Date: Thu, 28 Jan 1999 11:38:29 +0100
From: Xavier Leroy <Xavier.Leroy@inria.fr>
To: Michael Hicks <mwh@dsl.cis.upenn.edu>, caml-list@inria.fr
Subject: Re: one-time initialization
In-Reply-To: <199901280508.AAA21644@codex.cis.upenn.edu>; from Michael Hicks on Thu, Jan 28, 1999 at 12:08:23AM -0500
> I wonder if anyone knows how to optimize the following (simplified for the
> sake of dicussion) situation:
>
> let global = ref None
> let init i =
> global := Some i
> let f () =
> match (!global) with
> Some x -> x
> | None -> failwith "not initialized";;
> let g() =
> match (!global) with
> ...
>
> Essentially, there is some global state that is initialized once, and is
> used by all functions in the module. In a more realistic situation, this
> state might be initialized by reading in a file. Given that following
> initialization the global state never changes, it should be conceivable to
> eliminate the match and dereference;
The compiler certainly cannot do it for you, as it involves proving
that f and g are never applied before init is called...
> on my machine (pentium 166), the match
> and dereference result in about a 30% slowdown. I've fooled around with
> some things, but haven't found anything that performs better than this
> straightforward approach or is any more elegant.
Here are a couple of faster, but perhaps less modular approaches:
* Get rid of the ref and the option, and initialize "global" with the
right value the first time. That involves reading the file when
global is initialized, which might involve parsing (partially) the
command-line to find the right file name. E.g.:
let global =
let filename = <parse command line to find config file name, use default
file name if not found> in
read_config_file global
let f() = ... use global ...
* Keep the ref but get rid of the option. You then need a dummy value
of the type of your global state, and you're also no longer protected
against forgetting to initialize it.
let global = ref dummy_global_value
let init i = global := i
let f() = ... use !global ...
Regards,
- Xavier Leroy
This archive was generated by hypermail 2b29 : Sun Jan 02 2000 - 11:58:18 MET