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: unassig...@gcc.gnu.org
ReportedBy: ppluzhni...@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(),
