[libcxx] Add atomic_support.h header to src that handles needed atomic operations.
Summary:
In some places in libc++ we need to use the `__atomic_*` builtins. This patch adds a header that provides access to those builtins in a uniform way from within the dylib source.
If the compiler building the dylib does not support these builtins then a warning is issued.
Only relaxed loads are needed within the headers. A singe function to do these relaxed loads has been added to `<memory>`.
This patch applies the new atomic builtins to `__shared_count` and `call_once`.
Reviewers: mclow.lists
Subscribers: majnemer, jroelofs, cfe-commits
Differential Revision: http://reviews.llvm.org/D10406
llvm-svn: 241532
diff --git a/libcxx/src/mutex.cpp b/libcxx/src/mutex.cpp
index e56271d..5f8ba0a 100644
--- a/libcxx/src/mutex.cpp
+++ b/libcxx/src/mutex.cpp
@@ -12,6 +12,7 @@
#include "limits"
#include "system_error"
#include "cassert"
+#include "support/atomic_support.h"
_LIBCPP_BEGIN_NAMESPACE_STD
#ifndef _LIBCPP_HAS_NO_THREADS
@@ -220,6 +221,9 @@
static pthread_cond_t cv = PTHREAD_COND_INITIALIZER;
#endif
+/// NOTE: Changes to flag are done via relaxed atomic stores
+/// even though the accesses are protected by a mutex because threads
+/// just entering 'call_once` concurrently read from flag.
void
__call_once(volatile unsigned long& flag, void* arg, void(*func)(void*))
{
@@ -252,11 +256,11 @@
try
{
#endif // _LIBCPP_NO_EXCEPTIONS
- flag = 1;
+ __libcpp_relaxed_store(&flag, 1ul);
pthread_mutex_unlock(&mut);
func(arg);
pthread_mutex_lock(&mut);
- flag = ~0ul;
+ __libcpp_relaxed_store(&flag, ~0ul);
pthread_mutex_unlock(&mut);
pthread_cond_broadcast(&cv);
#ifndef _LIBCPP_NO_EXCEPTIONS
@@ -264,7 +268,7 @@
catch (...)
{
pthread_mutex_lock(&mut);
- flag = 0ul;
+ __libcpp_relaxed_store(&flag, 0ul);
pthread_mutex_unlock(&mut);
pthread_cond_broadcast(&cv);
throw;
