[libc++] Introduce `_LIBCPP_OVERRIDABLE_FUNC_VIS`

[smeenai]

Summary

This is a generalization of _LIBCPP_NEW_DELETE_VIS; the new macro name
captures the semantics better, and also allows us to get rid of the
_WIN32 check in include/new. No functional change.

[EricWF]

What happens on windows when operator new isn't overridden and has to be imported from the DLL? Does that work?

[EricWF]

src/new.cpp

163 ↗

(On Diff #78079)

You should be able to remove the macro on the definitions since they're redundant.

[smeenai]

In D26702#597377, @EricWF wrote:

What happens on windows when operator new isn't overridden and has to be imported from the DLL? Does that work?

Yup. If you have a function that isn't marked dllimport and it's not found locally, the linker will look for it in the libraries being linked to. dllimport just allows the function call to be more efficient (if a function is marked dllimport, the compiler can call the imported function directly, whereas if it isn't, the linker has to emit a fixup thunk and you end up with an extra function call for each call to the imported function). dllimport is necessary for data symbols though, but fortunately we're not dealing with any of those here.

src/new.cpp

163 ↗

(On Diff #78079)

Good point. Will do that before committing.

libcxx/trunk/docs/DesignDocs/VisibilityMacros.rst

 
 **_LIBCPP_HIDDEN**
   Mark a symbol as hidden so it will not be exported from shared libraries.
 
 **_LIBCPP_FUNC_VIS**
   Mark a symbol as being exported by the libc++ library. This attribute must
   be applied to the declaration of all functions exported by the libc++ dylib.
 
+**_LIBCPP_OVERRIDABLE_FUNC_VIS**
+  Mark a symbol as being exported by the libc++ library, but allow it to be
+  overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`.
+  This macro is applied to all `operator new` and `operator delete` overloads.
+
+  **Windows Behavior**: Any symbol marked `dllimport` cannot be overridden
+  locally, since `dllimport` indicates the symbol should be bound to a separate
+  DLL. All `operator new` and `operator delete` overloads are required to be
+  locally overridable, and therefore must not be marked `dllimport`. On Windows,
+  this macro therefore expands to `__declspec(dllexport)` when building the
+  library and has an empty definition otherwise.
+
 **_LIBCPP_INLINE_VISIBILITY**
   Mark a function as hidden and force inlining whenever possible.
 
 **_LIBCPP_ALWAYS_INLINE**
   A synonym for `_LIBCPP_INLINE_VISIBILITY`
 
 **_LIBCPP_TYPE_VIS**
   Mark a type's typeinfo and vtable as having default visibility.

 
 **_LIBCPP_EXCEPTION_ABI**
   Mark the member functions, typeinfo, and vtable of the type as being exported
   by the libc++ library. This macro must be applied to all *exception types*.
   Exception types should be defined directly in namespace `std` and not the
   versioning namespace. This allows throwing and catching some exception types
   between libc++ and libstdc++.
 
-**_LIBCPP_NEW_DELETE_VIS**
-  Mark a symbol as being exported by the libc++ library. This macro must be
-  applied to all `operator new` and `operator delete` overloads.
-
-  **Windows Behavior**: The `operator new` and `operator delete` overloads
-  should not be marked as `dllimport`; if they were, source files including the
-  `<new>` header (either directly or transitively) would lose the ability to use
-  local overloads of `operator new` and `operator delete`. On Windows, this
-  macro therefore expands to `__declspec(dllexport)` when building the library
-  and has an empty definition otherwise. A related caveat is that libc++ must be
-  included on the link line before `msvcrt.lib`, otherwise Microsoft's
-  definitions of `operator new` and `operator delete` inside `msvcrt.lib` will
-  end up being used instead of libc++'s.
-
 Links
 =====
 
