在VS 2015上，使用/Zc：threadSafeInit，静态局部变量的初始化是线程安全的，但MSDN说
线程安全的静态局部变量在内部使用线程本地存储 (TLS)，以便在静态初始化时提供高效执行。
Windows XP忽略.tls段，如果动态加载带有LoadLibrary的Dll，所以
- 为什么使用TLS，它如何有效地工作？
- std：：call_once 是否使用 TLS ？
- 如何在没有TLS的情况下实现线程安全初始化？
引文的链接在这里 线程安全的本地静态初始化
此功能的实现依赖于 Windows Vista 和更高版本的操作系统中的 Windows 操作系统支持功能。Windows XP、Windows Server 2003 和较旧的操作系统没有这种支持，因此它们无法获得效率优势。
我的测试代码：
class AA
{
public:
int m_a = 1;
};
AA* getAA()
{
static AA a;
return &a;
}
int main()
{
AA* pa = getAA();
return 0;
}
使用/Zc:threadSafeInit时，windbg 中的函数getAA分解为：
008c1000 55 push ebp
008c1001 8bec mov ebp,esp
008c1003 64a12c000000 mov eax,dword ptr fs:[0000002Ch]
008c1009 8b08 mov ecx,dword ptr [eax]
008c100b 8b15b8338c00 mov edx,dword ptr [testStatic!__favor+0x4 (008c33b8)]
008c1011 3b9104000000 cmp edx,dword ptr [ecx+4]
008c1017 7e2d jle testStatic!getAA+0x46 (008c1046)
008c1019 68b8338c00 push offset testStatic!__favor+0x4 (008c33b8)
008c101e e809020000 call testStatic!_Init_thread_header (008c122c)
008c1023 83c404 add esp,4
008c1026 833db8338c00ff cmp dword ptr [testStatic!__favor+0x4 (008c33b8)],0FFFFFFFFh
008c102d 7517 jne testStatic!getAA+0x46 (008c1046)
008c102f b9bc338c00 mov ecx,offset testStatic!a (008c33bc)
008c1034 e817000000 call testStatic!AA::AA (008c1050)
008c1039 68b8338c00 push offset testStatic!__favor+0x4 (008c33b8)
008c103e e89f010000 call testStatic!_Init_thread_footer (008c11e2)
008c1043 83c404 add esp,4
008c1046 b8bc338c00 mov eax,offset testStatic!a (008c33bc)
008c104b 5d pop ebp
008c104c c3 ret
使用/Zc:threadSafeInit-(关闭功能)，函数getAA反汇编为：
010e1000 55 push ebp
010e1001 8bec mov ebp,esp
010e1003 a180330e01 mov eax,dword ptr [testStatic!a+0x4 (010e3380)]
010e1008 83e001 and eax,1
010e100b 7519 jne testStatic!getAA+0x26 (010e1026)
010e100d 8b0d80330e01 mov ecx,dword ptr [testStatic!a+0x4 (010e3380)]
010e1013 83c901 or ecx,1
010e1016 890d80330e01 mov dword ptr [testStatic!a+0x4 (010e3380)],ecx
010e101c b97c330e01 mov ecx,offset testStatic!a (010e337c)
010e1021 e80a000000 call testStatic!AA::AA (010e1030)
010e1026 b87c330e01 mov eax,offset testStatic!a (010e337c)
010e102b 5d pop ebp
010e102c c3 ret
为什么静态局部变量的 MSVC 线程安全初始化使用 TLS
TLS 仅用作优化。对于静态初始化，它使用单个全局关键部分，或者(如果可用)SRW 锁。
您可以在VC++ CRT源crt/src/vcruntime/thread_safe_statics.cpp中看到它。
它是开源的，可作为 MSVC 安装的一部分提供。我在这里包含相关片段以供参考：
//
// thread_safe_statics.cpp
//
// Copyright (c) Microsoft Corporation. All rights reserved.
//
// Helper functions used by thread-safe static initialization.
//
#ifdef _M_CEE
#error This file cannot be built as managed
#endif
#include <vcstartup_internal.h>
#include <vcruntime_internal.h>
#include <limits.h>
static DWORD const xp_timeout = 100; // ms
static int const uninitialized = 0;
static int const being_initialized = -1;
static int const epoch_start = INT_MIN;
// Access to these variables is guarded in the below functions. They may only
// be modified while the lock is held. _Tss_epoch is readable from user
// code and is read without taking the lock.
extern "C"
{
int _Init_global_epoch = epoch_start;
__declspec(thread) int _Init_thread_epoch = epoch_start;
}
// On Vista or newer, the native CONDITION_VARIABLE type is used. On XP, we use a simple
// Windows event. This is not safe to use as a complete condition variable, but for the purposes
// of this feature the event is sufficient but not optimal. See the code in _Tss_wait
// below.
