[m-rev.] for review: faster merging of varsets

[Julien Fischer]

Hi Zoltan,

On Sun, 13 Jan 2019, Zoltan Somogyi wrote:

> For review by anyone.
>
> The diff implements the proposed faster algorithm just once,
> even though it is applicable to four predicates. I plan to address
> any review comments on that one predicate, and *then* copy
> the approach to the other three.
>
> If someone would undertake to run tuning benchmarks for me,
> I would write the required benchmarking code.

I'm happy to run them for you.

> (The compiler at the moment goes out of its way to avoid calling these
> predicates with argument sizes where the old algorithm is too slow,
> so I don't think it would be a good benchmark for this.
> The main objective of this diff is the avoidance of this need.)

...

> Make merging varsets asymptotically more efficient.
> 
> library/varset.m:
>     Add a faster implementation of merge_renaming and (later) related
>     predicates.

I would say add a "faster alternative implementation" since the existing
one is still present.

The rest looks fine.

Julien.