[libcxx] r283408 - Fix strict-aliasing violation in typeinfo::hash_code()

[Eric Fiselier via cfe-commits]

Author: ericwf
Date: Wed Oct  5 17:55:10 2016
New Revision: 283408

URL: http://llvm.org/viewvc/llvm-project?rev=283408&view=rev
Log:
Fix strict-aliasing violation in typeinfo::hash_code()

Summary:
The current implementation of `hash_code()` for uniqued RTTI strings violates strict aliasing by dereferencing a type-punned pointer. Specifically it generates a `const char**` pointer from the address of the `__name` member before casting it to `const size_t*` and dereferencing it to get the hash. This is really just a complex and incorrect way of writing `reinterpret_cast<size_t>(__name)`.

This patch changes the conversion sequence so that it no longer contains UB.


Reviewers: howard.hinnant, mclow.lists

Subscribers: rjmccall, cfe-commits

Differential Revision: https://reviews.llvm.org/D24012

Modified:
    libcxx/trunk/include/__config
    libcxx/trunk/include/typeinfo

Modified: libcxx/trunk/include/__config
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/__config?rev=283408&r1=283407&r2=283408&view=diff
==============================================================================
--- libcxx/trunk/include/__config (original)
+++ libcxx/trunk/include/__config Wed Oct  5 17:55:10 2016
@@ -694,12 +694,6 @@ template <unsigned> struct __static_asse
 #define _NOALIAS
 #endif
 
-#ifdef __GNUC__
-#define _LIBCPP_MAY_ALIAS __attribute__((__may_alias__))
-#else
-#define _LIBCPP_MAY_ALIAS
-#endif
-
 #if __has_feature(cxx_explicit_conversions) || defined(__IBMCPP__)
 #   define _LIBCPP_EXPLICIT explicit
 #else

Modified: libcxx/trunk/include/typeinfo
URL: http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/typeinfo?rev=283408&r1=283407&r2=283408&view=diff
==============================================================================
--- libcxx/trunk/include/typeinfo (original)
+++ libcxx/trunk/include/typeinfo Wed Oct  5 17:55:10 2016
@@ -77,8 +77,6 @@ class _LIBCPP_EXCEPTION_ABI type_info
     type_info& operator=(const type_info&);
     type_info(const type_info&);
 
-    typedef size_t _LIBCPP_MAY_ALIAS _ASizeT; // Avoid strict-aliasing issues.
-
 protected:
 #ifndef _LIBCPP_NONUNIQUE_RTTI_BIT
     const char* __type_name;
@@ -119,7 +117,7 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     size_t hash_code() const _NOEXCEPT
 #ifndef _LIBCPP_NONUNIQUE_RTTI_BIT
-        {return *reinterpret_cast<const _ASizeT *>(&__type_name);}
+        {return reinterpret_cast<size_t>(__type_name);}
 #else
         {if (!(__type_name & _LIBCPP_NONUNIQUE_RTTI_BIT)) return __type_name;
          const char *__ptr = name();