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269 lines
9.2 KiB
C
269 lines
9.2 KiB
C
1 year ago
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/*
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Simple DirectMedia Layer
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Copyright (C) 1997-2016 Sam Lantinga <slouken@libsdl.org>
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This software is provided 'as-is', without any express or implied
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warranty. In no event will the authors be held liable for any damages
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arising from the use of this software.
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Permission is granted to anyone to use this software for any purpose,
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including commercial applications, and to alter it and redistribute it
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freely, subject to the following restrictions:
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1. The origin of this software must not be misrepresented; you must not
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claim that you wrote the original software. If you use this software
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in a product, an acknowledgment in the product documentation would be
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appreciated but is not required.
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2. Altered source versions must be plainly marked as such, and must not be
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misrepresented as being the original software.
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3. This notice may not be removed or altered from any source distribution.
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*/
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/**
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* \file SDL_atomic.h
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*
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* Atomic operations.
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*
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* IMPORTANT:
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* If you are not an expert in concurrent lockless programming, you should
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* only be using the atomic lock and reference counting functions in this
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* file. In all other cases you should be protecting your data structures
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* with full mutexes.
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*
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* The list of "safe" functions to use are:
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* SDL_AtomicLock()
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* SDL_AtomicUnlock()
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* SDL_AtomicIncRef()
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* SDL_AtomicDecRef()
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*
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* Seriously, here be dragons!
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* ^^^^^^^^^^^^^^^^^^^^^^^^^^^
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*
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* You can find out a little more about lockless programming and the
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* subtle issues that can arise here:
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* http://msdn.microsoft.com/en-us/library/ee418650%28v=vs.85%29.aspx
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*
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* There's also lots of good information here:
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* http://www.1024cores.net/home/lock-free-algorithms
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* http://preshing.com/
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*
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* These operations may or may not actually be implemented using
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* processor specific atomic operations. When possible they are
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* implemented as true processor specific atomic operations. When that
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* is not possible the are implemented using locks that *do* use the
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* available atomic operations.
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*
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* All of the atomic operations that modify memory are full memory barriers.
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*/
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#ifndef _SDL_atomic_h_
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#define _SDL_atomic_h_
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#include "SDL_stdinc.h"
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#include "SDL_platform.h"
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#include "begin_code.h"
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/* Set up for C function definitions, even when using C++ */
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#ifdef __cplusplus
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extern "C" {
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#endif
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/**
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* \name SDL AtomicLock
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*
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* The atomic locks are efficient spinlocks using CPU instructions,
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* but are vulnerable to starvation and can spin forever if a thread
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* holding a lock has been terminated. For this reason you should
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* minimize the code executed inside an atomic lock and never do
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* expensive things like API or system calls while holding them.
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*
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* The atomic locks are not safe to lock recursively.
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*
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* Porting Note:
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* The spin lock functions and type are required and can not be
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* emulated because they are used in the atomic emulation code.
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*/
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/* @{ */
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typedef int SDL_SpinLock;
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/**
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* \brief Try to lock a spin lock by setting it to a non-zero value.
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*
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* \param lock Points to the lock.
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*
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* \return SDL_TRUE if the lock succeeded, SDL_FALSE if the lock is already held.
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*/
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extern DECLSPEC SDL_bool SDLCALL SDL_AtomicTryLock(SDL_SpinLock *lock);
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/**
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* \brief Lock a spin lock by setting it to a non-zero value.
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*
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* \param lock Points to the lock.
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*/
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extern DECLSPEC void SDLCALL SDL_AtomicLock(SDL_SpinLock *lock);
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/**
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* \brief Unlock a spin lock by setting it to 0. Always returns immediately
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*
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* \param lock Points to the lock.
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*/
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extern DECLSPEC void SDLCALL SDL_AtomicUnlock(SDL_SpinLock *lock);
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/* @} *//* SDL AtomicLock */
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/**
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* The compiler barrier prevents the compiler from reordering
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* reads and writes to globally visible variables across the call.
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*/
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#if defined(_MSC_VER) && (_MSC_VER > 1200)
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void _ReadWriteBarrier(void);
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#pragma intrinsic(_ReadWriteBarrier)
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#define SDL_CompilerBarrier() _ReadWriteBarrier()
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#elif (defined(__GNUC__) && !defined(__EMSCRIPTEN__)) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
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/* This is correct for all CPUs when using GCC or Solaris Studio 12.1+. */
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#define SDL_CompilerBarrier() __asm__ __volatile__ ("" : : : "memory")
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#else
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#define SDL_CompilerBarrier() \
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{ SDL_SpinLock _tmp = 0; SDL_AtomicLock(&_tmp); SDL_AtomicUnlock(&_tmp); }
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#endif
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/**
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* Memory barriers are designed to prevent reads and writes from being
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* reordered by the compiler and being seen out of order on multi-core CPUs.