//
// For Windows OS components: The OS supports APISets downlevel to Windows 7,
// and OS components that run downlevel to Windows 7 may build against APISets.
// However, these components cannot use CONDITION_VARIABLE directly because it
// is not available via APISets until Windows 8. Thus, for Windows OS components,
// we use the "ancient" code path and first try the APISet and then fall back to
// kernel32.dll.
#if defined _SCRT_ENCLAVE_BUILD || defined _CRT_APP || defined _CRT_WINDOWS_USE_VISTA_TSS
|| (!defined _CRT_WINDOWS && (defined _ONECORE || defined _M_ARM || defined _M_ARM64))
#define _USE_VISTA_THREAD_SAFE_STATICS 1
#else
#define _USE_VISTA_THREAD_SAFE_STATICS 0
#endif
static CONDITION_VARIABLE g_tss_cv;
#if _USE_VISTA_THREAD_SAFE_STATICS
static SRWLOCK g_tss_srw;
#else // ^^^ _USE_VISTA_THREAD_SAFE_STATICS ^^^ // vvv !_USE_VISTA_THREAD_SAFE_STATICS vvv //
static HANDLE g_tss_event;
static CRITICAL_SECTION g_tss_mutex;
static decltype(SleepConditionVariableCS)* g_sleep_condition_variable_cs;
static decltype(WakeAllConditionVariable)* g_wake_all_condition_variable;
static void __cdecl __scrt_initialize_thread_safe_statics_platform_specific() noexcept
{
// This can fail pre-Vista and that is ignored.
InitializeCriticalSectionAndSpinCount(&g_tss_mutex, 4000);
// CONDITION_VARIABLE is available via this APISet starting on Windows 8.
HMODULE kernel_dll = GetModuleHandleW(L"api-ms-win-core-synch-l1-2-0.dll");
if (kernel_dll == nullptr)
{
kernel_dll = GetModuleHandleW(L"kernel32.dll");
}
if (kernel_dll == nullptr)
{
__scrt_fastfail(FAST_FAIL_FATAL_APP_EXIT);
}
#define GET_PROC_ADDRESS(m, f) reinterpret_cast<decltype(f)*>(GetProcAddress(m, _CRT_STRINGIZE(f)))
auto const sleep_condition_variable_cs = GET_PROC_ADDRESS(kernel_dll, SleepConditionVariableCS);
auto const wake_all_condition_variable = GET_PROC_ADDRESS(kernel_dll, WakeAllConditionVariable);
#undef GET_PROC_ADDRESS
if (sleep_condition_variable_cs && wake_all_condition_variable)
{
g_sleep_condition_variable_cs = sleep_condition_variable_cs;
g_wake_all_condition_variable = wake_all_condition_variable;
}
else
{
g_tss_event = CreateEventW(NULL, TRUE, FALSE, NULL);
if (g_tss_event == nullptr)
{
__scrt_fastfail(FAST_FAIL_FATAL_APP_EXIT);
}
}
}
// Terminator for synchronization data structures.
static void __cdecl __scrt_uninitialize_thread_safe_statics() noexcept
{
DeleteCriticalSection(&g_tss_mutex);
if (g_tss_event != nullptr)
{
CloseHandle(g_tss_event);
}
}
// Initializer for synchronization data structures.
static int __cdecl __scrt_initialize_thread_safe_statics() noexcept
{
__scrt_initialize_thread_safe_statics_platform_specific();
// If CRT initialization was skipped then we should initialize the atexit tables.
// This will only be needed when using a managed DLL with /NOENTRY specified.
if (!__scrt_initialize_onexit_tables(__scrt_module_type::dll))
{
__scrt_fastfail(FAST_FAIL_FATAL_APP_EXIT);
}
atexit(__scrt_uninitialize_thread_safe_statics);
return 0;
}
_CRTALLOC(".CRT$XIC") static _PIFV __scrt_initialize_tss_var = __scrt_initialize_thread_safe_statics;
#endif // _USE_VISTA_THREAD_SAFE_STATICS
// Helper functions for accessing the mutex and condition variable. Can be replaced with
// more suitable data structures provided by the CRT, preferably ones that use the most
// efficient synchronization primitives available on the platform.
