Use __attribute__((internal_linkage)) when available.

[EricWF]

Summary

This patch has been a long time coming (Thanks @eugenis). It changes _LIBCPP_INLINE_VISIBILITY to use __attribute__((internal_linkage)) instead of __attribute__((visibility("hidden"), always_inline)).

The point of _LIBCPP_INLINE_VISIBILITY is to prevent inline functions from being exported from both the libc++ library and from user libraries. This helps libc++ better manage it's ABI.
Previously this was done by forcing inlining and modifying the symbols visibility. However inlining isn't guaranteed and symbol visibility only affects shared libraries making this an imperfect solution. internal_linkage improves this situation by making all symbols local to the TU they are emitted in, regardless of inlining or visibility. IIRC the effect of applying __attribute__((internal_linkage)) to an inline function is the same as applying static.

For more information about the attribute see: http://lists.llvm.org/pipermail/cfe-dev/2015-October/045580.html

Most of the work for this patch was done by @eugenis.

[EricWF]

Revert accidental change.

[eugenis]

Looks great.
Thank you for seeing it through!

[EricWF]

• Add always_inline to _LIBCPP_INLINE_VISIBILITY. In future I think only _LIBCPP_ALWAYS_INLINE should actually apply always_inline, but I'll make that change separately.
• Change _LIBCPP_ALWAYS_INLINE to use internal_linkage as well.

[EricWF]

I'll commit this change tomorrow barring any objections.

include/__config

 
 #define _LIBCPP_VERSION 4000
 
 #ifndef _LIBCPP_ABI_VERSION
 #define _LIBCPP_ABI_VERSION 1
 #endif
 
 #if defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
+#define _LIBCPP_ABI_EXTERN_TEMPLATE_SYMBOLS_VERSION 2
 // Change short string representation so that string data starts at offset 0,
 // improving its alignment in some cases.
 #define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 // Fix deque iterator type in order to support incomplete types.
 #define _LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
 // Fix undefined behavior in how std::list stores it's linked nodes.
 #define _LIBCPP_ABI_LIST_REMOVE_NODE_POINTER_UB
 // Fix undefined behavior in  how __tree stores its end and parent nodes.
 #define _LIBCPP_ABI_TREE_REMOVE_NODE_POINTER_UB
 // Fix undefined behavior in how __hash_table stores it's pointer types
 #define _LIBCPP_ABI_FIX_UNORDERED_NODE_POINTER_UB
 #define _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB
 #define _LIBCPP_ABI_FIX_UNORDERED_CONTAINER_SIZE_TYPE
 #define _LIBCPP_ABI_VARIADIC_LOCK_GUARD
 #elif _LIBCPP_ABI_VERSION == 1
+#define _LIBCPP_ABI_EXTERN_TEMPLATE_SYMBOLS_VERSION 1
 // Feature macros for disabling pre ABI v1 features. All of these options
 // are deprecated.
 #if defined(__FreeBSD__)
 #define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #endif
 #endif
 
 #ifdef _LIBCPP_TRIVIAL_PAIR_COPY_CTOR

 #  if __has_attribute(__type_visibility__)
 #    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __attribute__ ((__visibility__("default")))
 #  else
 #    define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 #  endif
 #endif
 
 #ifndef _LIBCPP_INLINE_VISIBILITY
-#define _LIBCPP_INLINE_VISIBILITY __attribute__ ((__visibility__("hidden"), __always_inline__))
-#endif
+# if __has_attribute(__internal_linkage__)
+#   define _LIBCPP_INLINE_VISIBILITY __attribute__((__internal_linkage__, __always_inline__))
+# else
+#   define _LIBCPP_INLINE_VISIBILITY __attribute__ ((__visibility__("hidden"), __always_inline__))
+# endif
+#endif
 
 #ifndef _LIBCPP_ALWAYS_INLINE
-#define _LIBCPP_ALWAYS_INLINE  __attribute__ ((__visibility__("hidden"), __always_inline__))
-#endif
+# if __has_attribute(__internal_linkage__)
+#   define _LIBCPP_ALWAYS_INLINE __attribute__((__internal_linkage__, __always_inline__))
+# else
+#   define _LIBCPP_ALWAYS_INLINE  __attribute__ ((__visibility__("hidden"), __always_inline__))
+# endif
+#endif
 
 #ifndef _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY
 # ifdef _LIBCPP_BUILDING_LIBRARY
 #   define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY __attribute__((__visibility__("default"), __always_inline__))
 # else
 #   define _LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY _LIBCPP_INLINE_VISIBILITY
 # endif
 #endif

src/string.cpp

 
 template class basic_string<char>;
 template class basic_string<wchar_t>;
 
 template
     string
     operator+<char, char_traits<char>, allocator<char> >(char const*, string const&);
 
+// These external instantiations are required to maintain dylib compatibility
+// for ABI v1 when using __attribute__((internal_linkage)) as opposed to
+// __attribute__((visibility("hidden"), always_inline)).
+#if _LIBCPP_ABI_EXTERN_TEMPLATE_SYMBOLS_VERSION == 1
+template basic_string<char>::iterator
+basic_string<char>::insert(basic_string<char>::const_iterator,
+                           char const *, char const *);
+
+template basic_string<wchar_t>::iterator
+basic_string<wchar_t>::insert(basic_string<wchar_t>::const_iterator,
+                              wchar_t const *, wchar_t const *);
+
+template basic_string<char> &
+basic_string<char>::replace(basic_string<char>::const_iterator,
+                               basic_string<char>::const_iterator,
+                               char const *, char const *);
+
+template basic_string<wchar_t> &
+basic_string<wchar_t>::replace(basic_string<wchar_t>::const_iterator,
+                               basic_string<wchar_t>::const_iterator,
+                               wchar_t const *, wchar_t const *);
+#endif // _LIBCPP_ABI_EXTERN_TEMPLATE_SYMBOLS_VERSION
+
 namespace
 {
 
 template<typename T>
 inline
 void throw_helper( const string& msg )
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS