https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113704
Bug ID: 113704
Summary: std::locale::locale<Facet>(const locale&, Facet*) is
inefficient
Product: gcc
Version: 11.4.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
template<typename _Facet>
locale::
locale(const locale& __other, _Facet* __f)
{
_M_impl = new _Impl(*__other._M_impl, 1);
__try
{ _M_impl->_M_install_facet(&_Facet::id, __f); }
__catch(...)
{
_M_impl->_M_remove_reference();
__throw_exception_again;
}
delete [] _M_impl->_M_names[0];
_M_impl->_M_names[0] = 0; // Unnamed.
}
This is very wasteful if the facet isn't already present in __other. First the
_Impl constructor allocates new copies of all the arrays in *__other._M_impl,
then _M_install_facets reallocates them with additional space for the new
facet.
We should check whether _Facet::id can be stored without reallocation. If it
can't, we should avoid allocating and copying all the arrays twice.
This matters for custom user facets which will always have a higher ID and
require reallocation.
