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Date: Wed, 14 Mar 2012 00:02:02 +0200
To: Raphael Proust <raphlalou@gmail.com>
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From: moj@tanssi.net (Matti Jokinen)
Subject: Re: [Caml-list] Very slow compilation

> When working with ocamlduce (a few years ago) the same problem was
> raised. A simple thing that can greatly reduce typing time is putting
> explicit type annotations. Although the verbosity is increased it is
> not that much of a burden if the annotated parts do not evolve too
> much.

In my experience, ocamlduceopt/ocamlduceopt.opt slows down regularly when
the source file approaches 1000 lines of OCamlDuce code.  The compilation
time then grows rapidly: in this machine (Thinkpad R5000) 1000 lines
took 20 seconds, 2000 lines 2 minutes, 3000 lines hanged the system.
Only OCamlDuce code causes slowdown, pure OCaml is compiled rapidly even
by OCamlDuce compilers.

However, it is not typechecking that takes time.  The time-consuming
step appears to be register allocation.  Try:

ocamldebug /usr/bin/ocamlduceopt -c big_ocamlduce_module.ml
run
... wait about two minutes and press control-C

You will probably find the compiler executing a function from modules
such as Interf, Coloring or Spill, or a lower level function called
from these modules.

I have never observed anything similar in OCaml, but ocamlduceopt
appears to use unmodified ocamlopt code generation modules.  I wonder
what is the critical difference between OCamlDuce code and typical
OCaml code at this level.

- Matti Jokinen