[m-dev.] More trailing bugs. :-)

[Warwick Harvey]

Fergus wrote:
> On 28-Jan-2000, Warwick Harvey <wharvey at cs.monash.edu.au> wrote:
> > My understanding of how the trailing operations should work, is that during 
> > forward execution, the ticket counter should never decrease.
> 
> This is not the case.
[...]

OK, you've convinced me there are some problems with this.

So I have another proposal.  :-)

The problem I described is not so much that the ticket counter / choicepoint 
ID is decremented, but rather that values are re-used.  What if we introduce 
a ticket "high water mark", and redefine MR_store_ticket() as follows:

        /*
        ** Called when we create a choice point
        ** (including semidet choice points).
        */
/* void MR_store_ticket(Word &); */
#define MR_store_ticket(save_trail_ptr)                         \
        do {                                                    \
                (save_trail_ptr) = (Word) MR_trail_ptr;         \
                MR_ticket_counter = ++MR_ticket_high_water;     \
        } while(0)

That avoids the ID re-use problem, and means we are allowed to decrement the 
counter, so that redundant trailing isn't crippled.  Can you see any 
problems with this approach?

Warwick

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