1/*
2 Simple DirectMedia Layer
3 Copyright (C) 1997-2018 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 * \brief Try to lock a spin lock by setting it to a non-zero value.
93 *
94 * \param lock Points to the lock.
95 *
96 * \return SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already held.
97 */
98extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
99
100/**
101 * \brief Lock a spin lock by setting it to a non-zero value.
102 *
103 * \param lock Points to the lock.
104 */
105extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
106
107/**
108 * \brief Unlock a spin lock by setting it to 0. Always returns immediately
109 *
110 * \param lock Points to the lock.
111 */
112extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
113
114/* @} *//* SDL AtomicLock */
115
116
117/**
118 * The compiler barrier prevents the compiler from reordering
119 * reads and writes to globally visible variables across the call.
120 */
121#if defined(_MSC_VER) && (_MSC_VER > 1200) && !defined(__clang__)
122void _ReadWriteBarrier(void);
123#pragma intrinsic(_ReadWriteBarrier)
124#define SDL_CompilerBarrier() _ReadWriteBarrier()
125#elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
126/* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
127#define SDL_CompilerBarrier() __asm__ __volatile__ ("" : : : "memory")
128#elif defined(__WATCOMC__)
129extern _inline void SDL_CompilerBarrier (void);
130#pragma aux SDL_CompilerBarrier = "" parm [] modify exact [];
131#else
132#define SDL_CompilerBarrier() \
133{ SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
134#endif
135
136/**
137 * Memory barriers are designed to prevent reads and writes from being
138 * reordered by the compiler and being seen out of order on multi-core CPUs.
139 *
140 * A typical pattern would be for thread A to write some data and a flag,
141 * and for thread B to read the flag and get the data. In this case you
142 * would insert a release barrier between writing the data and the flag,
143 * guaranteeing that the data write completes no later than the flag is
144 * written, and you would insert an acquire barrier between reading the
145 * flag and reading the data, to ensure that all the reads associated
146 * with the flag have completed.
147 *
148 * In this pattern you should always see a release barrier paired with
149 * an acquire barrier and you should gate the data reads/writes with a
150 * single flag variable.
151 *
152 * For more information on these semantics, take a look at the blog post:
153 * http://preshing.com/20120913/acquire-and-release-semantics
154 */
155extern DECLSPEC void SDLCALL SDL_MemoryBarrierReleaseFunction(void);
156extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquireFunction(void);
157
158#if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
159#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
160#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
161#elif defined(__GNUC__) && defined(__aarch64__)
162#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
163#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
164#elif defined(__GNUC__) && defined(__arm__)
165#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__)
166#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
167#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
168#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__)
169#ifdef __thumb__
170/* The mcr instruction isn't available in thumb mode, use real functions */
171#define SDL_MemoryBarrierRelease() SDL_MemoryBarrierReleaseFunction()
172#define SDL_MemoryBarrierAcquire() SDL_MemoryBarrierAcquireFunction()
173#else
174#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
175#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
176#endif /* __thumb__ */
177#else
178#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
179#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
180#endif /* __GNUC__ && __arm__ */
181#else
182#if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
183/* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
184#include <mbarrier.h>
185#define SDL_MemoryBarrierRelease() __machine_rel_barrier()
186#define SDL_MemoryBarrierAcquire() __machine_acq_barrier()
187#else
188/* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
189#define SDL_MemoryBarrierRelease() SDL_CompilerBarrier()
190#define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
191#endif
192#endif
193
194/**
195 * \brief A type representing an atomic integer value. It is a struct
196 * so people don't accidentally use numeric operations on it.
197 */
198typedef struct { int value; } SDL_atomic_t;
199
200/**
201 * \brief Set an atomic variable to a new value if it is currently an old value.
202 *
203 * \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
204 *
205 * \note If you don't know what this function is for, you shouldn't use it!
206*/
207extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
208
209/**
210 * \brief Set an atomic variable to a value.
211 *
212 * \return The previous value of the atomic variable.
213 */
214extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
215
216/**
217 * \brief Get the value of an atomic variable
218 */
219extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
220
221/**
222 * \brief Add to an atomic variable.
223 *
224 * \return The previous value of the atomic variable.
225 *
226 * \note This same style can be used for any number operation
227 */
228extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
229
230/**
231 * \brief Increment an atomic variable used as a reference count.
232 */
233#ifndef SDL_AtomicIncRef
234#define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
235#endif
236
237/**
238 * \brief Decrement an atomic variable used as a reference count.
239 *
240 * \return SDL_TRUE if the variable reached zero after decrementing,
241 * SDL_FALSE otherwise
242 */
243#ifndef SDL_AtomicDecRef
244#define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
245#endif
246
247/**
248 * \brief Set a pointer to a new value if it is currently an old value.
249 *
250 * \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
251 *
252 * \note If you don't know what this function is for, you shouldn't use it!
253*/
254extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
255
256/**
257 * \brief Set a pointer to a value atomically.
258 *
259 * \return The previous value of the pointer.
260 */
261extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
262
263/**
264 * \brief Get the value of a pointer atomically.
265 */
266extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
267
268/* Ends C function definitions when using C++ */
269#ifdef __cplusplus
270}
271#endif
272
273#include "close_code.h"
274
275#endif /* SDL_atomic_h_ */
276
277/* vi: set ts=4 sw=4 expandtab: */
278