一、关键点解析

1、_Lock_policy解析

2、__default_lock_policy的定义

3、__atomic_add_dispatch解析

4、__attribute__

5、__exchange_and_add_dispatch解析

6、_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE和_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER解析

7、内存屏障_GLIBCXX_READ_MEM_BARRIER和_GLIBCXX_WRITE_MEM_BARRIER

8、_Sp_counted_base的父类

9、头文件

二、源码解析

一、关键点解析

1、_Lock_policy解析

enum _Lock_policy { _S_single, _S_mutex, _S_atomic };

// Available locking policies:
// _S_single single-threaded code that doesn't need to be locked.
// _S_mutex multi-threaded code that requires additional support
// from gthr.h or abstraction layers in concurrence.h.
// _S_atomic multi-threaded code using atomic operations.

2、__default_lock_policy的定义

static const _Lock_policy __default_lock_policy =
#ifdef __GTHREADS
#if (defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2) \
&& defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4))
_S_atomic;
#else
_S_mutex;
#endif
#else
_S_single;
#endif

3、__atomic_add_dispatch解析

（1）、 __atomic_add_dispatch函数定义

static inline void
__attribute__ ((__unused__))
__atomic_add_dispatch(_Atomic_word* __mem, int __val)
{
#ifdef __GTHREADS//如果定义了多线程
if (__gthread_active_p())//如果多线程是可用的
__atomic_add(__mem, __val);
else
__atomic_add_single(__mem, __val);
#else
__atomic_add_single(__mem, __val);
#endif
}

（2）、其它一些定义

#ifdef _GLIBCXX_ATOMIC_BUILTINS
static inline _Atomic_word
__exchange_and_add(volatile _Atomic_word* __mem, int __val)
{ return __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }

static inline void
__atomic_add(volatile _Atomic_word* __mem, int __val)
{ __atomic_fetch_add(__mem, __val, __ATOMIC_ACQ_REL); }
#else
_Atomic_word
__attribute__ ((__unused__))
__exchange_and_add(volatile _Atomic_word*, int) throw ();

void
__attribute__ ((__unused__))
__atomic_add(volatile _Atomic_word*, int) throw ();
#endif

static inline void
__atomic_add_single(_Atomic_word* __mem, int __val)
{ *__mem += __val; }

4、attribute

GNU C 的一大特色就是__attribute__ 机制。__attribute__ 可以设置函数属性（Function Attribute ）、变量属性（Variable Attribute ）和类型属性（Type Attribute ）。

__attribute__ 书写特征是：__attribute__ 前后都有两个下划线，并切后面会紧跟一对原括弧，括弧里面是相应的__attribute__ 参数。

__attribute__ 语法格式为：__attribute__ ((attribute-list))

5、__exchange_and_add_dispatch解析

static inline _Atomic_word
__attribute__ ((__unused__))
__exchange_and_add_dispatch(_Atomic_word* __mem, int __val)
{
#ifdef __GTHREADS
if (__gthread_active_p())
return __exchange_and_add(__mem, __val);
else
return __exchange_and_add_single(__mem, __val);
#else
return __exchange_and_add_single(__mem, __val);
#endif
}

Both of these functions are declared in the header file <ext/atomicity.h>, and are in namespace __gnu_cxx.

__exchange_and_add_dispatch

Adds the second argument's value to the first argument. Returns the old value.
__atomic_add_dispatch

Adds the second argument's value to the first argument. Has no return value.

6、_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE和_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER解析

// Macros for race detectors.
// _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(A) and
// _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(A) should be used to explain
// atomic (lock-free) synchronization to race detectors:
// the race detector will infer a happens-before arc from the former to the
// latter when they share the same argument pointer.
//
// The most frequent use case for these macros (and the only case in the
// current implementation of the library) is atomic reference counting:
// void _M_remove_reference()
// {
// _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&this->_M_refcount);
// if (__gnu_cxx::__exchange_and_add_dispatch(&this->_M_refcount, -1) <= 0)
// {
// _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&this->_M_refcount);
// _M_destroy(__a);
// }
// }
// The annotations in this example tell the race detector that all memory
// accesses occurred when the refcount was positive do not race with
// memory accesses which occurred after the refcount became zero.

All synchronization primitives used in the library internals need to be understood by race detectors so that they do not produce false reports.

