https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108846
--- Comment #22 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #16)
> const ptrdiff_t _Num = __last - __first;
> - if (_Num)
> + if (__builtin_expect(_Num > 1, true))
> __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
> + else if (_Num == 1)
Richi suggested that we could avoid these runtime branches (which hurt
optimization, see PR 109445) if we knew how many bytes of tail padding there
are in _Tp, e.g.,
__builtin_memmove(__result, __first, sizeof(_Tp) * _Num - __padding_bytes);
This would mean we copy whole objects for the first Num - 1 elements, and then
only copy the non-padding bytes for the last element. This would be conforming,
because there's no guarantee that padding bits are copied by assignment anyway,
and std::copy is specified in terms of assignment.
When copying a single base-class object (the subject of this bug) the last
element is the only element, so we solve the problem.
We don't have a built-in to tell us the number of padding bytes, but we might
be able to use something like this:
template<typename T>
constexpr size_t
potentially_overlapping_tail_padding()
{
if constexpr (is_final<T>::value)
return 0;
else
{
struct D1 : T
{
char c1;
};
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Winvalid-offsetof"
return sizeof(T) - offsetof(D1, c1);
#pragma GCC diagnostic pop
}
}
For pre-C++17 we would need to use tag dispatching and __is_final instead of
if-constexpr, but that's doable. For pre-C++14 it can't be constexpr, but
that's OK too.