 * `[cfe-dev] Visibility in libc++ - 1 <http://lists.llvm.org/pipermail/cfe-dev/2013-July/030610.html>`_
 * `[cfe-dev] Visibility in libc++ - 2 <http://lists.llvm.org/pipermail/cfe-dev/2013-August/031195.html>`_
 * `[libcxx] Visibility fixes for Windows <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20130805/085461.html>`_

libcxx/trunk/include/__config

 #endif // __clang__ || __GNUC__ || _MSC_VER || __IBMCPP__
 
 
 #ifdef _WIN32
 #if defined(_LIBCPP_DISABLE_DLL_IMPORT_EXPORT)
 # define _LIBCPP_DLL_VIS
 # define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 # define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
+# define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #elif defined(_LIBCPP_BUILDING_LIBRARY)
 # define _LIBCPP_DLL_VIS __declspec(dllexport)
 # define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 # define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
+# define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_DLL_VIS
 #else
 # define _LIBCPP_DLL_VIS __declspec(dllimport)
 # define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
 # define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
+# define _LIBCPP_OVERRIDABLE_FUNC_VIS
 #endif
 
 #define _LIBCPP_TYPE_VIS            _LIBCPP_DLL_VIS
 #define _LIBCPP_FUNC_VIS            _LIBCPP_DLL_VIS
 #define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS
 #define _LIBCPP_HIDDEN
 #define _LIBCPP_TYPE_VIS_ONLY
 #define _LIBCPP_FUNC_VIS_ONLY

 #ifndef _LIBCPP_TYPE_VIS_ONLY
 # define _LIBCPP_TYPE_VIS_ONLY _LIBCPP_TYPE_VIS
 #endif
 
 #ifndef _LIBCPP_FUNC_VIS_ONLY
 # define _LIBCPP_FUNC_VIS_ONLY _LIBCPP_FUNC_VIS
 #endif
 
+#ifndef _LIBCPP_OVERRIDABLE_FUNC_VIS
+# define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_FUNC_VIS
+#endif
+
 #ifndef _LIBCPP_EXCEPTION_ABI
 #define _LIBCPP_EXCEPTION_ABI __attribute__ ((__visibility__("default")))
 #endif
 
 #ifndef _LIBCPP_ENUM_VIS
 #  if __has_attribute(__type_visibility__)
 #    define _LIBCPP_ENUM_VIS __attribute__ ((__type_visibility__("default")))
 #  else

libcxx/trunk/include/new

 struct _LIBCPP_TYPE_VIS nothrow_t {};
 extern _LIBCPP_FUNC_VIS const nothrow_t nothrow;
 typedef void (*new_handler)();
 _LIBCPP_FUNC_VIS new_handler set_new_handler(new_handler) _NOEXCEPT;
 _LIBCPP_FUNC_VIS new_handler get_new_handler() _NOEXCEPT;
 
 }  // std
 
-#if defined(_WIN32) && !defined(_LIBCPP_BUILDING_LIBRARY)
-# define _LIBCPP_NEW_DELETE_VIS
-#else
-# define _LIBCPP_NEW_DELETE_VIS _LIBCPP_FUNC_VIS
-#endif
-
 #if !__has_feature(cxx_noexcept)
 #define _THROW_BAD_ALLOC throw(std::bad_alloc)
 #else
 #define _THROW_BAD_ALLOC
 #endif
 
-_LIBCPP_NEW_DELETE_VIS void* operator new(std::size_t __sz) _THROW_BAD_ALLOC;
-_LIBCPP_NEW_DELETE_VIS void* operator new(std::size_t __sz, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
-_LIBCPP_NEW_DELETE_VIS void  operator delete(void* __p) _NOEXCEPT;
-_LIBCPP_NEW_DELETE_VIS void  operator delete(void* __p, const std::nothrow_t&) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz) _THROW_BAD_ALLOC;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, const std::nothrow_t&) _NOEXCEPT;
 #ifndef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
-_LIBCPP_NEW_DELETE_VIS void  operator delete(void* __p, std::size_t __sz) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, std::size_t __sz) _NOEXCEPT;
 #endif
 
