[PATCH v2] libstdc++: On Windows, retrieve thread-local variables via functions
LIU Hao
lh_mouse@126.com
Thu Dec 4 09:30:52 GMT 2025
The previous patch was incorrect, because it defined static references to thread-local variables and in
that case they would no longer be thread-local.
Instead of that, those getter functions are called directly in `_Prepare_execution`.
---
For Windows, GCC can be configured with `--enable-tls` to enable native TLS.
The native TLS implementation has a limitation that it is incapable of
exporting thread-local variables from DLLs. Therefore, they are retrieved
via getter functions instead.
libstdc++-v3/ChangeLog:
* config/os/mingw32-w64/os_defines.h (_GLIBCXX_NO_EXTERN_THREAD_LOCAL): New
macro.
* include/std/mutex (__get_once_callable): Declare function when
_GLIBCXX_NO_EXTERN_THREAD_LOCAL.
(__get_once_call): Likewise.
(once_flag::_Prepare_execution::_Prepare_execution): Retrieve thread-local
pointers via functions when _GLIBCXX_NO_EXTERN_THREAD_LOCAL.
(once_flag::_Prepare_execution::~_Prepare_execution): Likewise.
* src/c++11/mutex.cc (__get_once_callable): Define function when
_GLIBCXX_NO_EXTERN_THREAD_LOCAL.
(__get_once_call): Likewise.
Signed-off-by: LIU Hao <lh_mouse@126.com>
---
.../config/os/mingw32-w64/os_defines.h | 4 +++
libstdc++-v3/include/std/mutex | 26 +++++++++++++++++--
libstdc++-v3/src/c++11/mutex.cc | 14 ++++++++++
3 files changed, 42 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/config/os/mingw32-w64/os_defines.h
b/libstdc++-v3/config/os/mingw32-w64/os_defines.h
index 893cd704891b..70106fe792c6 100644
--- a/libstdc++-v3/config/os/mingw32-w64/os_defines.h
+++ b/libstdc++-v3/config/os/mingw32-w64/os_defines.h
@@ -96,4 +96,8 @@
// See libstdc++/94268
#define _GLIBCXX_BUFSIZ 4096
+// Use functions to access thread-local variables from a different module.
+// Windows does not support exporting thread-local data.
+#define _GLIBCXX_NO_EXTERN_THREAD_LOCAL 1
+
#endif
diff --git a/libstdc++-v3/include/std/mutex b/libstdc++-v3/include/std/mutex
index d4fc4c646488..703d8ca69997 100644
--- a/libstdc++-v3/include/std/mutex
+++ b/libstdc++-v3/include/std/mutex
@@ -817,9 +817,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
# ifdef _GLIBCXX_HAVE_TLS
// If TLS is available use thread-local state for the type-erased callable
// that is being run by std::call_once in the current thread.
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
+ void*&
+ __get_once_callable() noexcept;
+
+ __typeof__(void (*)())&
+ __get_once_call() noexcept;
+
+# else // ! _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
extern __thread void* __once_callable;
extern __thread void (*__once_call)();
+# endif // ! _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
// RAII type to set up state for pthread_once call.
struct once_flag::_Prepare_execution
{
@@ -827,17 +839,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
explicit
_Prepare_execution(_Callable& __c)
{
- // Store address in thread-local pointer:
+ // Store address in thread-local pointer, and trampoline function to
+ // invoke the closure via thread-local pointer.
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+ __get_once_callable() = std::__addressof(__c);
+ __get_once_call() = [] { (*static_cast<_Callable*>(__get_once_callable()))(); };
+# else
__once_callable = std::__addressof(__c);
- // Trampoline function to invoke the closure via thread-local pointer:
__once_call = [] { (*static_cast<_Callable*>(__once_callable))(); };
+# endif
}
~_Prepare_execution()
{
// PR libstdc++/82481
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+ __get_once_callable() = nullptr;
+ __get_once_call() = nullptr;
+# else
__once_callable = nullptr;
__once_call = nullptr;
+# endif
}
_Prepare_execution(const _Prepare_execution&) = delete;
diff --git a/libstdc++-v3/src/c++11/mutex.cc b/libstdc++-v3/src/c++11/mutex.cc
index d5da5c66ae99..82f0afa4cb4e 100644
--- a/libstdc++-v3/src/c++11/mutex.cc
+++ b/libstdc++-v3/src/c++11/mutex.cc
@@ -34,6 +34,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__thread void* __once_callable;
__thread void (*__once_call)();
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
+ // When thread-local variables can't be exported, these functions are called
+ // to retrieve these variables.
