std::optional defaut constructor

[Marc Glisse]

On Thu, 4 Jun 2020, Ville Voutilainen wrote:

> Right, I was just wondering whether we can reasonably verify in a test 
> that the whole shebang is not zeroed. That may need a tree-dump scan in 
> the test, and probably should go into PR86173 anyway, so I'm not saying 
> such a thing needs to be a part of this fix.

The optimized dumps changed with the patch:

-  <retval> = {};
+  MEM[(struct optional *)&<retval>] ={v} {CLOBBER};
    MEM[(union _Storage *)&<retval>] ={v} {CLOBBER};
+  MEM[(struct _Optional_payload_base *)&<retval>]._M_engaged = 0;
    return <retval>;

checking for the absence of "<retval> = {}", or the presence of 
_M_engaged, may be robust enough across platforms. It doesn't really 
guarantee that nothing writes to the buffer though.

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.

-- 
Marc Glisse