[libcxx] Don't use pthread initializers in constexpr constructors

[elram]

Summary

Some pthread implementations use volatile types in their structs.
C++11 does not allow initializing volatile types in constexpr constructors.

This fixes building libcxx on musl-libc and NetBSD.

[bcraig]

@joerg. I think this has been fixed in newer versions of NetBSD.

Sure would be nice if someone could fix this on the MUSL side of things.

See https://bugs.musicpd.org/view.php?id=4110 for fun conversations about mutex, PTHREAD_MUTEX_INIT and constexpr.

Also, the first revision of this review:
http://reviews.llvm.org/D19344

[EricWF]

C++03 does not support default member initializers but libc++ provides both of these classes an extensions in C++03.
C++03 also does not support defaulted special members.

I think the current patch looks good for C++11, but a fallback implementation is needed for C++03.

[EricWF]

LGTM.

[rmaprath]

Small side question: so how are __m_ and __cv_ initialized in this setup? is it that pthread_mutex_lock() and the like would be able to handle an un-initialized blob of memory (in a thread-safe way)? Because I don't see a call to pthread_mutex_init() anywhere in the code-path.

[EricWF]

In D21637#471151, @rmaprath wrote:

Small side question: so how are __m_ and __cv_ initialized in this setup? is it that pthread_mutex_lock() and the like would be able to handle an un-initialized blob of memory (in a thread-safe way)? Because I don't see a call to pthread_mutex_init() anywhere in the code-path.

__m_ and __cv_ are initialized during non-static data member initialization. They didn't previously call pthread_mutex_lock(), instead they directly initialized 'm_' and 'cv_' in the constructors initializer list.

And according to the current spec we are allowed to use PTHREAD_MUTEX_INITIALIZER to initialize non-static mutexes.
http://pubs.opengroup.org/onlinepubs/9699919799/

In cases where default mutex attributes are appropriate, the macro PTHREAD_MUTEX_INITIALIZER can be used to initialize mutexes.
The effect shall be equivalent to dynamic initialization by a call to pthread_mutex_init() with parameter attr specified as NULL, except that no error checks are performed.

[rmaprath]

In D21637#471154, @EricWF wrote:

In D21637#471151, @rmaprath wrote:

Small side question: so how are __m_ and __cv_ initialized in this setup? is it that pthread_mutex_lock() and the like would be able to handle an un-initialized blob of memory (in a thread-safe way)? Because I don't see a call to pthread_mutex_init() anywhere in the code-path.

__m_ and __cv_ are initialized during non-static data member initialization. They didn't previously call pthread_mutex_lock(), instead they directly initialized 'm_' and 'cv_' in the constructors initializer list.

And according to the current spec we are allowed to use PTHREAD_MUTEX_INITIALIZER to initialize non-static mutexes.
http://pubs.opengroup.org/onlinepubs/9699919799/

In cases where default mutex attributes are appropriate, the macro PTHREAD_MUTEX_INITIALIZER can be used to initialize mutexes.
The effect shall be equivalent to dynamic initialization by a call to pthread_mutex_init() with parameter attr specified as NULL, except that no error checks are performed.

Ah! I've misread the patch - missed the assignment in the constructor (for the _LIBCPP_HAS_NO_CONSTEXPR case). Sorted now.

Thanks.

/ Asiri

[elram]

I don't have commit access.
Can someone do it for me please?

[mclow.lists]

Committed as revision 275819.

include/__mutex_base

 
 class _LIBCPP_TYPE_VIS _LIBCPP_THREAD_SAFETY_ANNOTATION(capability("mutex")) mutex
 {
+#ifndef _LIBCPP_HAS_NO_CONSTEXPR
+    __libcpp_mutex_t __m_ = _LIBCPP_MUTEX_INITIALIZER;
+#else
     __libcpp_mutex_t __m_;
+#endif
 
 public:
     _LIBCPP_INLINE_VISIBILITY
 #ifndef _LIBCPP_HAS_NO_CONSTEXPR
-    constexpr mutex() _NOEXCEPT : __m_(_LIBCPP_MUTEX_INITIALIZER) {}
+    constexpr mutex() _NOEXCEPT _LIBCPP_DEFAULT
 #else
     mutex() _NOEXCEPT {__m_ = (__libcpp_mutex_t)_LIBCPP_MUTEX_INITIALIZER;}
 #endif
-     ~mutex();
 
 private:
     mutex(const mutex&);// = delete;

 
 class _LIBCPP_TYPE_VIS condition_variable
 {
-    __libcpp_condvar_t  __cv_;
+#ifndef _LIBCPP_HAS_NO_CONSTEXPR
+    __libcpp_condvar_t __cv_ = _LIBCPP_CONDVAR_INITIALIZER;
+#else
+    __libcpp_condvar_t __cv_;
+#endif
+
 public:
     _LIBCPP_INLINE_VISIBILITY
 #ifndef _LIBCPP_HAS_NO_CONSTEXPR
-    constexpr condition_variable() : __cv_(_LIBCPP_CONDVAR_INITIALIZER) {}
+    constexpr condition_variable() _NOEXCEPT _LIBCPP_DEFAULT
 #else
-    condition_variable() {__cv_ = (__libcpp_condvar_t)_LIBCPP_CONDVAR_INITIALIZER;}
+    condition_variable() _NOEXCEPT {__cv_ = (__libcpp_condvar_t)_LIBCPP_CONDVAR_INITIALIZER;}
 #endif
     ~condition_variable();