// This is not intended to be a recursive lock.
extern "C" void __cdecl _Init_thread_lock()
{
#if _USE_VISTA_THREAD_SAFE_STATICS
AcquireSRWLockExclusive(&g_tss_srw);
#else // ^^^ _USE_VISTA_THREAD_SAFE_STATICS ^^^ // vvv !_USE_VISTA_THREAD_SAFE_STATICS vvv
EnterCriticalSection(&g_tss_mutex);
#endif // _USE_VISTA_THREAD_SAFE_STATICS
}
extern "C" void __cdecl _Init_thread_unlock()
{
#if _USE_VISTA_THREAD_SAFE_STATICS
ReleaseSRWLockExclusive(&g_tss_srw);
#else // ^^^ _USE_VISTA_THREAD_SAFE_STATICS ^^^ // vvv !_USE_VISTA_THREAD_SAFE_STATICS vvv
LeaveCriticalSection(&g_tss_mutex);
#endif // _USE_VISTA_THREAD_SAFE_STATICS
}
// Wait on the condition variable. In the XP implementation using only a Windows event
// we can't guarantee that we'll ever actually receive the notification signal, so we
// must use a non-infinite timeout. This is not optimal: we may wake up early if the
// initializer is long-running, or we may miss the signal and not wake up until the
// timeout expires. The signal may be missed because the sleeping threads may be
// stolen by the kernel to service an APC, or due to the race condition between the
// unlock call and the WaitForSingleObject call.
extern "C" void __cdecl _Init_thread_wait(DWORD const timeout)
{
#if _USE_VISTA_THREAD_SAFE_STATICS
SleepConditionVariableSRW(&g_tss_cv, &g_tss_srw, timeout, 0);
#else // ^^^ _USE_VISTA_THREAD_SAFE_STATICS ^^^ // vvv !_USE_VISTA_THREAD_SAFE_STATICS vvv //
if (g_sleep_condition_variable_cs)
{
// Vista+ code is first because it's most likely
g_sleep_condition_variable_cs(&g_tss_cv, &g_tss_mutex, timeout);
return;
}
_ASSERT(timeout != INFINITE);
_Init_thread_unlock();
WaitForSingleObjectEx(g_tss_event, timeout, FALSE);
_Init_thread_lock();
#endif // _USE_VISTA_THREAD_SAFE_STATICS
}
extern "C" void __cdecl _Init_thread_notify()
{
#if _USE_VISTA_THREAD_SAFE_STATICS
WakeAllConditionVariable(&g_tss_cv);
#else // ^^^ _USE_VISTA_THREAD_SAFE_STATICS ^^^ // vvv !_USE_VISTA_THREAD_SAFE_STATICS vvv
if (g_wake_all_condition_variable)
{
// Vista+ code is first because it's most likely
g_wake_all_condition_variable(&g_tss_cv);
return;
}
SetEvent(g_tss_event);
ResetEvent(g_tss_event);
#endif // _USE_VISTA_THREAD_SAFE_STATICS
}
// Control access to the initialization expression. Only one thread may leave
// this function before the variable has completed initialization, this thread
// will perform initialization. All other threads are blocked until the
// initialization completes or fails due to an exception.
extern "C" void __cdecl _Init_thread_header(int* const pOnce) noexcept
{
_Init_thread_lock();
if (*pOnce == uninitialized)
{
*pOnce = being_initialized;
}
else
{
while (*pOnce == being_initialized)
{
// Timeout can be replaced with an infinite wait when XP support is
// removed or the XP-based condition variable is sophisticated enough
// to guarantee all waiting threads will be woken when the variable is
// signalled.
_Init_thread_wait(xp_timeout);
if (*pOnce == uninitialized)
{
*pOnce = being_initialized;
_Init_thread_unlock();
return;
}
}
_Init_thread_epoch = _Init_global_epoch;
}
_Init_thread_unlock();
}
// Abort processing of the initializer due to an exception. Reset the state
// to uninitialized and release waiting threads (one of which will take over
// initialization, any remaining will again sleep).
extern "C" void __cdecl _Init_thread_abort(int* const pOnce) noexcept
{
_Init_thread_lock();
*pOnce = uninitialized;
_Init_thread_unlock();
_Init_thread_notify();
}
// Called by the thread that completes initialization of a variable.
// Increment the global and per thread counters, mark the variable as
// initialized, and release waiting threads.
extern "C" void __cdecl _Init_thread_footer(int* const pOnce) noexcept
{
_Init_thread_lock();
++_Init_global_epoch;
*pOnce = _Init_global_epoch;
_Init_thread_epoch = _Init_global_epoch;
_Init_thread_unlock();
_Init_thread_notify();
}
需要锁的原因源于 ISO C++ 标准 ([stmt.dcl]/4)，该标准要求块范围的static局部变量以线程安全的方式精确初始化一次(MSDN 文章中有关/Zc：threadSafeInit 的部分中也对此进行了解释，该文章可用于放宽此要求作为编译器扩展)。