[Bug libstdc++/59048] operator== between std::string and const char* slower than strcmp

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59048

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|2013-11-08 00:00:00         |2017-05-19
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #16 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.

I don't think the char* overload can ever match the string overload because the
former has to compute the length of the char* argument and (possibly also)
compare the strings, while the latter can avoid the first step because the
lengths of both arguments are precomputed.

But the char* overload can be sped up by avoiding string::compare which always
traverses the strings, when operator== only has to do that if their lengths are
the same.

It also doesn't help that string::compare is not expanded inline (because
std::string is declared extern).

In my tests, the following definition speeds the operator up by a factor of 5,
making it about 20% faster than strcmp:

  template<typename _CharT>
    inline bool
    operator==(const basic_string<_CharT>& __lhs,
        const _CharT* __rhs)
  {
    return !__lhs.compare (__rhs);

    typedef typename basic_string<_CharT>::size_type size_type;
    const size_type __len1 = __lhs.length ();
    const size_type __len2 = __builtin_strlen (__rhs);

    return __len1 == __len2
      && !char_traits<_CharT>::compare (__lhs.data (), __rhs,
                                        __len1 < __len2 ? __len1 : __len2);
  }