Re: [Caml-list] Accelerating compilation

[Gabriel Scherer <gabriel.scherer@gmail.com>]

Those are all good points, but they would be just as appropriate as
feature requests in the OCaml bugtracker (
http://caml.inria.fr/mantis/ ).

I think it should not be too difficult to add a flag to the compiler
to only produce .cmi files out of its source inputs, and not also a
.cmo (or also a .cmx) as is currently done. If you (or anyone
interested in contributing) are interested in providing a patch for
this, I'm more than ready to review the patch to help upstream
integration. I'd consider this "junior job" difficulty.

Integrating this into build systems may be trickier because, as Markus
noted, just asking for the .cmi of the main module of your project
will recursively build .cmi from the .mli of its dependencies, without
checking that the .ml matches them. You could add a phony rule/stamp
.impl.cmi that is handled differently by the build system (basically
like .cmo, except they would be empty).

Your points (2) and (3) are well noted. I've personally been thinking
about an optional -use-timestamp option for ocamlbuild, but devoted my
own contribution time to other things this summer.


Markus Mottl wrote:
> I'm not sure what you are referring to, OCamlBuild does already
> support parallel builds.

While there is support for parallel compilation in ocamlbuild (...
except on Windows, I'm afraid), I've often found out that it doesn't
parallelize much in practice because of current implementation
limitations -- but if you have success stories about this I'd be glad
to hear about them. Improving on this is on the medium-term
ocamlbuild-development TODO list.

On Fri, Sep 6, 2013 at 3:56 PM, Romain Bardou <romain.bardou@inria.fr> wrote:
> Hello list,
>
> As my project grows bigger, it is becoming much less efficient to use
> the type-checker to quickly find places in my code which must be updated
> (e.g. pattern-matching). Here is a wishlist for improvements.
>
> 1) Separate typing and code generation, in ocamlc and in ocamlopt
>
> For instance, provide an option -typing-only which would mean "only
> produce the .cmi but do not produce the .cmo or the .cmx". The compiler
> would only need the .cmi of the dependencies, not their .cmo or .cmx.
> This would make it possible to have a Makefile target, or an Ocamlbuild
> option, to just type.
>
> Also, provide an option -do-not-retype which would mean "if the .cmi
> exists, load it instead of type-checking again". This would allow the
> build process to first type-check (using -typing-only) and then generate
> the code without type-checking again (using -do-not-retype). Of course
> the build system should be very careful to ensure the .cmi is
> up-to-date. This option could also help when compiling both in bytecode
> and in native code. This option is not necessary to just find errors
> quickly, though.
>
> 2) Be able to disable Ocamlbuild's digest mechanism and use dates and
> file sizes instead
>
> If I am not mistaken, this is one of the main reasons why Ocamlbuild is
> slower that make. It does help to prevent useless recompilation, but
> what good does it make to prevent a useless recompilation once in a
> while if it is at the cost of losing a lot of time in all other cases?
> I'm sure it is project-dependent though so it should only be an option,
> say, -do-not-hash, or -comparison-mode dateandsize.
>
> 3) Parallel compilation in Ocamlbuild
>
> Of course it would help but it is not easy to implement so I'm just
> putting it there to be exhaustive.
>
> I think the most important point is the first one, as the other two
> depend on the build system, and they have been debated already. I'm not
> aware of any discussion about separing typing and code generation
> though. What do you think about that?
>
> Cheers,
>
> --
> Romain Bardou
>
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