[mercury-users] Compiler optimization for large number of factual clauses.

[Julien Fischer]

On Fri, 29 Jun 2007, Bartlomiej Szymczak wrote:

> Consider the following Mercury code:
>
> :-type c --->
>       c_1;
>       c_2;
> ....(thousands of entries)
>       c_99999.
>
> :-pred rel(c,c).
> :-mode rel(in,in) is semidet.
> :-mode rel(in,out) is nondet.
> :-mode rel(out,in) is nondet.
>
> rel(c_1,c_2).
> rel(c_1,c_3).
> rel(c_3,c_1).
> ....(thousands of entries)
> rel(c_99999,c_3).
>
> I want to ask how fast the "rel" predicate is. Will the compiler
> optimize it by adding some hash table based lookup?

In low-level C grades recent versions of the Mercury compiler (> 0.13.x)
will compile predicates that consist of facts into code that does table
lookups.  (With version 0.13.x and below you have to use fact tables
to obtain similar code.)

> Are all three modes equivalently efficient?

The compiler generates the separate code for each mode of a predicate.
It will attempt to make the code for each mode as efficient as
possible.

Julien.
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