[m-dev.] for review: retry in trailing grades

[Fergus Henderson]

On 05-Oct-1999, Zoltan Somogyi <zs at cs.mu.OZ.AU> wrote:
> 
> Make the retry command work in trailing grades (e.g. for HAL).
...
> compiler/llds.m:
> 	Add a new reason for resetting the trail: a retry in the debugger.
...
> runtime/mercury_trail.h:
> 	Add the new reason why the trail may be reset.

You should modify all the places in the Mercury distribution
which switch on the trail reset reason --
in particular extras/trailed_update/var.m and extras/clpr/clpr.m,
but the may be others.

> +        case MR_TYPECTOR_REP_HP:
> +            fatal_error("Tyson hasn't yet moved the code"
> +			    "for copying saved heap pointers here");

I don't think it is a good idea to refer to individuals in error messages.
I would change that to `fatal_error("Sorry, not implemented:
copying of saved heap pointers")'; if you must mention Tyson by name,
then it should go in a comment in the source, not in a runtime error
message that our users might encounter.

> +        case MR_TYPECTOR_REP_TRAIL_PTR:
> +        case MR_TYPECTOR_REP_TICKET:
> +	    /* we do not yet compress the trail when doing gc */
> +            new_data = data;
> +            break;

That comment probably deserves an "XXX".

> +++ mercury_trail.h	1999/09/29 05:20:32
> @@ -32,16 +32,18 @@
>  ** MR_store_ticket()
>  **	called when creating a choice point, or before a commit
>  ** MR_reset_ticket()
> -**	called when resuming forward execution after failing (MR_undo),
> -**	or after a commit (MR_commit), or after a "soft commit"
> -**	[one that doesn't prune away all the alternative solutions,
> -**	but which does require us to commit to this goal being solvable]
> -**	in an if-then-else with a nondet condition, or in solutions/2
> -**	(MR_solve).
> +**	called under the following circumstances, with different parameters:
> +**	- when resuming forward execution after failing (MR_undo);
> +**	- after a commit (MR_commit);
> +**	- after a "soft commit" [one that doesn't prune away all the
> +**	  alternative solutions, but which does require us to commit to
> +**	  this goal being solvable] in an if-then-else with a nondet condition,
> +**	  or in solutions/2 (MR_solve);
> +**	- when executing a `retry' command in the debugger (MR_retry).

Actually there is another case which should be documented there:
	- when throwing an exception (MR_exception)

There are a few other places where the trailing documentation has
not been updated to reflect that we know support exceptions.
I suppose that could be fixed as a separate change.

> +++ mercury_trace.c	1999/09/30 06:40:05
> @@ -384,6 +384,22 @@
>  				MR_LONG_LVAL_NUMBER(location));
>  		MR_saved_sp(saved_regs) -= entry->MR_sle_stack_slots;
>  		MR_trace_event_number = MR_event_num_stackvar(this_frame);
> +
> +#ifdef	MR_USE_TRAIL
> +		if (entry->MR_sle_maybe_trail >= 0) {
> +			Word	ticket_counter;
> +			Word	trail_ptr;
> +
> +			trail_ptr = MR_based_stackvar(this_frame,
> +					entry->MR_sle_maybe_trail);
> +			ticket_counter = MR_based_stackvar(this_frame,
> +					entry->MR_sle_maybe_trail+1);
> +			MR_reset_ticket(trail_ptr, MR_retry);
> +			MR_discard_tickets_to(ticket_counter);
> +		} else {
> +			fatal_error("retry cannot restore the trail");
> +		}
> +#endif

Hmm... are the Mercury registers valid here?

What will happen if the trail function tries to call back into Mercury code?

-- 
Fergus Henderson <fjh at 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 at 128.250.37.3        |     -- the last words of T. S. Garp.
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