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From: David B. Held (dheld_at_[hidden])
Date: 2003-02-01 17:49:52

"David Abrahams" <dave_at_[hidden]> wrote in message
> [...]
> What question are you asking? I think all NDAs on the vc7.1 betas
> are expired, so I can just run a test...

Actually, I wanted someone on the compiler team to give me a set of
circumstances under which EBO is guaranteed to work. If there is no
way to get MI to fit into those circumstances, then we can give up.

> [...]
> BTW, VC++ is not the only kid on the block, and the same
> argument applies to all the other players.

Yes, but you seem to imply that other Win32 compilers try to be
object-compatible with VC++, so it seems they should exhibit the
same behaviour. If there is some way to get the desired optimization,
with any luck, the other compilers will perform it as well. Or maybe
this is wishful thinking?

> [...]
> What is optimally_inherit? Latest round of changes in what?

Optimally_inherit is a device that is the "dual" of compressed_pair<>.
Andrei suggested it when this issue first came up. Whilst
compressed_pair<> aggregates when a type is non-empty, and
inherits when it's empty, optimally_inherit inherits when the type
is non-empty, and just calls c'tors on temporaries when the type is
empty. This way, empty bases don't actually enter the inheritance
tree. Unfortunately, EBO interacts with non-empty bases as well
as empty ones, and that is our problem.

The "latest round of changes" is the set of changes that moves
cleanup from named procedures (destroy(), release(), etc.) to d'tors.
For this reason, I suspected that non-trivial user-defined d'tors were
causing VC++ to drop EBO. But commenting out the d'tors just to
check the size did not restore EBO. So now I would like a VC++
compiler team member (preferably the one(s) who wrote the EBO
code) to tell me when EBO works, and when it doesn't, or as much
about that as possible. The optimally_inherit trick works just fine
on BCB, and gcc didn't need it to begin with (that big stud of a
compiler!). If we can get VC++ and VC++ wannabes to invoke
EBO again, we can at least cover a good amount of the market, no?
I know that optimally_inherit *does* work under VC++ for some
cases. I just don't know exactly what those cases are, and I don't
really want to spend a week trying to discover it, especially as that
won't help people who are writing custom policies.


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