Re: Q: access to subprogram

[fjh@mundook.cs.mu.OZ.AU (Fergus Henderson)]

bobduff@world.std.com (Robert A Duff) writes:

>If you *always* represent access-to-subprogram values with extra data
>telling where the generic instance data is, then it can work, but that's
>distributed overhead for the case where you didn't use generics.
>
>Make sense?

After much thought -- nope, I'm still confused ;-)

Mercury's shared-generics implementation doesn't have any distributed
overhead when compared to "Mercury minus generics".  I don't think it
has any distributed overhead when compared to Pascal either (i.e.
I think Pascal could be extended with Mercury-style generics without
imposing any distributed overhead.)  But there is distributed overhead
compared to C, if you're not willing to adopt the GNU C trampoline
approach or something along those lines.

I'm still confused about whether there is distributed overhead compared
to Ada.

You can't represent higher-order types (Mercury's equivalent to
access-to-subprogram types) using a single code address, you
need a data address as well -- presuming you don't use trampolines.
But I thought Ada already imposed that requirement, since you can
take the address of nested procedures in Ada? (can't you?)

The way the Mercury implementation handles taking the address of a
generic instance in a shared generic is to package up the generic
instance data in basically the same way that the stack frame data in an
enclosing procedure is packed up when you take the address of a nested
procedure.  So as long as you are already committed to being able to
take the address of nested procedures, there is no distributed
overhead -- I think.

--
Fergus Henderson <fjh@cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh@128.250.37.3         |     -- the last words of T. S. Garp.