EricWF created this revision.
EricWF added reviewers: mclow.lists, howard.hinnant.
EricWF added a subscriber: cfe-commits.
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.
https://reviews.llvm.org/D24012
Files:
include/__config
include/typeinfo
Index: include/typeinfo
===================================================================
--- include/typeinfo
+++ include/typeinfo
@@ -74,8 +74,6 @@
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;
@@ -116,7 +114,7 @@
_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();
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -691,12 +691,6 @@
#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
Index: include/typeinfo
===================================================================
--- include/typeinfo
+++ include/typeinfo
@@ -74,8 +74,6 @@
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;
@@ -116,7 +114,7 @@
_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();
Index: include/__config
===================================================================
--- include/__config
+++ include/__config
@@ -691,12 +691,6 @@
#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
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