OpenE2K/gcc
2
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

libstdc++: Do not use volatile for __gnu_cxx::rope reference counting

Browse Source 
The rope extension uses a volatile variable for its reference count.
This is not only unnecessary for correctness (volatile provides neither
atomicity nor memory visibility, and the variable is only modified while
a lock is held) but it now causes deprecated warnings with
-Wsystem-headers due to the use of ++ and -- operators.

It would be possible to use __gnu_cxx::__exchange_and_add in _M_incr and
_M_decr when __atomic_is_lock_free(sizeof(_RC_t), &_M_ref_count) is
true, rather than locking a mutex. That would probably be a significant
improvement for multi-threaded and single-threaded code (because
__exchange_and_add will use non-atomic ops when possible, and even in MT
code it should be faster than the mutex lock/unlock pair). However,
mixing objects compiled with the old and new code would result in
inconsistent synchronization being used for the reference count.

libstdc++-v3/ChangeLog:

	* include/ext/rope (_Refcount_Base::_M_ref_count): Remove
	volatile qualifier.
	(_Refcount_Base::_M_decr()): Likewise.
This commit is contained in:
Jonathan Wakely 2020-10-29 14:47:17 +00:00
parent 3c9b99ef71
commit d067bd7293
1 changed files with 2 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
libstdc++-v3/include/ext/rope
View File

@ -452,7 +452,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typedef std::size_t _RC_t;
// The data member _M_ref_count
volatile _RC_t _M_ref_count;
_RC_t _M_ref_count;
// Constructor
#ifdef __GTHREAD_MUTEX_INIT
@ -489,7 +489,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_decr()
{
__gthread_mutex_lock(&_M_ref_count_lock);
volatile _RC_t __tmp = --_M_ref_count;
_RC_t __tmp = --_M_ref_count;
__gthread_mutex_unlock(&_M_ref_count_lock);
return __tmp;
}