HELP EFFICIENCY                                 Chris Mellish  Sept 1982
                                 Revised by Kathryn Seifert  August 1986

Hints about how to make Prolog programs more efficient

Keywords: efficiency, system, compilation


--  EFFICIENCY IN PROLOG ----------------------------------------------

If you are very worried about the efficiency of your Prolog programs,
the following hints may be useful:

1) COMPILE A WHOLE PROCEDURE IN ONE GO

   If the Prolog system reads all the clauses for some predicate as a
   contiguous chunk in a file (or from the terminal), it can make a
   significantly faster procedure from them than if it reads them
   interspersed with other clauses, or if clauses are added and deleted
   to them by use of 'assert' and 'retract'.

2) USE CUTS

   The use of a cut where a procedure is determinate reduces the amount
   of space occupied by the running program (and hence garbage
   collection time). It also reduces the time spent by the system trying
   possibilities that cannot possibly succeed.

3) ALLOCATE DISCRIMINATORY ARGUMENTS TO THE FRONT

   When Prolog matches a goal against the head of a clause, it attempts
   to match the arguments from left to right. So if the choice of a
   which clause is appropriate depends mainly on some particular
   argument, it makes sense to make that the first argument.

4) MAKE USE OF POP PROCEDURES

   There may be a POP-11 procedure to do just what you want, either in
   the POP-11 system, or in the POP-11 library. If so, it may be faster
   than the corresponding Prolog procedure. See PLOGHELP * PLOGTOPOP and
   PLOGHELP * MIXED_LANGUAGES  for how to call POP-11 procedures from
   Prolog. If you use PROLOG_EVAL (see PLOGHELP * PROLOG_EVAL in the
   first goal of a clause (or the first goal after a cut), the result is
   almost as fast as calling the relevant POP-11 procedures directly
   from POP-11.

5) USE 'PROLOG_NO_CLAUSES' AND 'PROLOG_SYSPREDICATES'

   The built-in predicates 'prolog_no_clauses' and
   'prolog_syspredicates' can be used to cut down on compilation and
   running time.  See PLOGHELP * SYSTEM for information on these
   predicates.

6) See PLOGHELP * SYSTEM for more information about tuning.

7) Avoid unwitting production of ratios as a result of using "/" with
   integers. See REF * EFFICIENCY

-- RELATED DOCUMENTATION ----------------------------------------------

PLOGHELP * SYSTEM
 Predicate which affect the way the Prolog system operates

REF * EFFICIENCY
 This gives many tips relating to efficiency in POPLOG, mostly
 relevant to POP-11 programmers. However, advanced PROLOG programmers
 may be able to make good use of the information.

REF * FASTPROCS
 Gives information about fast non-checking procedures available for
 system programming.

HELP * POPMEMLIM
  Explains some of the background to PLOGHELP * SYSTEM

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