u/sym_num

I implemented a prototype mode inference system for N-Prolog.

The system analyzes predicates and infers input/output modes automatically using a two-pass analysis. The current prototype can already infer modes for predicates such as partition/4, fact/2, and even qsort/3.

The long-term goal is to identify predicates that behave like deterministic functions and replace them with optimized C functions while preserving normal Prolog semantics.

Preliminary experiments show speed improvements of around 3–4x for analyzable predicates.

I wrote a technical overview here:

N-Prolog Mode Inference Prototype | by Kenichi Sasagawa | May, 2026 | Medium

I’ve been experimenting with a new optimization idea for N-Prolog.

The basic idea is:

• detect predicates that are effectively functional,
• infer their input/output modes statically,
• and compile them into simpler C-style function calls.

As a first experiment, I manually wrote optimized C code using N-Prolog’s embedded C feature and compared the performance against the normal predicate execution path.

Benchmark: qsort repeated 10,000 times.

N-Prolog 5.19:

?- measure(run(qsort,10000)).
Elapsed Time=0.326355 (second)  15749722(LIPS)

SWI-Prolog:

?- time(run(qsort,10000)).
% 6,160,005 inferences, 0.358 CPU in 0.358 seconds
% (100% CPU, 17189389 Lips)

The optimized version became roughly 3–4x faster than the ordinary predicate-oriented execution path.

The motivation is that many Prolog programs contain predicates that are logically relational, but operationally behave almost like functions. qsort is a typical example.

If such predicates can be detected automatically through mode inference and static analysis, they may be compiled into much simpler execution paths while still preserving ordinary Prolog semantics for general predicates.

I’m currently working on the design of the mode inference engine itself.

Notes and experimental code are in mode.pl if anyone is interested.

I’d also be interested to hear whether similar ideas were explored historically in Prolog systems beyond the usual Mercury-style mode systems.

Recently I had an interesting realization while practicing drums.

I’ve been trying to play Burn like Ian Paice, and I discovered that fast drumming is not about brute force. The key is eliminating unnecessary motion and using rebound efficiently.

Oddly enough, this led me to rethink optimization in my Prolog compiler.

About a year ago I introduced TCO into N-Prolog and achieved decent performance improvements for nqueens. However, qsort still resisted optimization because predicates like partition/4 appear relational:

partition([X|L], Y, [X|L1], L2) :-
    X < Y, !, partition(L, Y, L1, L2).

But then I realized something important:

These predicates are only partially relational.

For example:

?- partition(X,2,[1],[2,3]).
X = [1,2,3].

This “reverse execution” works only incompletely. In practice, predicates like this behave much more like one-way functions than true relations.

I started calling them mut (“mutants”): predicates syntactically written in Prolog form, but whose computation direction is essentially fixed.

That opens an interesting possibility.

Instead of compiling everything into general relational machinery with full unification/backtracking overhead, mut predicates might be statically analyzed and translated into simple C functions.

In other words:

• ordinary relational predicates remain Prolog
• function-like predicates become optimized function code

I suspect operators such as <, arithmetic constraints, and mode usage may allow automatic direction inference.

Ironically, I originally tried to overcome this performance wall with parallelism. Now I’m beginning to think that removing unnecessary generality may be the more important optimization.

Drumming and compiler optimization turned out to share the same lesson:
eliminate unnecessary movement.

The Road to Speeding Up N-Prolog. After finishing the stress tests and… | by Kenichi Sasagawa | May, 2026 | Medium