std::optional defaut constructor

[Ville Voutilainen]

On Thu, 4 Jun 2020 at 11:53, Marc Glisse <marc.glisse@inria.fr> wrote:
>
> On Thu, 4 Jun 2020, Ville Voutilainen wrote:
>
> > On Thu, 4 Jun 2020 at 11:00, Marc Glisse <marc.glisse@inria.fr> wrote:
> >> Maybe create a buffer, fill it with some non-zero values (-1?), then call
> >> placement new, and read some value in the middle of the buffer, possibly
> >> with some protection against optimizations? Ah, no, actual constructors
> >> are fine, it is only the inlined initialization that happens with the
> >> defaulted constructor that zeroes things.
> >
> > The zero-init is part of value-initialization of a class type with a
> > defaulted default constructor, so value-initialization with placement
> > new should indeed show us whether the target buffer is zeroed.
>
> Ah, yes, I had forgotten the empty () at the end of the operator new line
> when testing. Now the patch makes this runtime test go from abort to
> success at -O0 (with optimizations, the memset is removed as dead code). I
> am still not sure we want this kind of test though. And I added launder
> more to quiet a warning than with confidence that it does the right thing.
>
> #include <optional>
> struct A {
>    int a[1024];
> };
> typedef std::optional<A> O;
> int main(){
>    unsigned char t[sizeof(O)];
>    __builtin_memset(t, -1, sizeof(t));
>    new(t)O();
>    if(std::launder(t)[512] != (unsigned char)(-1)) __builtin_abort();
> }

Yeah, I think the patch is OK with or without the test. As a side
note, you don't need the launder
if the check uses the pointer value returned by placement-new.