std::optional defaut constructor

[Marc Glisse]

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();
}

-- 
Marc Glisse