On Thu, 27 Jun 2024 at 13:49, Jonathan Wakely <jwak...@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.
