Fix PR27658 - Make ~mutex trivial when possible.
Currently std::mutex has a constexpr constructor, but a non-trivial
destruction.
The constexpr constructor is required to ensure the construction of a
mutex with static storage duration happens at compile time, during
constant initialization, and not during dynamic initialization.
This means that static mutex's are always initialized and can be used
safely during dynamic initialization without the "static initialization
order fiasco".
A trivial destructor is important for similar reasons. If a mutex is
used during dynamic initialization it might also be used during program
termination. If a static mutex has a non-trivial destructor it will be
invoked during termination. This can introduce the "static
deinitialization order fiasco".
Additionally, function-local statics emit a guard variable around
non-trivially destructible types. This results in horrible codegen and
adds a runtime cost to every call to that function. non-local static's
also result in slightly worse codegen but it's not as big of a problem.
Example codegen can be found here: https://goo.gl/3CSzbM
Note: This optimization is not safe with every pthread implementation.
Some implementations allocate on the first call to pthread_mutex_lock
and free the allocation in pthread_mutex_destroy.
Also, changing the triviality of the destructor is not an ABI break.
At least to the best of my knowledge :-)
llvm-svn: 365273
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index 2a8ff2c..56d7b98 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -22,6 +22,7 @@
locale.cpp
memory.cpp
mutex.cpp
+ mutex_destructor.cpp
new.cpp
optional.cpp
random.cpp
diff --git a/libcxx/src/mutex.cpp b/libcxx/src/mutex.cpp
index 33a8197..0d69d7c 100644
--- a/libcxx/src/mutex.cpp
+++ b/libcxx/src/mutex.cpp
@@ -25,10 +25,7 @@
const try_to_lock_t try_to_lock = {};
const adopt_lock_t adopt_lock = {};
-mutex::~mutex() _NOEXCEPT
-{
- __libcpp_mutex_destroy(&__m_);
-}
+// ~mutex is defined elsewhere
void
mutex::lock()
diff --git a/libcxx/src/mutex_destructor.cpp b/libcxx/src/mutex_destructor.cpp
new file mode 100644
index 0000000..3a6b6b5
--- /dev/null
+++ b/libcxx/src/mutex_destructor.cpp
@@ -0,0 +1,51 @@
+//===--------------------- mutex_destructor.cpp ---------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Define ~mutex.
+//
+// On some platforms ~mutex has been made trivial and the definition is only
+// provided for ABI compatibility.
+//
+// In order to avoid ODR violations within libc++ itself, we need to ensure
+// that *nothing* sees the non-trivial mutex declaration. For this reason
+// we re-declare the entire class in this file instead of using
+// _LIBCPP_BUILDING_LIBRARY to change the definition in the headers.
+
+#include "__config"
+#include "__threading_support"
+
+#if !defined(_LIBCPP_HAS_NO_THREADS)
+#if _LIBCPP_ABI_VERSION == 1 || !defined(_LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION)
+#define NEEDS_MUTEX_DESTRUCTOR
+#endif
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#ifdef NEEDS_MUTEX_DESTRUCTOR
+class _LIBCPP_TYPE_VIS mutex
+{
+ __libcpp_mutex_t __m_ = _LIBCPP_MUTEX_INITIALIZER;
+
+public:
+ _LIBCPP_ALWAYS_INLINE _LIBCPP_INLINE_VISIBILITY
+ constexpr mutex() = default;
+ mutex(const mutex&) = delete;
+ mutex& operator=(const mutex&) = delete;
+ ~mutex() noexcept;
+};
+
+
+mutex::~mutex() _NOEXCEPT
+{
+ __libcpp_mutex_destroy(&__m_);
+}
+
+#endif // !_LIBCPP_HAS_NO_THREADS
+_LIBCPP_END_NAMESPACE_STD
+