From d067bd72936aaaa7e947e919fc869143539ae023 Mon Sep 17 00:00:00 2001 From: Jonathan Wakely Date: Thu, 29 Oct 2020 14:47:17 +0000 Subject: [PATCH] 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. --- libstdc++-v3/include/ext/rope | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/libstdc++-v3/include/ext/rope b/libstdc++-v3/include/ext/rope index fb7bdb0..08e510b 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; } -- 2.7.4