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 * 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 <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);
189extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
190
191#if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
192#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
193#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
194#elif defined(__GNUC__) && defined(__aarch64__)
195#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
196#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
197#elif defined(__GNUC__) && defined(__arm__)
198#if 0 /* defined(__LINUX__) || defined(__ANDROID__) */
199/* Information from:
200 https://chromium.googlesource.com/chromium/chromium/+/trunk/base/atomicops_internals_arm_gcc.h#19
201
202 The Linux kernel provides a helper function which provides the right code for a memory barrier,
203 hard-coded at address 0xffff0fa0
204*/
205typedef void (*SDL_KernelMemoryBarrierFunc)();
206#define SDL_MemoryBarrierRelease() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
207#define SDL_MemoryBarrierAcquire() ((SDL_KernelMemoryBarrierFunc)0xffff0fa0)()
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/* "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(__riscv) && __riscv_xlen == 64)
248 #define SDL_CPUPauseInstruction() __asm__ __volatile__(".insn i 0x0F, 0, x0, x0, 0x010");
249#elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
250 #define SDL_CPUPauseInstruction() _mm_pause() /* this is actually "rep nop" and not a SIMD instruction. No inline asm in MSVC x86-64! */
251#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
252 #define SDL_CPUPauseInstruction() __yield()
253#elif defined(__WATCOMC__) && defined(__386__)
254 extern __inline void SDL_CPUPauseInstruction(void);
255 #pragma aux SDL_CPUPauseInstruction = ".686p" ".xmm2" "pause"
256#else
257 #define SDL_CPUPauseInstruction()
258#endif
259
260
261/**
262 * \brief A type representing an atomic integer value. It is a struct
263 * so people don't accidentally use numeric operations on it.
264 */
265typedef struct { int value; } SDL_atomic_t;
266
267/**
268 * Set an atomic variable to a new value if it is currently an old value.
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 oldval the old value
275 * \param newval the new value
276 * \returns SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
277 *
278 * \since This function is available since SDL 3.0.0.
279 *
280 * \sa SDL_AtomicCASPtr
281 * \sa SDL_AtomicGet
282 * \sa SDL_AtomicSet
283 */
284extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
285
286/**
287 * Set an atomic variable to a value.
288 *
289 * This function also acts as a full memory barrier.
290 *
291 * ***Note: If you don't know what this function is for, you shouldn't use
292 * it!***
293 *
294 * \param a a pointer to an SDL_atomic_t variable to be modified
295 * \param v the desired value
296 * \returns the previous value of the atomic variable.
297 *
298 * \since This function is available since SDL 3.0.0.
299 *
300 * \sa SDL_AtomicGet
301 */
302extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
303
304/**
305 * Get the value of an atomic variable.
306 *
307 * ***Note: If you don't know what this function is for, you shouldn't use
308 * it!***
309 *
310 * \param a a pointer to an SDL_atomic_t variable
311 * \returns the current value of an atomic variable.
312 *
313 * \since This function is available since SDL 3.0.0.
314 *
315 * \sa SDL_AtomicSet
316 */
317extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
318
319/**
320 * Add to an atomic variable.
321 *
322 * This function also acts as a full memory barrier.
323 *
324 * ***Note: If you don't know what this function is for, you shouldn't use
325 * it!***
326 *
327 * \param a a pointer to an SDL_atomic_t variable to be modified
328 * \param v the desired value to add
329 * \returns the previous value of the atomic variable.
330 *
331 * \since This function is available since SDL 3.0.0.
332 *
333 * \sa SDL_AtomicDecRef
334 * \sa SDL_AtomicIncRef
335 */
336extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
337
338/**
339 * \brief Increment an atomic variable used as a reference count.
340 */
341#ifndef SDL_AtomicIncRef
342#define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
343#endif
344
345/**
346 * \brief Decrement an atomic variable used as a reference count.
347 *
348 * \return SDL_TRUE if the variable reached zero after decrementing,
349 * SDL_FALSE otherwise
350 */
351#ifndef SDL_AtomicDecRef
352#define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
353#endif
354
355/**
356 * Set a pointer to a new value if it is currently an old value.
357 *
358 * ***Note: If you don't know what this function is for, you shouldn't use
359 * it!***
360 *
361 * \param a a pointer to a pointer
362 * \param oldval the old pointer value
363 * \param newval the new pointer value
364 * \returns SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
365 *
366 * \since This function is available since SDL 3.0.0.
367 *
368 * \sa SDL_AtomicCAS
369 * \sa SDL_AtomicGetPtr
370 * \sa SDL_AtomicSetPtr
371 */
372extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
373
374/**
375 * Set a pointer to a value atomically.
376 *
377 * ***Note: If you don't know what this function is for, you shouldn't use
378 * it!***
379 *
380 * \param a a pointer to a pointer
381 * \param v the desired pointer value
382 * \returns the previous value of the pointer.
383 *
384 * \since This function is available since SDL 3.0.0.
385 *
386 * \sa SDL_AtomicCASPtr
387 * \sa SDL_AtomicGetPtr
388 */
389extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
390
391/**
392 * Get the value of a pointer atomically.
393 *
394 * ***Note: If you don't know what this function is for, you shouldn't use
395 * it!***
396 *
397 * \param a a pointer to a pointer
398 * \returns the current value of a pointer.
399 *
400 * \since This function is available since SDL 3.0.0.
401 *
402 * \sa SDL_AtomicCASPtr
403 * \sa SDL_AtomicSetPtr
404 */
405extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
406
407/* Ends C function definitions when using C++ */
408#ifdef __cplusplus
409}
410#endif
411
412#include <SDL3/SDL_close_code.h>
413
414#endif /* SDL_atomic_h_ */
SDL_bool SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval)
void SDL_MemoryBarrierAcquireFunction(void)
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_atomic_t *a)
int SDL_SpinLock
Definition: SDL_atomic.h:94
#define SDL_CPUPauseInstruction()
Definition: SDL_atomic.h:257
SDL_bool SDL_AtomicTryLock(SDL_SpinLock *lock)
void SDL_AtomicUnlock(SDL_SpinLock *lock)
int SDL_AtomicAdd(SDL_atomic_t *a, int v)
void * SDL_AtomicSetPtr(void **a, void *v)
int SDL_AtomicSet(SDL_atomic_t *a, int v)
void SDL_AtomicLock(SDL_SpinLock *lock)
typedef void(APIENTRYP PFNGLDRAWRANGEELEMENTSPROC)(GLenum mode
const GLdouble * v
Definition: SDL_opengl.h:2057
GLboolean GLboolean GLboolean GLboolean a
SDL_bool
Definition: SDL_stdinc.h:130
A type representing an atomic integer value. It is a struct so people don't accidentally use numeric ...
Definition: SDL_atomic.h:265