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From: "krzysio.kurek at wp dot pl" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/94960] New: extern template prevents inlining of
 standard library objects
Date: Tue, 05 May 2020 20:09:58 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 9.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94960

            Bug ID: 94960
           Summary: extern template prevents inlining of standard library
                    objects
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krzysio.kurek at wp dot pl
  Target Milestone: ---

Consider this example
void foo()
{
  std::string(1, 0);
}
(https://godbolt.org/z/AlkBBJ)
This function creates a string using the `basic_string(size_t, CharT)`
constructor and then discards it. This particular constructor uses _M_const=
ruct
internally, which is declared as an out of line member function. Because of
this, and because the function isn't marked as `inline`, when the compiler
reaches the `extern template class basic_string<char>;`, it foregoes trying=
 to
find the definition for _M_construct, instead generating a call to it, caus=
ing
foo() to fully instantiate a string object and then delete it, since the
compiler can't find _M_construct within its own translation unit.

This problem applies to every member function of any class which has an ext=
ern
template, is defined out of line and is not marked as `inline`.=