From e115af2777f6532c8943e95baf7011e2f92f781a Mon Sep 17 00:00:00 2001
From: Howard Hinnant <hhinnant@apple.com>
Date: Tue, 18 Sep 2012 21:34:12 +0000
Subject: [PATCH] I've added Apple-only behavior that looks for libstdc++ in
 the same process and if found, checks the string stored in <stdexcept>
 exception objects to see if it is the gcc empty string singleton before
 manipulating the reference count.  This is done so that if such an exception
 is created with a zero-length string in libstdc++, libc++abi won't try to
 delete the memory.  This is part of a ongoing process to make libc++
 exceptions ABI-compatible with libstdc++-4.2 exceptions, to the point that
 each library can catch exceptions thrown by the other.  If other parties
 would also like this behavior, the #if __APPLE__ can be broadened.

llvm-svn: 164170
---
 libcxxabi/src/stdexcept.cpp | 50 ++++++++++++++++++++++++++++++++-----
 1 file changed, 44 insertions(+), 6 deletions(-)

diff --git a/libcxxabi/src/stdexcept.cpp b/libcxxabi/src/stdexcept.cpp
index 5ea92fb5744a..7071915addba 100644
--- a/libcxxabi/src/stdexcept.cpp
+++ b/libcxxabi/src/stdexcept.cpp
@@ -14,6 +14,11 @@
 #include <cstdint>
 #include <cstddef>
 
+#if __APPLE__
+#include <dlfcn.h>
+#include <mach-o/dyld.h>
+#endif
+
 // Note:  optimize for size
 
 #pragma GCC visibility push(hidden)
@@ -33,6 +38,27 @@ private:
                                                                        sizeof(count_t));
 
     count_t& count() const _NOEXCEPT {return (count_t&)(*(str_ - sizeof(count_t)));}
+
+#if __APPLE__
+    static
+    const void*
+    compute_gcc_empty_string_storage() _LIBCPP_CANTTHROW
+    {
+        void* handle = dlopen("libstdc++.dylib", RTLD_LAZY);
+        if (handle == 0)
+            return 0;
+        return (const char*)dlsym(handle, "_ZNSs4_Rep20_S_empty_rep_storageE") + offset;
+    }
+    
+    static
+    const void*
+    get_gcc_empty_string_storage() _LIBCPP_CANTTHROW
+    {
+        static const void* p = compute_gcc_empty_string_storage();
+        return p;
+    }
+#endif
+
 public:
     explicit __libcpp_nmstr(const char* msg);
     __libcpp_nmstr(const __libcpp_nmstr& s) _LIBCPP_CANTTHROW;
@@ -56,7 +82,10 @@ inline
 __libcpp_nmstr::__libcpp_nmstr(const __libcpp_nmstr& s)
     : str_(s.str_)
 {
-    __sync_add_and_fetch(&count(), 1);
+#if __APPLE__
+    if (str_ != get_gcc_empty_string_storage())
+#endif
+        __sync_add_and_fetch(&count(), 1);
 }
 
 __libcpp_nmstr&
@@ -64,17 +93,26 @@ __libcpp_nmstr::operator=(const __libcpp_nmstr& s)
 {
     const char* p = str_;
     str_ = s.str_;
-    __sync_add_and_fetch(&count(), 1);
-    if (__sync_add_and_fetch((count_t*)(p-sizeof(count_t)), count_t(-1)) < 0)
-        delete [] (p-offset);
+#if __APPLE__
+    if (str_ != get_gcc_empty_string_storage())
+#endif
+        __sync_add_and_fetch(&count(), 1);
+#if __APPLE__
+    if (p != get_gcc_empty_string_storage())
+#endif
+        if (__sync_add_and_fetch((count_t*)(p-sizeof(count_t)), count_t(-1)) < 0)
+            delete [] (p-offset);
     return *this;
 }
 
 inline
 __libcpp_nmstr::~__libcpp_nmstr()
 {
-    if (__sync_add_and_fetch(&count(), count_t(-1)) < 0)
-        delete [] (str_ - offset);
+#if __APPLE__
+    if (str_ != get_gcc_empty_string_storage())
+#endif
+        if (__sync_add_and_fetch(&count(), count_t(-1)) < 0)
+            delete [] (str_ - offset);
 }
 
 }