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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/98465] [11 Regression] Bogus -Wstringop-overread
 with -std=gnu++20 -O2 and std::string::insert
Date: Thu, 04 Feb 2021 14:03:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98465

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |redi at gcc dot gnu.org

--- Comment #18 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
So, if adding some attribute or whatever to std::string etc. members doesn't
look doable/desirable, can't we e.g. implement _M_disjunct using some built=
in
that would allow gcc to optimize it better (and later fold it into how it is
implemented now)?

It is currently:

      // True if _Rep and source do not overlap.
      bool
      _M_disjunct(const _CharT* __s) const _GLIBCXX_NOEXCEPT
      {
        return (less<const _CharT*>()(__s, _M_data())
                || less<const _CharT*>()(_M_data() + this->size(), __s));
      }

and when std::basic_string uses the normal allocators, _M_dataplus._M_p
can point either to some heap allocated object or to _M_local_buf inside of
*this.
So, can some template stuff ensure that the builtin would be only used when
using a standard allocator and not something that can say have a global:
char buffer[0x10000000]; and allocate from that?
Pass to the builtin all the needed info (e.g.
return __builtin_whatever_disjunct (_M_data(), this->size(), _M_local_data(=
),
__s);
) and let it use points to info to fold it to false if the last pointer can=
't
point to either _M_local_data() pointed object or heap memory, otherwise fo=
ld
into pointer sized casts of the pointers and integral comparisons as before=
?=