[PATCH] PR libstdc++/59439 optimize uses of classic ("C") std::locale

[Jonathan Wakely]

On 26/09/18 17:17 +0100, Jonathan Wakely wrote:
>The global locale::_Impl that represents the "C" locale is never
>destroyed, so there is no need to keep track of reference count updates
>for that object. This greatly reduce contention between threads that
>refer to the classic locale. Since the global std::locale initially uses
>the classic locale, this benefits the common case for any code using the
>global locale, such as construction/destruction of iostream objects.
>
>All these updates are done inside libstdc++.so so there's no need to
>worry about users' objects having inlined old versions of the code which
>still update the reference count for the classic locale.
>
>	PR libstdc++/59439
>       * src/c++98/locale.cc (locale::locale(const locale&)): Bypass
>       reference count updates for the classic locale.
>       (locale::~locale()): Likewise.  (locale::operator=(const
>       locale&)): Likewise.
>       * src/c++98/locale_init.cc (locale::locale()): Likewise.
>       (locale::global(const locale&)): Likewise.
>
>Tested x86_64-linux, not committed yet.
>
>Does anybody see any problems with this change?

Committed to trunk.



>commit 131d4c26876a5a884fe4408deaf054e01ba90ffb
>Author: Jonathan Wakely <jwakely@redhat.com>
>Date:   Wed Sep 26 16:34:42 2018 +0100
>
>    PR libstdc++/59439 optimize uses of classic ("C") std::locale
>
>    The global locale::_Impl that represents the "C" locale is never
>    destroyed, so there is no need to keep track of reference count updates
>    for that object. This greatly reduce contention between threads that
>    refer to the classic locale. Since the global std::locale initially uses
>    the classic locale, this benefits the common case for any code using the
>    global locale, such as construction/destruction of iostream objects.
>
>    All these updates are done inside libstdc++.so so there's no need to
>    worry about users' objects having inlined old versions of the code which
>    still update the reference count for the classic locale.
>
>            PR libstdc++/59439
>            * src/c++98/locale.cc (locale::locale(const locale&)): Bypass
>            reference count updates for the classic locale.
>            (locale::~locale()): Likewise.  (locale::operator=(const
>            locale&)): Likewise.
>            * src/c++98/locale_init.cc (locale::locale()): Likewise.
>            (locale::global(const locale&)): Likewise.
>
>diff --git a/libstdc++-v3/src/c++98/locale.cc b/libstdc++-v3/src/c++98/locale.cc
>index 148bf59658e..fe06d297039 100644
>--- a/libstdc++-v3/src/c++98/locale.cc
>+++ b/libstdc++-v3/src/c++98/locale.cc
>@@ -77,7 +77,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>   locale::locale(const locale& __other) throw()
>   : _M_impl(__other._M_impl)
>-  { _M_impl->_M_add_reference(); }
>+  {
>+    if (_M_impl != _S_classic)
>+      _M_impl->_M_add_reference();
>+  }
>
>   // This is used to initialize global and classic locales, and
>   // assumes that the _Impl objects are constructed correctly.
>@@ -86,7 +89,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>   { }
>
>   locale::~locale() throw()
>-  { _M_impl->_M_remove_reference(); }
>+  {
>+    if (_M_impl != _S_classic)
>+      _M_impl->_M_remove_reference();
>+  }
>
>   bool
>   locale::operator==(const locale& __rhs) const throw()
>@@ -112,8 +118,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>   const locale&
>   locale::operator=(const locale& __other) throw()
>   {
>-    __other._M_impl->_M_add_reference();
>-    _M_impl->_M_remove_reference();
>+    if (__other._M_impl != _S_classic)
>+      __other._M_impl->_M_add_reference();
>+    if (_M_impl != _S_classic)
>+      _M_impl->_M_remove_reference();
>     _M_impl = __other._M_impl;
>     return *this;
>   }
>diff --git a/libstdc++-v3/src/c++98/locale_init.cc b/libstdc++-v3/src/c++98/locale_init.cc
>index c9078c015c3..b580a9f9d58 100644
>--- a/libstdc++-v3/src/c++98/locale_init.cc
>+++ b/libstdc++-v3/src/c++98/locale_init.cc
>@@ -257,9 +257,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     //   fall back to lock protected access to both _S_global and
>     //   its reference count.
>     _M_impl = _S_global;
>-    if (_M_impl == _S_classic)
>-      _M_impl->_M_add_reference();
>-    else
>+    if (_M_impl != _S_classic)
>       {
>         __gnu_cxx::__scoped_lock sentry(get_locale_mutex());
>         _S_global->_M_add_reference();
>@@ -275,7 +273,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     {
>       __gnu_cxx::__scoped_lock sentry(get_locale_mutex());
>       __old = _S_global;
>-      __other._M_impl->_M_add_reference();
>+      if (__other._M_impl != _S_classic)
>+	__other._M_impl->_M_add_reference();
>       _S_global = __other._M_impl;
>       const string __other_name = __other.name();
>       if (__other_name != "*")
>@@ -284,7 +283,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>     // Reference count sanity check: one reference removed for the
>     // subsition of __other locale, one added by return-by-value. Net
>-    // difference: zero. When the returned locale object's destrutor
>+    // difference: zero. When the returned locale object's destructor
>     // is called, then the reference count is decremented and possibly
>     // destroyed.
>     return locale(__old);