From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id AE9623858027; Wed, 1 Dec 2021 15:07:55 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org AE9623858027 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-5691] libstdc++: Optimize ref-count updates in COW std::string X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/master X-Git-Oldrev: e9089e4fa9f726405a83d8ce46baeda469ca1a1d X-Git-Newrev: 2b83bc6097fd138b2dc81f3c8f1786c9079dcd67 Message-Id: <20211201150755.AE9623858027@sourceware.org> Date: Wed, 1 Dec 2021 15:07:55 +0000 (GMT) X-BeenThere: libstdc++-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 01 Dec 2021 15:07:55 -0000 https://gcc.gnu.org/g:2b83bc6097fd138b2dc81f3c8f1786c9079dcd67 commit r12-5691-g2b83bc6097fd138b2dc81f3c8f1786c9079dcd67 Author: Jonathan Wakely Date: Tue Nov 30 13:21:55 2021 +0000 libstdc++: Optimize ref-count updates in COW std::string Most ref-count updates in the COW string are done via the functions in , which will use non-atomic ops when the program is known to be single-threaded. The _M_is_leaked() and _M_is_shared() functions use __atomic_load_n directly, because doesn't provide a load operation. Those functions can check the __is_single_threaded() predicate to avoid using __atomic_load_n when not needed. The move constructor for the fully-dynamic-string increments the ref-count by either 2 or 1, for leaked or non-leaked strings respectively. That can be changed to use a non-atomic store of 1 for all non-shared strings. It can be non-atomic because even if the program is multi-threaded, conflicting access to the rvalue object while it's being moved from would be data race anyway. It can store 1 directly for all non-shared strings because it doesn't matter whether the initial refcount was -1 or 0, it should be 1 after the move constructor creates a second owner. libstdc++-v3/ChangeLog: * include/bits/cow_string.h (basic_string::_M_is_leaked): Use non-atomic load when __is_single_threaded() is true. (basic_string::_M_is_shared): Likewise. (basic_string::(basic_string&&)) [_GLIBCXX_FULLY_DYNAMIC_STRING]: Use non-atomic store when rvalue is not shared. Diff: --- libstdc++-v3/include/bits/cow_string.h | 24 ++++++++++++------------ 1 file changed, 12 insertions(+), 12 deletions(-) diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h index ced395b80b8..4fae1d02981 100644 --- a/libstdc++-v3/include/bits/cow_string.h +++ b/libstdc++-v3/include/bits/cow_string.h @@ -105,7 +105,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION * destroy the empty-string _Rep object. * * All but the last paragraph is considered pretty conventional - * for a C++ string implementation. + * for a Copy-On-Write C++ string implementation. */ // 21.3 Template class basic_string template @@ -207,10 +207,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION // so we need to use an atomic load. However, _M_is_leaked // predicate does not change concurrently (i.e. the string is either // leaked or not), so a relaxed load is enough. - return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0; -#else - return this->_M_refcount < 0; + if (!__gnu_cxx::__is_single_threaded()) + return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0; #endif + return this->_M_refcount < 0; } bool @@ -222,10 +222,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION // but one reference concurrently with this check, so we need this // load to be acquire to synchronize with release fetch_and_add in // _M_dispose. - return __atomic_load_n(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0; -#else - return this->_M_refcount > 0; + if (!__gnu_cxx::__is_single_threaded()) + return __atomic_load_n(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0; #endif + return this->_M_refcount > 0; } void @@ -629,12 +629,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION #else // Rather than allocate an empty string for the rvalue string, // just share ownership with it by incrementing the reference count. - // If the rvalue string was "leaked" then it was the unique owner, - // so need an extra increment to indicate shared ownership. - if (_M_rep()->_M_is_leaked()) - __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 2); - else + // If the rvalue string was the unique owner then there are exactly + // two owners now. + if (_M_rep()->_M_is_shared()) __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 1); + else + _M_rep()->_M_refcount = 1; #endif }