On 22/06/18 00:28 +0100, Jonathan Wakely wrote:
The explicit instantiation declarations for std::basic_string are
disabled for C++17 (and later) so that basic_string symbols get
implicitly instantiated in every translation unit that needs them. On
targets that don't support STB_GNU_UNIQUE this leads to multiple copies
of the empty rep symbol for COW strings. In order to detect whether a
COW string needs to deallocate its storage it compares the address with
the empty rep. When there are multiple copies of the empty rep object
the address is not unique, and so string destructors try to delete the
empty rep, which crashes.
In order to guarantee uniqueness of the _S_empty_rep_storage symbol this
patch adds an explicit instantiation declaration for just that symbol.
This means the other symbols are still implicitly instantiated in C++17
code, but for the empty rep the definition in the library gets used.
Separately, there is no need for C++17 code to implicitly instantiate
the I/O functions for strings, so this also restores the explicit
instantiation declarations for those functions.
PR libstdc++/86138
* include/bits/basic_string.tcc:
[__cplusplus > 201402 && !_GLIBCXX_USE_CXX11_ABI]
(basic_string<char>::_Rep::_S_empty_rep_storage)
(basic_string<wchar_t>::_Rep::_S_empty_rep_storage): Add explicit
instantiation declarations.
[__cplusplus > 201402] (operator>>, operator<<, getline): Re-enable
explicit instantiation declarations.
* testsuite/21_strings/basic_string/cons/char/86138.cc: New.
* testsuite/21_strings/basic_string/cons/wchar_t/86138.cc: New.
Tested x86_64-linux, committed to trunk.
If this passes testing on Cygwin I'll also backport it to gcc-7 and
gcc-8, as the explicit instantiation declarations are disabled for
C++17 on those branches.
The new tests are failing with _GLIBCXX_ASSERTIONS or _GLIBCXX_DEBUG
defined, because enabling assertions suppresses the explicit
instantiation declarations. However the COW empty reps and the I/O
functions don't contain any assertions, so don't need to be implicitly
instantiated.
Tested x86_64-linux, committed to trunk.
commit 22bd831a1bc38ba3e114c6166e96817fe03f006b
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Tue Jun 26 22:14:29 2018 +0100
Declare some explicit instantiations for strings in Debug Mode
The empty reps and the I/O functions do not need to be implicitly
instantiated to enable assertions, so declare the explicit
instantiations when _GLIBCXX_EXTERN_TEMPLATE == -1 (i.e. when
_GLIBCXX_ASSERTIONS is defined).
PR libstdc++/86138
* include/bits/basic_string.tcc: [_GLIBCXX_EXTERN_TEMPLATE < 0]
Declare explicit instantiations of COW empty reps and I/O functions.
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 9fbea84c4af..04b68ca0202 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -1597,13 +1597,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// Inhibit implicit instantiations for required instantiations,
// which are defined via explicit instantiations elsewhere.
-#if _GLIBCXX_EXTERN_TEMPLATE > 0
+#if _GLIBCXX_EXTERN_TEMPLATE
// The explicit instantiations definitions in src/c++11/string-inst.cc
// are compiled as C++14, so the new C++17 members aren't instantiated.
// Until those definitions are compiled as C++17 suppress the declaration,
// so C++17 code will implicitly instantiate std::string and std::wstring
// as needed.
-# if __cplusplus <= 201402L
+# if __cplusplus <= 201402L && _GLIBCXX_EXTERN_TEMPLATE > 0
extern template class basic_string<char>;
# elif ! _GLIBCXX_USE_CXX11_ABI
// Still need to prevent implicit instantiation of the COW empty rep,
@@ -1626,7 +1626,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
getline(basic_istream<char>&, string&);
#ifdef _GLIBCXX_USE_WCHAR_T
-# if __cplusplus <= 201402L
+# if __cplusplus <= 201402L && _GLIBCXX_EXTERN_TEMPLATE > 0
extern template class basic_string<wchar_t>;
# elif ! _GLIBCXX_USE_CXX11_ABI
extern template basic_string<wchar_t>::size_type
@@ -1646,7 +1646,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
basic_istream<wchar_t>&
getline(basic_istream<wchar_t>&, wstring&);
#endif // _GLIBCXX_USE_WCHAR_T
-#endif // _GLIBCXX_EXTERN_TEMPLATE > 0
+#endif // _GLIBCXX_EXTERN_TEMPLATE
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
