[PATCH 2/3] libstdc++: Optimize __uninitialized_default using memset

[Maciej Cencora]

You could include some of the bigger classes by checking whether the class
type is bit_cast-able to std::array of bytes, and that bitcasted output is
equal to value-initialized array.

Regards,
Maciej

czw., 27 cze 2024 o 14:50 Jonathan Wakely <jwakely@redhat.com> napisał(a):

> On Thu, 27 Jun 2024 at 13:49, Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> > On Thu, 27 Jun 2024 at 13:40, Maciej Cencora wrote:
> > >
> > > Hi,
> > >
> > > not sure whether I've missed some conditional that would exclude this
> case, but your change seems to incorrectly handle trivial types that have a
> non-zero bit pattern of value-initialized object, e.g. pointer to member.
> >
> > Good point. I started working on this optimization after last week's
> > https://gcc.gnu.org/r15-1550-g139d65d1f5a60a where is_trivial is
> > appropriate, because valarray only supports numeric types, not
> > pointers to members.
> >
> > But the uninitialized memory algos don't have that restriction, so
> > need to be more careful.
> >
> > I think memset is OK for arithmetic types, enums, pointers, nullptr_t,
> > and trivial classes ... except when those trivial classes contain
> > pointers to members. Which we can't tell.
> >
> > So maybe we can only do it for is_trivial && ((is_scalar and not
> > is_member_pointer) or (is_class and is_empty)).
>
> I suppose any trivial class type smaller than sizeof(int T::*) would
> be OK, because it can't hold a member pointer if it's smaller than
> one.
>
>
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