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*
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* A typical pattern would be for thread A to write some data and a flag,
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* and for thread B to read the flag and get the data. In this case you
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* would insert a release barrier between writing the data and the flag,
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* guaranteeing that the data write completes no later than the flag is
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* written, and you would insert an acquire barrier between reading the
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* flag and reading the data, to ensure that all the reads associated
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* with the flag have completed.
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*
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* In this pattern you should always see a release barrier paired with
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* an acquire barrier and you should gate the data reads/writes with a
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* single flag variable.
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*
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* For more information on these semantics, take a look at the blog post:
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* http://preshing.com/20120913/acquire-and-release-semantics
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*/
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#if defined(__GNUC__) && (defined(__powerpc__) || defined(__ppc__))
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#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("lwsync" : : : "memory")
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#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("lwsync" : : : "memory")
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#elif defined(__GNUC__) && defined(__arm__)
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#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__)
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#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("dmb ish" : : : "memory")
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#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("dmb ish" : : : "memory")
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#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__)
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#ifdef __thumb__
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/* The mcr instruction isn't available in thumb mode, use real functions */
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extern DECLSPEC void SDLCALL SDL_MemoryBarrierRelease();
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extern DECLSPEC void SDLCALL SDL_MemoryBarrierAcquire();
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#else
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#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
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#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5" : : "r"(0) : "memory")
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#endif /* __thumb__ */
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#else
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#define SDL_MemoryBarrierRelease() __asm__ __volatile__ ("" : : : "memory")
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#define SDL_MemoryBarrierAcquire() __asm__ __volatile__ ("" : : : "memory")
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#endif /* __GNUC__ && __arm__ */
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#else
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#if (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x5120))
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/* This is correct for all CPUs on Solaris when using Solaris Studio 12.1+. */
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#include <mbarrier.h>
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#define SDL_MemoryBarrierRelease() __machine_rel_barrier()
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#define SDL_MemoryBarrierAcquire() __machine_acq_barrier()
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#else
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/* This is correct for the x86 and x64 CPUs, and we'll expand this over time. */
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#define SDL_MemoryBarrierRelease() SDL_CompilerBarrier()
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#define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
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#endif
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#endif
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/**
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* \brief A type representing an atomic integer value. It is a struct
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* so people don't accidentally use numeric operations on it.
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*/
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typedef struct { int value; } SDL_atomic_t;
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/**
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* \brief Set an atomic variable to a new value if it is currently an old value.
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*
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* \return SDL_TRUE if the atomic variable was set, SDL_FALSE otherwise.
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*
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* \note If you don't know what this function is for, you shouldn't use it!
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*/
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extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCAS(SDL_atomic_t *a, int oldval, int newval);
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/**
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* \brief Set an atomic variable to a value.
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*
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* \return The previous value of the atomic variable.
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*/
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extern DECLSPEC int SDLCALL SDL_AtomicSet(SDL_atomic_t *a, int v);
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/**
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* \brief Get the value of an atomic variable
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*/
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extern DECLSPEC int SDLCALL SDL_AtomicGet(SDL_atomic_t *a);
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/**
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* \brief Add to an atomic variable.
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*
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* \return The previous value of the atomic variable.
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*
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* \note This same style can be used for any number operation
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*/
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extern DECLSPEC int SDLCALL SDL_AtomicAdd(SDL_atomic_t *a, int v);
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/**
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* \brief Increment an atomic variable used as a reference count.
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*/
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#ifndef SDL_AtomicIncRef
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#define SDL_AtomicIncRef(a) SDL_AtomicAdd(a, 1)
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#endif
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/**
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* \brief Decrement an atomic variable used as a reference count.
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*
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* \return SDL_TRUE if the variable reached zero after decrementing,
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* SDL_FALSE otherwise
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*/
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#ifndef SDL_AtomicDecRef
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#define SDL_AtomicDecRef(a) (SDL_AtomicAdd(a, -1) == 1)
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#endif
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/**
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* \brief Set a pointer to a new value if it is currently an old value.
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*
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* \return SDL_TRUE if the pointer was set, SDL_FALSE otherwise.
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*
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* \note If you don't know what this function is for, you shouldn't use it!
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*/
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extern DECLSPEC SDL_bool SDLCALL SDL_AtomicCASPtr(void **a, void *oldval, void *newval);
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/**
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* \brief Set a pointer to a value atomically.
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*
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* \return The previous value of the pointer.
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*/
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extern DECLSPEC void* SDLCALL SDL_AtomicSetPtr(void **a, void* v);
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/**
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* \brief Get the value of a pointer atomically.
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*/
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extern DECLSPEC void* SDLCALL SDL_AtomicGetPtr(void **a);
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/* Ends C function definitions when using C++ */
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#ifdef __cplusplus
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}
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#endif
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#include "close_code.h"
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#endif /* _SDL_atomic_h_ */
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/* vi: set ts=4 sw=4 expandtab: */
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