Two annotation macros are used to explain low-level synchronization to race detectors: _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE() and _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(). By default, these macros are defined empty -- anyone who wants to use a race detector needs to redefine them to call an appropriate API. Since these macros are empty by default when the library is built, redefining them will only affect inline functions and template instantiations which are compiled in user code. This allows annotation of templates such as shared_ptr, but not code which is only instantiated in the library. Code which is only instantiated in the library needs to be recompiled with the annotation macros defined. That can be done by rebuilding the entire libstdc++.so file but a simpler alternative exists for ELF platforms such as GNU/Linux, because ELF symbol interposition allows symbol7s defined in the shared library to be overridden by symbols with the same name that appear earlier in the runtime search path. This means you only need to recompile the functions that are affected by the annotation macros, which can be done by recompiling individual files.

7、内存屏障_GLIBCXX_READ_MEM_BARRIER和_GLIBCXX_WRITE_MEM_BARRIER

CPU乱序执行在单线程环境下是一种很好的优化手段，但是在多线程环境下，就会出现数据不一致的问题，因此就可以通过内存屏障这个机制来处理这个问题。

1.写内存屏障(Store Memory Barrier)：在指令后插入Store Barrier，能让写入缓存中最新数据更新写入主内存中，让其他线程可见。强制写入主内存，这种显示调用，不会让CPU去进行指令重排序
2.读内存屏障(Load Memory Barrier)：在指令后插入Load Barrier，可以让高速缓存中的数据失效，强制重新从主内存中加载数据。也是不会让CPU去进行指令重排。

#ifndef _GLIBCXX_READ_MEM_BARRIER
#define _GLIBCXX_READ_MEM_BARRIER __asm __volatile ("":::"memory")
#endif
#ifndef _GLIBCXX_WRITE_MEM_BARRIER
#define _GLIBCXX_WRITE_MEM_BARRIER __asm __volatile ("":::"memory")
#endif

__asm __volatile ("":::"memory")为汇编代码

creates a compiler level memory barrier forcing optimizer to not re-order memory accesses across the barrier.

For example, if you need to access some address in a specific order (probably because that memory area is actually backed by a different device rather than a memory) you need to be able tell this to the compiler otherwise it may just optimize your steps for the sake of efficiency.

memory强制gcc编译器假设RAM所有内存单元均被汇编指令修改，这样cpu中的registers和cache中已缓存的内存单元中的数据将作废。cpu将不得不在需要的时候重新读取内存中的数据。这就阻止了cpu又将registers，cache中的数据用于去优化指令。

这里就是跟编译器说已经更改了内存了。

8、_Sp_counted_base的父类

是继承_Mutex_base 类的（_Mutex_base 类解析可以参考GNU C++ 智能指针2-- 解析_Mutex_base类_kupePoem的专栏-CSDN博客）