-_LIBCPP_NEW_DELETE_VIS void* operator new[](std::size_t __sz) _THROW_BAD_ALLOC;
-_LIBCPP_NEW_DELETE_VIS void* operator new[](std::size_t __sz, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
-_LIBCPP_NEW_DELETE_VIS void  operator delete[](void* __p) _NOEXCEPT;
-_LIBCPP_NEW_DELETE_VIS void  operator delete[](void* __p, const std::nothrow_t&) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](std::size_t __sz) _THROW_BAD_ALLOC;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](std::size_t __sz, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, const std::nothrow_t&) _NOEXCEPT;
 #ifdef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
-_LIBCPP_NEW_DELETE_VIS void  operator delete[](void* __p, std::size_t __sz) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, std::size_t __sz) _NOEXCEPT;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_ALIGNED_ALLOCATION
-_LIBCPP_NEW_DELETE_VIS void* operator new(std::size_t __sz, std::align_val_t) _THROW_BAD_ALLOC;
-_LIBCPP_NEW_DELETE_VIS void* operator new(std::size_t __sz, std::align_val_t, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
-_LIBCPP_NEW_DELETE_VIS void  operator delete(void* __p, std::align_val_t) _NOEXCEPT;
-_LIBCPP_NEW_DELETE_VIS void  operator delete(void* __p, std::align_val_t, const std::nothrow_t&) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz, std::align_val_t) _THROW_BAD_ALLOC;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new(std::size_t __sz, std::align_val_t, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, std::align_val_t) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, std::align_val_t, const std::nothrow_t&) _NOEXCEPT;
 #ifndef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
-_LIBCPP_NEW_DELETE_VIS void  operator delete(void* __p, std::size_t __sz, std::align_val_t) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete(void* __p, std::size_t __sz, std::align_val_t) _NOEXCEPT;
 #endif
 
-_LIBCPP_NEW_DELETE_VIS void* operator new[](std::size_t __sz, std::align_val_t) _THROW_BAD_ALLOC;
-_LIBCPP_NEW_DELETE_VIS void* operator new[](std::size_t __sz, std::align_val_t, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
-_LIBCPP_NEW_DELETE_VIS void  operator delete[](void* __p, std::align_val_t) _NOEXCEPT;
-_LIBCPP_NEW_DELETE_VIS void  operator delete[](void* __p, std::align_val_t, const std::nothrow_t&) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](std::size_t __sz, std::align_val_t) _THROW_BAD_ALLOC;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void* operator new[](std::size_t __sz, std::align_val_t, const std::nothrow_t&) _NOEXCEPT _NOALIAS;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, std::align_val_t) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, std::align_val_t, const std::nothrow_t&) _NOEXCEPT;
 #ifndef _LIBCPP_HAS_NO_SIZED_DEALLOCATION
-_LIBCPP_NEW_DELETE_VIS void  operator delete[](void* __p, std::size_t __sz, std::align_val_t) _NOEXCEPT;
+_LIBCPP_OVERRIDABLE_FUNC_VIS void  operator delete[](void* __p, std::size_t __sz, std::align_val_t) _NOEXCEPT;
 #endif
 #endif
 
 inline _LIBCPP_INLINE_VISIBILITY void* operator new  (std::size_t, void* __p) _NOEXCEPT {return __p;}
 inline _LIBCPP_INLINE_VISIBILITY void* operator new[](std::size_t, void* __p) _NOEXCEPT {return __p;}
 inline _LIBCPP_INLINE_VISIBILITY void  operator delete  (void*, void*) _NOEXCEPT {}
 inline _LIBCPP_INLINE_VISIBILITY void  operator delete[](void*, void*) _NOEXCEPT {}
 

