Re: Q: access to subprogram

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

rogoff@sccm.Stanford.EDU (Brian Rogoff) writes:

>It was one of the refs in Norman Cohen's precis of the discussion, from 
>Randy Brukardt at RR Software. Here is the relevant excerpt:
>
>--------------------------------------------------------------------------- 
>To put the problem in a nutshell:
> 
>If 'Access is allowed in generic bodies, generic code sharing requires
>distributed overhead.  (Or compilers have to do body analysis of generics
>in order to prevent sharing on them).

Having implemented shared generic handling for Mercury, without any
distributed overhead as far as I am aware, I'm keen to understand in
detail how this problem arises in Ada.  

>To explain this in detail, I'd have to give a crash course on the construction
>of shared generics.  However, here is a simplified version of the problem:
>A shared generic passes in some way a pointer to a data area identifying the
>generic, and containing its local objects.  [Note that this exists in any but
>the most simplified versions of sharing - any sharing algorithm which allows
>data in the generic specification or body would have the problem].
>However, an access-to-subprogram does not know that the routine comes from
>a generic, or even which generic.  The solution to this problem in the
>specification is to construct a 'wrapper' routine at instantiation time,
>which knows about the appropriate generic instantiation.

I think I follow you so far...

>However, a wrapper cannot be built for 'Accesses in the generic body
>inside the generic body. 

Can you explain in a bit more detail when this would be necessary,
and why it is not possible?

--
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"
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