9、头文件

类位于shared_ptr_base.h中

二、源码解析

  template<_Lock_policy _Lp = __default_lock_policy>class _Sp_counted_base: public _Mutex_base<_Lp>{public:  _Sp_counted_base() noexcept: _M_use_count(1), _M_weak_count(1) { }virtual~_Sp_counted_base() noexcept{ }// Called when _M_use_count drops to zero, to release the resources// managed by *this.virtual void_M_dispose() noexcept = 0;// Called when _M_weak_count drops to zero.virtual void_M_destroy() noexcept{ delete this; }virtual void*_M_get_deleter(const std::type_info&) noexcept = 0;void_M_add_ref_copy(){ __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }void_M_add_ref_lock();bool_M_add_ref_lock_nothrow();void_M_release() noexcept{// Be race-detector-friendly.  For more info see bits/c++config._GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1){_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);_M_dispose();// There must be a memory barrier between dispose() and destroy()// to ensure that the effects of dispose() are observed in the// thread that runs destroy().// See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.htmlif (_Mutex_base<_Lp>::_S_need_barriers){_GLIBCXX_READ_MEM_BARRIER;_GLIBCXX_WRITE_MEM_BARRIER;}// Be race-detector-friendly.  For more info see bits/c++config._GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count,-1) == 1){_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);_M_destroy();}}}void_M_weak_add_ref() noexcept{ __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }void_M_weak_release() noexcept{// Be race-detector-friendly. For more info see bits/c++config._GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1){_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);if (_Mutex_base<_Lp>::_S_need_barriers){// See _M_release(),// destroy() must observe results of dispose()_GLIBCXX_READ_MEM_BARRIER;_GLIBCXX_WRITE_MEM_BARRIER;}_M_destroy();}}long_M_get_use_count() const noexcept{// No memory barrier is used here so there is no synchronization// with other threads.return __atomic_load_n(&_M_use_count, __ATOMIC_RELAXED);}private:  _Sp_counted_base(_Sp_counted_base const&) = delete;_Sp_counted_base& operator=(_Sp_counted_base const&) = delete;_Atomic_word  _M_use_count;     // #shared_Atomic_word  _M_weak_count;    // #weak + (#shared != 0)};template<>inline void_Sp_counted_base<_S_single>::_M_add_ref_lock(){if (_M_use_count == 0)__throw_bad_weak_ptr();++_M_use_count;}template<>inline void_Sp_counted_base<_S_mutex>::_M_add_ref_lock(){__gnu_cxx::__scoped_lock sentry(*this);if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0){_M_use_count = 0;__throw_bad_weak_ptr();}}template<> inline void_Sp_counted_base<_S_atomic>::_M_add_ref_lock(){// Perform lock-free add-if-not-zero operation._Atomic_word __count = _M_get_use_count();do{if (__count == 0)__throw_bad_weak_ptr();// Replace the current counter value with the old value + 1, as// long as it's not changed meanwhile. }while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,true, __ATOMIC_ACQ_REL, __ATOMIC_RELAXED));}template<>inline bool_Sp_counted_base<_S_single>::_M_add_ref_lock_nothrow(){if (_M_use_count == 0)return false;++_M_use_count;return true;}template<>inline bool_Sp_counted_base<_S_mutex>::_M_add_ref_lock_nothrow(){__gnu_cxx::__scoped_lock sentry(*this);if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0){_M_use_count = 0;return false;}return true;}template<>inline bool_Sp_counted_base<_S_atomic>::_M_add_ref_lock_nothrow(){// Perform lock-free add-if-not-zero operation._Atomic_word __count = _M_get_use_count();do{if (__count == 0)return false;// Replace the current counter value with the old value + 1, as// long as it's not changed meanwhile.}while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,true, __ATOMIC_ACQ_REL,__ATOMIC_RELAXED));return true;}template<>inline void_Sp_counted_base<_S_single>::_M_add_ref_copy(){ ++_M_use_count; }template<>inline void_Sp_counted_base<_S_single>::_M_release() noexcept{if (--_M_use_count == 0){_M_dispose();if (--_M_weak_count == 0)_M_destroy();}}template<>inline void_Sp_counted_base<_S_single>::_M_weak_add_ref() noexcept{ ++_M_weak_count; }template<>inline void_Sp_counted_base<_S_single>::_M_weak_release() noexcept{if (--_M_weak_count == 0)_M_destroy();}template<>inline long_Sp_counted_base<_S_single>::_M_get_use_count() const noexcept{ return _M_use_count; }

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GNU C++ 智能指针3-- 解析_Sp_counted_base类

一、关键点解析

1、_Lock_policy解析

2、__default_lock_policy的定义

3、__atomic_add_dispatch解析

4、attribute

5、__exchange_and_add_dispatch解析

6、_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE和_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER解析

7、内存屏障_GLIBCXX_READ_MEM_BARRIER和_GLIBCXX_WRITE_MEM_BARRIER

8、_Sp_counted_base的父类

9、头文件

二、源码解析

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GNU C++ 智能指针3-- 解析_Sp_counted_base类

一、关键点解析

1、_Lock_policy解析

2、__default_lock_policy的定义

3、__atomic_add_dispatch解析

4、__attribute__

5、__exchange_and_add_dispatch解析

6、_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE和_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER解析

7、 内存屏障_GLIBCXX_READ_MEM_BARRIER和_GLIBCXX_WRITE_MEM_BARRIER

8、_Sp_counted_base的父类

9、头文件

二、源码解析

GNU C++ 智能指针3-- 解析_Sp_counted_base类相关推荐

最新文章

热门文章

4、attribute

7、内存屏障_GLIBCXX_READ_MEM_BARRIER和_GLIBCXX_WRITE_MEM_BARRIER