libcxx/trunk/src/new.cpp

 #endif
 
 #ifndef __GLIBCXX__
 
 // Implement all new and delete operators as weak definitions
 // in this shared library, so that they can be overridden by programs
 // that define non-weak copies of the functions.
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void *
 operator new(std::size_t size) _THROW_BAD_ALLOC
 {
     if (size == 0)
         size = 1;
     void* p;
     while ((p = ::malloc(size)) == 0)
     {
         // If malloc fails and there is a new_handler,
         // call it to try free up memory.
         std::new_handler nh = std::get_new_handler();
         if (nh)
             nh();
         else
 #ifndef _LIBCPP_NO_EXCEPTIONS
             throw std::bad_alloc();
 #else
             break;
 #endif
     }
     return p;
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void *
 operator new(std::size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC
 {
     if (size == 0)
         size = 1;
     if (static_cast<size_t>(alignment) < sizeof(void*))
       alignment = std::align_val_t(sizeof(void*));
     void* p;
#else
             p = nullptr; // posix_memalign doesn't initialize 'p' on failure
             break;
 #endif
         }
     }
     return p;
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void*
 operator new(size_t size, const std::nothrow_t&) _NOEXCEPT
 {
     void* p = 0;
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
     {
 #endif  // _LIBCPP_NO_EXCEPTIONS
         p = ::operator new(size);
 #ifndef _LIBCPP_NO_EXCEPTIONS
     }
     catch (...)
     {
     }
 #endif  // _LIBCPP_NO_EXCEPTIONS
     return p;
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void*
 operator new(size_t size, std::align_val_t alignment, const std::nothrow_t&) _NOEXCEPT
 {
     void* p = 0;
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
     {
 #endif  // _LIBCPP_NO_EXCEPTIONS
         p = ::operator new(size, alignment);
 #ifndef _LIBCPP_NO_EXCEPTIONS
     }
     catch (...)
     {
     }
 #endif  // _LIBCPP_NO_EXCEPTIONS
     return p;
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void*
 operator new[](size_t size) _THROW_BAD_ALLOC
 {
     return ::operator new(size);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void*
 operator new[](size_t size, std::align_val_t alignment) _THROW_BAD_ALLOC
 {
     return ::operator new(size, alignment);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void*
 operator new[](size_t size, const std::nothrow_t&) _NOEXCEPT
 {
     void* p = 0;
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
     {
 #endif  // _LIBCPP_NO_EXCEPTIONS
         p = ::operator new[](size);
 #ifndef _LIBCPP_NO_EXCEPTIONS
     }
     catch (...)
     {
     }
 #endif  // _LIBCPP_NO_EXCEPTIONS
     return p;
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void*
 operator new[](size_t size, std::align_val_t alignment, const std::nothrow_t&) _NOEXCEPT
 {
     void* p = 0;
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
     {
 #endif  // _LIBCPP_NO_EXCEPTIONS
         p = ::operator new[](size, alignment);
 #ifndef _LIBCPP_NO_EXCEPTIONS
     }
     catch (...)
     {
     }
 #endif  // _LIBCPP_NO_EXCEPTIONS
     return p;
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete(void* ptr) _NOEXCEPT
 {
     if (ptr)
         ::free(ptr);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete(void* ptr, std::align_val_t) _NOEXCEPT
 {
     if (ptr)
         ::free(ptr);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete(void* ptr, const std::nothrow_t&) _NOEXCEPT
 {
     ::operator delete(ptr);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete(void* ptr, std::align_val_t alignment, const std::nothrow_t&) _NOEXCEPT
 {
     ::operator delete(ptr, alignment);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete(void* ptr, size_t) _NOEXCEPT
 {
     ::operator delete(ptr);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete(void* ptr, size_t, std::align_val_t alignment) _NOEXCEPT
 {
     ::operator delete(ptr, alignment);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete[] (void* ptr) _NOEXCEPT
 {
     ::operator delete(ptr);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete[] (void* ptr, std::align_val_t alignment) _NOEXCEPT
 {
     ::operator delete(ptr, alignment);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete[] (void* ptr, const std::nothrow_t&) _NOEXCEPT
 {
     ::operator delete[](ptr);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete[] (void* ptr, std::align_val_t alignment, const std::nothrow_t&) _NOEXCEPT
 {
     ::operator delete[](ptr, alignment);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete[] (void* ptr, size_t) _NOEXCEPT
 {
     ::operator delete[](ptr);
 }
 
-_LIBCPP_WEAK _LIBCPP_NEW_DELETE_VIS
+_LIBCPP_WEAK
 void
 operator delete[] (void* ptr, size_t, std::align_val_t alignment) _NOEXCEPT
 {
     ::operator delete[](ptr, alignment);
 }
 
 #endif // !__GLIBCXX__