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[libcxx] Add atomic_support.h header to src that handles needed atomic operations.

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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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@241532 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eric Fiselier
2015-07-07 00:27:16 +00:00
parent faaf5ee349
commit c6e466911f
6 changed files with 271 additions and 13 deletions
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10
src/mutex.cpp
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@@ -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_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
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 @@ __call_once(volatile unsigned long& flag, void* arg, void(*func)(void*))
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 @@ __call_once(volatile unsigned long& flag, void* arg, void(*func)(void*))
catch (...)
{
pthread_mutex_lock(&mut);
flag = 0ul;
__libcpp_relaxed_store(&flag, 0ul);
pthread_mutex_unlock(&mut);
pthread_cond_broadcast(&cv);
throw;