Lambda lifting: only lift functions that have free variables

[OlivierNicole]

This addresses the second remark in #1792: lifted functions that have no free variables don't need to be wrapped.

[hhugo]

What's the status of this ?

[OlivierNicole]

It’s a small improvement, currently only used for --effects=double-translation, and is ready for review.

[vouillon]

In this functor, one should be able to lift without wrapper both f and g.

module F (X : sig end) = struct
  let f x = prerr_endline x
  let x = 1
  let g x = f x; f x
end

Sequences of functions are not handled:

module G (X : sig end) = struct
  let f x = prerr_endline x
  let g x = f x; f x
end

Neither are mutually recursive functions:

module H (X : sig end) = struct
  let rec h x y =
    if x > 0 then i x y else prerr_endline y
  and i x y =
    h (x - 1) y
end

Maybe one should use Strongly_connected_components to distinguish functions that are really mutually recursive from functions that just follow one-another?

[OlivierNicole]

We could, it would be a further improvement on the gain that the present state of this PR brings.