[m-rev.] for review: IL back-end: use value types for environments

[Tyson Dowd]

On 17-Jul-2001, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> On 15-Jul-2001, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> > On 14-Jul-2001, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> > > On 14-Jul-2001, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> > > > 
> > > > For the IL back-end, use value types rather than class types
> > > > for the environment structures used for nondeterministic code.
> > > 
> > > This doesn't work, because you can't use `refany' types as fields of structures.
> > > 
> > > We had the same problem with ordinary managed reference types, and worked around
> > > that by putting a value rather than a reference in the environment struct,
> > > and using copy-in copy-out.
> > > 
> > > But that work-around won't help here, since there's no generic type we can use
> > > for the value.
> > > 
> > > :-(
> > 
> > Plan B is to use unmanaged pointers rather than 'refany'.
> > 
> > (Still not 100% sure if this plan is feasible,
> > since I haven't implemented it yet.)
> 
> I've implemented it, and it works.
> 
> > This will of course be completely unverifiable.
> > So it ought to be optional, with the other option being to
> > allocate nondet environments on the heap (as we do currently).
> 
> I plan to implement that compiler switch in the long run.
> But I think it might be best to just check in this change as is for now.
> 
> Comments?

I don't know whether it's a good idea to make the code less verifiable
than it already is.  Particularly if you can't even switch it on or off.

Does this code achieve anything at all apart from moving the goalposts
of verifiable code further away?  (I'm hoping the answer is yes, but you
haven't given any rationale for the change yet). 

If you are going to say efficiency, I'm going to say there are *lots* of
other things we could work on that are fully verifiable that could give
us better efficiency.

(I'd have no problem with this code if it were switchable, or if you
cleaned up the code for doing heap allocated environments at the same
time).

Tyson.
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