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.
Tested x86_64-linux. Committed to trunk.
commit d067bd72936aaaa7e947e919fc869143539ae023
Author: Jonathan Wakely <[email protected]>
Date: Thu Oct 29 14:47:17 2020
libstdc++: Do not use volatile for __gnu_cxx::rope reference counting
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.
diff --git a/libstdc++-v3/include/ext/rope b/libstdc++-v3/include/ext/rope
index fb7bdb0d6f4..08e510bb0dc 100644
--- a/libstdc++-v3/include/ext/rope
+++ b/libstdc++-v3/include/ext/rope
@@ -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;
}
