[libc++] Only check for GCC's empty string storage on macOS and iOS

[ldionne]

Summary

We don't need to do that on other Apple platforms, since they never
shipped libstdc++. I also added a comment extracted from the original
commit by Howard Hinnant (e115af2777f6).

libcxx/src/include/refstring.h

 //===------------------------ __refstring ---------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP_REFSTRING_H
 #define _LIBCPP_REFSTRING_H
 
 #include <__config>
 #include <stdexcept>
 #include <cstddef>
 #include <cstring>
-#ifdef __APPLE__
-#include <dlfcn.h>
-#include <mach-o/dyld.h>
+#include "atomic_support.h"
+
+// MacOS and iOS used to ship with libstdc++, and still support old applications
+// linking against libstdc++. The libc++ and libstdc++ exceptions are supposed
+// to be ABI compatible, such that they can be thrown from one library and caught
+// in the other.
+//
+// For that reason, we must look for libstdc++ in the same process and if found,
+// check the string stored in the exception object to see if it is the GCC empty
+// string singleton before manipulating the reference count. This is done so that
+// if an exception is created with a zero-length string in libstdc++, libc++abi
+// won't try to delete the memory.
+#if defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || \
+    defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__)
+#   define _LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE
+#   include <dlfcn.h>
+#   include <mach-o/dyld.h>
 #endif
-#include "atomic_support.h"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace __refstring_imp { namespace {
 typedef int count_t;
 
 struct _Rep_base {
     std::size_t len;
     std::size_t cap;
     count_t     count;
 };
 
 inline _Rep_base* rep_from_data(const char *data_) noexcept {
     char *data = const_cast<char *>(data_);
     return reinterpret_cast<_Rep_base *>(data - sizeof(_Rep_base));
 }
 
 inline char * data_from_rep(_Rep_base *rep) noexcept {
     char *data = reinterpret_cast<char *>(rep);
     return data + sizeof(*rep);
 }
 
-#if defined(__APPLE__)
+#if defined(_LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE)
 inline
 const char* compute_gcc_empty_string_storage() _NOEXCEPT
 {
     void* handle = dlopen("/usr/lib/libstdc++.6.dylib", RTLD_NOLOAD);
     if (handle == nullptr)
         return nullptr;
     void* sym = dlsym(handle, "_ZNSs4_Rep20_S_empty_rep_storageE");
     if (sym == nullptr)
if (__uses_refcount()) {
         if (__libcpp_atomic_add(&rep->count, count_t(-1)) < 0) {
             ::operator delete(rep);
         }
     }
 }
 
 inline
 bool __libcpp_refstring::__uses_refcount() const {
-#ifdef __APPLE__
+#if defined(_LIBCPP_CHECK_FOR_GCC_EMPTY_STRING_STORAGE)
     return __imp_ != get_gcc_empty_string_storage();
 #else
     return true;
 #endif
 }
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif //_LIBCPP_REFSTRING_H