[libc++] Optimize std::string's external instantiations in an ABI compatible way

[ldionne]

Summary

This patch introduces the mechanism through which libc++ can add, remove, or modify externally instantiated methods of std::string.
Under an unstable ABI, adding and removing external definitions is trivial. Under the stable ABI we are much more limited.

If we want to allow inlining of a external method in ABI v1 we can remove the extern template declaration while
retaining the extern template definition New code will no longer generate out-of-line calls to that method, but
old code will continue to work since the definition is still present.

At the same time, we can define but not declare new out-of-line methods in the stable ABI. Staging these definitions
for use in the future, after all supported dylibs contain them.

This allows code to perform and inline std::string roughly the same across both ABI's. Meaning std::string
should retain the same performance characteristics regardless of ABI.

ABI breaking changes are guarded by _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS, but n

[mvels]

libcxx/include/__string

79

We should probably not outline this ctor before we do the 'init_long_external'

libcxx/src/string.cpp

21

nit : empty line added

27

Should we add a comment that these entries should never be removed, ever? This for reviewers less familiar with ABI subtleties.

[ldionne]

I think this can be abandoned since the work has been done in another patch. Is that correct? If so, please close to clean up the queue.

[ldionne]

Commandeering to abandon since the work has been done differently.

libcxx/include/__config

 #  define _LIBCPP_ABI_OPTIMIZED_LOCALE_NUM_GET
 // Use the smallest possible integer type to represent the index of the variant.
 // Previously libc++ used "unsigned int" exclusively.
 #  define _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
 // Unstable attempt to provide a more optimized std::function
 #  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 // All the regex constants must be distinct and nonzero.
 #  define _LIBCPP_ABI_REGEX_CONSTANTS_NONZERO
+// Re-worked external template instantiations for std::string with a focus on
+// performance and fast-path inlining.
+#  define _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support
 // applications compiled against older libraries. This is unnecessary with
 // COFF dllexport semantics, since dllexport forces a non-inline definition
 // of inline functions to be emitted anyway. Our own non-inline copy would
 // conflict with the dllexport-emitted copy, so we disable it.
 #    define _LIBCPP_DEPRECATED_ABI_LEGACY_LIBRARY_DEFINITIONS_FOR_INLINE_FUNCTIONS

libcxx/include/__string

 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+
+#ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS
+#define _LIBCPP_STRING_EXTERN_TEMPLATE_LIST_ADDITIONAL(_Func, _CharType) \
+  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init_long_external(value_type const*, size_type))
+#else
+#define _LIBCPP_STRING_EXTERN_TEMPLATE_LIST_ADDITIONAL(_Func, _CharType)
+#endif
+
 // The the extern template ABI lists are kept outside of <string> to improve the
 // readability of that header.
 #define _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(value_type const*, size_type, size_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&, std::allocator<_CharType> const&)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_last_not_of(value_type const*, size_type, size_type) const) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::~basic_string()) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_not_of(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, size_type, value_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(value_type)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS const _CharType& basic_string<_CharType>::at(size_type) const) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_of(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, size_type, value_type)) \
#define _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(basic_string const&, size_type, size_type)) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(value_type const*) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*) const) \
   _Func(_LIBCPP_FUNC_VIS _CharType& basic_string<_CharType>::at(size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, basic_string const&, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*, size_type) const) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(basic_string const&)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, basic_string const&, size_type, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::iterator basic_string<_CharType>::insert(basic_string::const_iterator, value_type)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::resize(size_type, value_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type))
-
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type)) \
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LIST_ADDITIONAL(_Func, _CharType)
 
 // char_traits
 
 template <class _CharT>
 struct _LIBCPP_TEMPLATE_VIS char_traits
 {
     typedef _CharT    char_type;
     typedef int       int_type;

libcxx/include/string

template<class _InputIterator, class = _EnableIf<__is_cpp17_input_iterator<_InputIterator>::value> >
         basic_string(_InputIterator __first, _InputIterator __last, const allocator_type& __a);
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     basic_string(initializer_list<_CharT> __il);
     _LIBCPP_INLINE_VISIBILITY
     basic_string(initializer_list<_CharT> __il, const _Allocator& __a);
 #endif  // _LIBCPP_CXX03_LANG
 
-    inline ~basic_string();
+
+    ~basic_string();
 
     _LIBCPP_INLINE_VISIBILITY
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
 
     basic_string& operator=(const basic_string& __str);
 
     template <class _Tp, class = _EnableIf<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
     basic_string& operator=(const _Tp& __t)

libcxx/src/string.cpp

 #include "cerrno"
 #include "limits"
 #include "stdexcept"
 #include <stdio.h>
 #include "__debug"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS __basic_string_common<true>;
 
 _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
 _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
 
+#define _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_Func, _CharType) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&)) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::~basic_string()) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(basic_string const&))
+
+#ifndef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS
+  // These functions are no longer declared as external in ABI v1, but still need
+  // to be defined in the dylib to maintain ABI compatibility with older libc++ versions.
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE, char)
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE, wchar_t)
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
+#endif
+
 template
     string
     operator+<char, char_traits<char>, allocator<char> >(char const*, string const&);
 
 namespace
 {
 
 template<typename T>