[cxx-abi-dev] Guaranteed copy elision and tail padding

[John McCall]

> On Jul 21, 2016, at 11:02 AM, Jason Merrill <jason at redhat.com> wrote:
> P0135 seems to require that we elide the copy when using the result of
> a function returning by value to initialize a base class subobject,
> but the ABI doesn't currently require that such a function avoid
> clobbering tail padding when initializing its return object.
> Thoughts?

This is not possible in general.  A function returning by value returns a complete
object, i.e. one with its own virtual base subobjects.  We have no choice but to
emit that to a temporary and move out of the non-virtual subobject.

The next semantic question is whether it's compatible with NRVO, i.e. whether
there are guarantees about the existence of padding on named local variables.

Assuming that it's possible in some definable cases (and I think you could
easily revise the standard to make it only apply to classes without v-bases), 
it seems abstractly reasonable.  Certainly it's generally preferable to avoid
a high-level copy/move + destroy pair than to use a larger store at the end
of very specific initializers.

As an implementor, I think I'm most worried about how this + NRVO would
mess up our existing peepholes that assume the existence of tail padding
on certain complete objects.

John.