Exceptions store the message as reference counted string for

compatibility to libstdc++. Move the implementation into a header for
easier sharing with libc++abi. Merge a number of improvements from that
version. Provide a POD definition for <stdexcept>'s public use to avoid
cast dances. Discussed with Marshall Clow.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@207695 91177308-0d34-0410-b5e6-96231b3b80d8

include/__refstring

@@ -0,0 +1,139 @@
+//===------------------------ __refstring ---------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___REFSTRING
+#define _LIBCPP___REFSTRING
+
+#include <__config>
+#include <cstddef>
+#include <cstring>
+#if __APPLE__
+#include <dlfcn.h>
+#include <mach-o/dyld.h>
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+class _LIBCPP_HIDDEN __libcpp_refstring
+{
+private:
+    const char* str_;
+
+    typedef int count_t;
+
+    struct _Rep_base
+    {
+        std::size_t len;
+        std::size_t cap;
+        count_t     count;
+    };
+
+    static
+    _Rep_base*
+    rep_from_data(const char *data_) _NOEXCEPT
+    {
+        char *data = const_cast<char *>(data_);
+        return reinterpret_cast<_Rep_base *>(data - sizeof(_Rep_base));
+    }
+    static
+    char *
+    data_from_rep(_Rep_base *rep) _NOEXCEPT
+    {
+        char *data = reinterpret_cast<char *>(rep);
+        return data + sizeof(*rep);
+    }
+
+#if __APPLE__
+    static
+    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)
+            return nullptr;
+        return data_from_rep(reinterpret_cast<_Rep_base *>(sym));
+    }
+
+    static
+    const char*
+    get_gcc_empty_string_storage() _NOEXCEPT
+    {
+        static const char* p = compute_gcc_empty_string_storage();
+        return p;
+    }
+
+    bool
+    uses_refcount() const
+    {
+        return str_ != get_gcc_empty_string_storage();
+    }
+#else
+    bool
+    uses_refcount() const
+    {
+        return true;
+    }
+#endif
+
+public:
+    explicit __libcpp_refstring(const char* msg) {
+        std::size_t len = strlen(msg);
+        _Rep_base* rep = static_cast<_Rep_base *>(::operator new(sizeof(*rep) + len + 1));
+        rep->len = len;
+        rep->cap = len;
+        rep->count = 0;
+        char *data = data_from_rep(rep);
+        std::memcpy(data, msg, len + 1);
+        str_ = data;
+    }
+
+    __libcpp_refstring(const __libcpp_refstring& s) _NOEXCEPT : str_(s.str_)
+    {
+        if (uses_refcount())
+            __sync_add_and_fetch(&rep_from_data(str_)->count, 1);
+    }
+
+    __libcpp_refstring& operator=(const __libcpp_refstring& s) _NOEXCEPT
+    {
+        bool adjust_old_count = uses_refcount();
+        struct _Rep_base *old_rep = rep_from_data(str_);
+        str_ = s.str_;
+        if (uses_refcount())
+            __sync_add_and_fetch(&rep_from_data(str_)->count, 1);
+        if (adjust_old_count)
+        {
+            if (__sync_add_and_fetch(&old_rep->count, count_t(-1)) < 0)
+            {
+                ::operator delete(old_rep);
+            }
+        }
+        return *this;
+    }
+
+    ~__libcpp_refstring()
+    {
+        if (uses_refcount())
+        {
+            _Rep_base* rep = rep_from_data(str_);
+            if (__sync_add_and_fetch(&rep->count, count_t(-1)) < 0)
+            {
+                ::operator delete(rep);
+            }
+        }
+    }
+
+    const char* c_str() const _NOEXCEPT {return str_;}
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //_LIBCPP___REFSTRING