+ void*&
+ __get_once_callable() noexcept
+ { return __once_callable; }
+
+ __typeof__(void (*)())&
+ __get_once_call() noexcept
+ { return __once_call; }
+
+# endif // _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
extern "C" void __once_proxy()
{
// The caller stored a function pointer in __once_call. If it requires
--
2.52.0
-------------- next part --------------
From 108c58d31a7c2afdd691f0c19132c92f7da50d1d Mon Sep 17 00:00:00 2001
From: LIU Hao <lh_mouse@126.com>
Date: Wed, 3 Dec 2025 11:10:46 +0800
Subject: [PATCH] libstdc++: On Windows, retrieve thread-local variables via
functions
For Windows, GCC can be configured with `--enable-tls` to enable native TLS.
The native TLS implementation has a limitation that it is incapable of
exporting thread-local variables from DLLs. Therefore, they are retrieved
via getter functions instead.
libstdc++-v3/ChangeLog:
* config/os/mingw32-w64/os_defines.h (_GLIBCXX_NO_EXTERN_THREAD_LOCAL): New
macro.
* include/std/mutex (__get_once_callable): Declare function when
_GLIBCXX_NO_EXTERN_THREAD_LOCAL.
(__get_once_call): Likewise.
(once_flag::_Prepare_execution::_Prepare_execution): Retrieve thread-local
pointers via functions when _GLIBCXX_NO_EXTERN_THREAD_LOCAL.
(once_flag::_Prepare_execution::~_Prepare_execution): Likewise.
* src/c++11/mutex.cc (__get_once_callable): Define function when
_GLIBCXX_NO_EXTERN_THREAD_LOCAL.
(__get_once_call): Likewise.
Signed-off-by: LIU Hao <lh_mouse@126.com>
---
.../config/os/mingw32-w64/os_defines.h | 4 +++
libstdc++-v3/include/std/mutex | 26 +++++++++++++++++--
libstdc++-v3/src/c++11/mutex.cc | 14 ++++++++++
3 files changed, 42 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/config/os/mingw32-w64/os_defines.h b/libstdc++-v3/config/os/mingw32-w64/os_defines.h
index 893cd704891b..70106fe792c6 100644
--- a/libstdc++-v3/config/os/mingw32-w64/os_defines.h
+++ b/libstdc++-v3/config/os/mingw32-w64/os_defines.h
@@ -96,4 +96,8 @@
// See libstdc++/94268
#define _GLIBCXX_BUFSIZ 4096
+// Use functions to access thread-local variables from a different module.
+// Windows does not support exporting thread-local data.
+#define _GLIBCXX_NO_EXTERN_THREAD_LOCAL 1
+
#endif
diff --git a/libstdc++-v3/include/std/mutex b/libstdc++-v3/include/std/mutex
index d4fc4c646488..703d8ca69997 100644
--- a/libstdc++-v3/include/std/mutex
+++ b/libstdc++-v3/include/std/mutex
@@ -817,9 +817,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
# ifdef _GLIBCXX_HAVE_TLS
// If TLS is available use thread-local state for the type-erased callable
// that is being run by std::call_once in the current thread.
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
+ void*&
+ __get_once_callable() noexcept;
+
+ __typeof__(void (*)())&
+ __get_once_call() noexcept;
+
+# else // ! _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
extern __thread void* __once_callable;
extern __thread void (*__once_call)();
+# endif // ! _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
// RAII type to set up state for pthread_once call.
struct once_flag::_Prepare_execution
{
@@ -827,17 +839,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
explicit
_Prepare_execution(_Callable& __c)
{
- // Store address in thread-local pointer:
+ // Store address in thread-local pointer, and trampoline function to
+ // invoke the closure via thread-local pointer.
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+ __get_once_callable() = std::__addressof(__c);
+ __get_once_call() = [] { (*static_cast<_Callable*>(__get_once_callable()))(); };
+# else
__once_callable = std::__addressof(__c);
- // Trampoline function to invoke the closure via thread-local pointer:
__once_call = [] { (*static_cast<_Callable*>(__once_callable))(); };
+# endif
}
~_Prepare_execution()
{
// PR libstdc++/82481
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+ __get_once_callable() = nullptr;
+ __get_once_call() = nullptr;
+# else
__once_callable = nullptr;
__once_call = nullptr;
+# endif
}
_Prepare_execution(const _Prepare_execution&) = delete;
diff --git a/libstdc++-v3/src/c++11/mutex.cc b/libstdc++-v3/src/c++11/mutex.cc
index d5da5c66ae99..82f0afa4cb4e 100644
--- a/libstdc++-v3/src/c++11/mutex.cc
+++ b/libstdc++-v3/src/c++11/mutex.cc
@@ -34,6 +34,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__thread void* __once_callable;
__thread void (*__once_call)();
+# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
+ // When thread-local variables can't be exported, these functions are called
+ // to retrieve these variables.
+ void*&
+ __get_once_callable() noexcept
+ { return __once_callable; }
+
+ __typeof__(void (*)())&
+ __get_once_call() noexcept
+ { return __once_call; }
+
+# endif // _GLIBCXX_NO_EXTERN_THREAD_LOCAL
+
extern "C" void __once_proxy()
{
// The caller stored a function pointer in __once_call. If it requires
--
2.52.0
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