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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/108846] std::copy, std::copy_n and std::copy_backward
  on potentially overlapping subobjects
Date: Thu, 20 Apr 2023 14:12:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108846

--- Comment #23 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
(In reply to Jonathan Wakely from comment #22)
>=20
> Richi suggested that we could avoid these runtime branches (which hurt
> optimization, see PR 109445) if we knew how many bytes of tail padding th=
ere
> are in _Tp [...]
> We don't have a built-in to tell us the number of [trailing] padding byte=
s, but we
> might be able to use something like this

Three thoughts that might be helpful:
- There may be padding in the middle of an object, and I'm not confident th=
at
the Standard actually forbids it from being used. Of course your approach w=
orks
fine on the Itanium ABI, and probably works everywhere that actually exists=
. If
you've got chapter+verse proving that it must work *everywhere*, I'd apprec=
iate
seeing it, just for my own information.
- If GCC were ever to add a builtin for this notion, IMO the proper name wo=
uld
be `__datasizeof(T)`. See
https://danakj.github.io/2023/01/15/trivially-relocatable.html#data-size
- You can implement your library trait like this; see
https://quuxplusone.github.io/blog/2023/03/04/trivial-swap-prize-update/#st=
ep-1.-std-swap-can-t-assume-it

    template <class _Tp>
    struct __libcpp_datasizeof {
        struct _Up {
            [[__no_unique_address__]] _Tp __t_;
            char __c_;
        };
        static const size_t value =3D __builtin_offsetof(_Up, __c_);
    };

Unfortunately it looks like GCC doesn't support
`__attribute__((__no_unique_address__))` in C++03 mode. (Neither does Clang.
What is up with that!)

Your suggested trait implementation is slightly wrong for `is_final` types:=
 you
return 0 but really a final type _can_ have usable tail padding. See
https://godbolt.org/z/P6x459MEq=