Question

I remember once reading that there were at least two other alternatives invented roughly at the same time as the WAM. Any pointers?

Answer 1

Prior to the WAM, there was the ZIP by Clocksin. Its design is still very interesting. SWI-Prolog uses it. And also B-Prolog has slowly migrated from a WAM design towards the ZIP. Of course, on that way many new innovations were developed. Another alternative is the VAM.

A comparison as of 1993 is:

http://www.complang.tuwien.ac.at/ulrich/papers/PDF/binwam-nov93.pdf

In the meantime, the most interesting architectural developments are related to B-Prolog.

WAM vs. ZIP

The key difference between the WAM and the ZIP is the precise interface for a predicate’s arguments. In the WAM, the arguments are all passed via registers, that is, either real registers or at least fixed locations in memory. The ZIP passes all arguments via the stack.

Let’s consider a minimal example:

p(R1,R2,R3,L1,L2,L3) :-  % WAM                % ZIP
                         % store L1..L3       % nothing
                         % nothing            % push R1..R3
                         % init X1..X3        % push X1..X3
   q(R1,R2,R3,X1,X2,X3),
                         % put unsafe X1..X3  % push X1..X3
                         % load       L1..L3  % push L1..L3
   r(X1,X2,X3,L1,L2,L3).

Prior to calling q:

The WAM does not need to do any action for arguments that are passed on to the first goal at the very same positions (R1..R3). This is particularly interesting for binary clauses – that is, clauses with exactly one regular goal at the end. Here the WAM excels.

The other arguments L1..L3 need to be stored locally. So for these arguments, the register interface did not do anything good.

The ZIP on the other hand does not need to save arguments – they are already saved on the stack. This is not only good for clauses with more than one goal, but also for other interrupting goals like constraints or interrupts.

As a downside, the ZIP must push again R1..R3.

Both have to initialize X1..X3 and store them on the stack.

Calling q:

When calling q, the WAM has to allocate stack space for X1..X3 and L1..L3 thus 6 cells, whereas the ZIP needs R1..R3,L1..L3,X1..X3. So here, the WAM is more space efficient. Also, the WAM permits environment trimming (for more complex situations) which is next-to-impossible for the ZIP.

Prior to calling r:

This r is the last call, and systems try to free the space for this clause, provided no choice point is present.

For the WAM, the existential variables X1..X3 have to be checked for being still uninstantiated local variables (put_unsafe), and if, they are moved onto the heap – that’s expensive, but occurs rarely. L1..L3 are just loaded. That’s all, the WAM can now safely deallocate the local frame. So last call optimization is dirt cheap.

For the ZIP, everything has to be pushed as usual. Then only, an extra scan has to examine all the values on the stack and moves them accordingly. That’s rather expensive. Some optimizations are possible, but it is still much more than what the WAM does. ((A possible improvement would be to push arguments in reverse order. Then the variables L1..L3 might be left in their location. So these variables would not need any handling. I have not seen such an implementation (yet).))