G M: The attached patch is for libcxx's new.cpp and __config files. The patch's intent is to make new.cpp compile using MS's cl.exe compiler without changing the meaning of anything for any other compiler.

The issue this patch seeks to address is that MS's compiler (cl.exe) doesn't support the __attribute__((__weak__)) or __atribute__((__visibility__("default")) syntax; so a solution must be found where cl.exe doesn't see this syntax.

This patch seeks to solve this problem by changing code patterned like this:
__attribute__((__weak__, __visibility__("default")))
void* operator new(size_t size, const std::nothrow_t&) _NOEXCEPT { /*snip*/; return p; }

to code like this:
_LIBCPP_WEAK
void* operator new(size_t size, const std::nothrow_t&) _NOEXCEPT { return p; }

Howard:  Thanks for all the comments regarding the default visibility
tag on the definition.  I agree it isn't needed, and that there are lots
of other places where it is missing.  That being said, I'm not wanting
to rock the boat on that issue right now.  So I've added it back to the
definition via _LIBCPP_FUNC_VIS.  A later pass dedicated just to this
issue can bring things in to a consistent state one way or the other. 
Note that we do not want to have the exact same attributes on the
declaration and defintion in this case.  The declaration should not be
marked weak, whereas the definition should (which is what G M's patch
did). I've fully tested on OS X to ensure that the resultant attribute
syntax actually works.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@192007 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -434,6 +434,7 @@ using namespace _LIBCPP_NAMESPACE __attribute__((__strong__));
 #define _LIBCPP_END_NAMESPACE_STD  }
 #define _VSTD std
 
+#  define _LIBCPP_WEAK
 namespace std {
 }
 
@@ -602,4 +603,8 @@ template <unsigned> struct __static_assert_check {};
 #  endif
 #endif
 
+#ifndef _LIBCPP_WEAK
+#  define _LIBCPP_WEAK __attribute__((__weak__))
+#endif
+
 #endif  // _LIBCPP_CONFIG

src/new.cpp

@@ -39,7 +39,7 @@
 // in this shared library, so that they can be overriden by programs
 // that define non-weak copies of the functions.
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void *
 operator new(std::size_t size)
 #if !__has_feature(cxx_noexcept)
@@ -66,7 +66,7 @@ operator new(std::size_t size)
     return p;
 }
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void*
 operator new(size_t size, const std::nothrow_t&) _NOEXCEPT
 {
@@ -85,7 +85,7 @@ operator new(size_t size, const std::nothrow_t&) _NOEXCEPT
     return p;
 }
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void*
 operator new[](size_t size)
 #if !__has_feature(cxx_noexcept)
@@ -95,7 +95,7 @@ operator new[](size_t size)
     return ::operator new(size);
 }
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void*
 operator new[](size_t size, const std::nothrow_t&) _NOEXCEPT
 {
@@ -114,7 +114,7 @@ operator new[](size_t size, const std::nothrow_t&) _NOEXCEPT
     return p;
 }
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void
 operator delete(void* ptr) _NOEXCEPT
 {
@@ -122,21 +122,21 @@ operator delete(void* ptr) _NOEXCEPT
         ::free(ptr);
 }
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void
 operator delete(void* ptr, const std::nothrow_t&) _NOEXCEPT
 {
     ::operator delete(ptr);
 }
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void
 operator delete[] (void* ptr) _NOEXCEPT
 {
     ::operator delete (ptr);
 }
 
-__attribute__((__weak__, __visibility__("default")))
+_LIBCPP_WEAK _LIBCPP_FUNC_VIS
 void
 operator delete[] (void* ptr, const std::nothrow_t&) _NOEXCEPT
 {