AtomicInt *a, int oldval, int newval);
290
291/**
292 * Set an atomic variable to a value.
293 *
294 * This function also acts as a full memory barrier.
295 *
296 * ***Note: If you don't know what this function is for, you shouldn't use
297 * it!***
298 *
299 * \param a a pointer to an SDL_AtomicInt variable to be modified
300 * \param v the desired value
301 * \returns the previous value of the atomic variable.
302 *
303 * \since This function is available since SDL 3.0.0.
304 *
305 * \sa SDL_AtomicGet
306 */
307extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_AtomicInt *a, int v);
308
309/**
310 * Get the value of an atomic variable.
311 *
312 * ***Note: If you don't know what this function is for, you shouldn't use
313 * it!***
314 *
315 * \param a a pointer to an SDL_AtomicInt variable
316 * \returns the current value of an atomic variable.
317 *
318 * \since This function is available since SDL 3.0.0.
319 *
320 * \sa SDL_AtomicSet
321 */
322extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_AtomicInt *a);
323
324/**
325 * Add to an atomic variable.
326 *
327 * This function also acts as a full memory barrier.
328 *
329 * ***Note: If you don't know what this function is for, you shouldn't use
330 * it!***
331 *
332 * \param a a pointer to an SDL_AtomicInt variable to be modified
333 * \param v the desired value to add
334 * \returns the previous value of the atomic variable.
335 *
336 * \since This function is available since SDL 3.0.0.
337 *
338 * \sa SDL_AtomicDecRef
339 * \sa SDL_AtomicIncRef
340 */
341extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_AtomicInt *a, int v);
342
343/**
344 * Increment an atomic variable used as a reference count.
345 */
346#ifndef SDL_AtomicIncRef
347#define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
348#endif
349
350/**
351 * Decrement an atomic variable used as a reference count.
352 *
353 * \return SDL_TRUE if the variable reached zero after decrementing,
354 * SDL_FALSE otherwise
355 */
356#ifndef SDL_AtomicDecRef
357#define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
358#endif
359
360/**
361 * Set a pointer to a new value if it is currently an old value.
362 *
363 * ***Note: If you don't know what this function is for, you shouldn't use
364 * it!***
365 *
366 * \param a a pointer to a pointer
367 * \param oldval the old pointer value
368 * \param newval the new pointer value
369 * \returns SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
370 *
371 * \since This function is available since SDL 3.0.0.
372 *
373 * \sa SDL_AtomicCAS
374 * \sa SDL_AtomicGetPtr
375 * \sa SDL_AtomicSetPtr
376 */
377extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
378
379/**
380 * Set a pointer to a value atomically.
381 *
382 * ***Note: If you don't know what this function is for, you shouldn't use
383 * it!***
384 *
385 * \param a a pointer to a pointer
386 * \param v the desired pointer value
387 * \returns the previous value of the pointer.
388 *
389 * \since This function is available since SDL 3.0.0.
390 *
391 * \sa SDL_AtomicCASPtr
392 * \sa SDL_AtomicGetPtr
393 */
394extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
395
396/**
397 * Get the value of a pointer atomically.
398 *
399 * ***Note: If you don't know what this function is for, you shouldn't use
400 * it!***
401 *
402 * \param a a pointer to a pointer
403 * \returns the current value of a pointer.
404 *
405 * \since This function is available since SDL 3.0.0.
406 *
407 * \sa SDL_AtomicCASPtr
408 * \sa SDL_AtomicSetPtr
409 */
410extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
411
412/* Ends C function definitions when using C++ */
413#ifdef __cplusplus
414}
415#endif
416
417#include <SDL3/SDL_close_code.h>
418
419#endif /* SDL_atomic_h_ */
void SDL_MemoryBarrierAcquireFunction(void)
int SDL_AtomicSet(SDL_AtomicInt *a, int v)
SDL_bool SDL_AtomicCASPtr(void **a, void *oldval, void *newval)
void * SDL_AtomicGetPtr(void **a)
#define SDL_CompilerBarrier()
Definition SDL_atomic.h:163
void SDL_MemoryBarrierReleaseFunction(void)
int SDL_AtomicGet(SDL_AtomicInt *a)
int SDL_SpinLock
Definition SDL_atomic.h:94
#define SDL_CPUPauseInstruction()
Definition SDL_atomic.h:261
SDL_bool SDL_AtomicTryLock(SDL_SpinLock *lock)
void SDL_AtomicUnlock(SDL_SpinLock *lock)
int SDL_AtomicAdd(SDL_AtomicInt *a, int v)
SDL_bool SDL_AtomicCAS(SDL_AtomicInt *a, int oldval, int newval)
void * SDL_AtomicSetPtr(void **a, void *v)
void SDL_AtomicLock(SDL_SpinLock *lock)
unsigned int SDL_bool
Definition SDL_stdinc.h:136