[Bug c++/46830] New: s.swap(s) triggers overlapped memcpy() for versa_string

["ppluzhnikov at google dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46830

           Summary: s.swap(s) triggers overlapped memcpy() for
                    versa_string
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ppluzhnikov@google.com


Consider this test case:

//--- cut ---
#include <ext/vstring.h>
#include <ext/vstring_fwd.h>

typedef __gnu_cxx::__vstring string;

int main()
{
  string s = "Hello";
  s.swap(s);
}
//--- cut ---

When built with current GCC SVN (rev 167521), and run under Valgrind,
it produces:

==21887== Source and destination overlap in memcpy(0x7ffefff390, 0x7ffefff390,
16)
==21887==    at 0x4C2C364: memcpy (valgrind/memcheck/mc_replace_strmem.c:523)
==21887==    by 0x400B92: std::char_traits<char>::copy(char*, char const*,
unsigned long)
(/t/gcc-svn-install/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/bits/char_traits.h:268)
==21887==    by 0x400DD4: __gnu_cxx::__sso_string_base<char,
std::char_traits<char>, std::allocator<char>
>::_M_swap(__gnu_cxx::__sso_string_base<char, std::char_traits<char>,
std::allocator<char> >&)
(/t/gcc-svn-install/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/ext/sso_string_base.h:248)
==21887==    by 0x400C20: __gnu_cxx::__versa_string<char,
std::char_traits<char>, std::allocator<char>,
__gnu_cxx::__sso_string_base>::swap(__gnu_cxx::__versa_string<char,
std::char_traits<char>, std::allocator<char>, __gnu_cxx::__sso_string_base>&)
(/t/gcc-svn-install/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.6.0/../../../../include/c++/4.6.0/ext/vstring.h:1475)
==21887==    by 0x400ACC: main (/g/tmp/t.cc:9)

I can't find any prohibition in C++ standards on calling s.swap(s),
nor any indication in POSIX standards that memcpy(p, p, ...) is allowed.

Proposed fix:

Index: libstdc++-v3/include/ext/sso_string_base.h
===================================================================
--- libstdc++-v3/include/ext/sso_string_base.h  (revision 167521)
+++ libstdc++-v3/include/ext/sso_string_base.h  (working copy)
@@ -232,6 +232,9 @@
     __sso_string_base<_CharT, _Traits, _Alloc>::
     _M_swap(__sso_string_base& __rcs)
     {
+      if (this == &__rcs)
+       return;
+
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 431. Swapping containers with unequal allocators.
       std::__alloc_swap<_CharT_alloc_type>::_S_do_it(_M_get_allocator(),