Re: [Ocaml-lib-devel] write_double

[Richard J. <ri...@an...>]

On Sun, Jun 08, 2008 at 09:39:39PM +0200, Florent Monnier wrote:
> > In fact, thinking about this a bit more clearly, there's no need to
> > set the tag at all if all you do is call 'String.unsafe_get', so this
> > works:
> >
> >   let hairy_string_of_float (d : float) =
> >     (Obj.magic d : string)
> >
> > Be really careful that you don't let the returned "string" escape into
> > the wild though :-) It still looks like a double to the garbage
> > collector and it's not a well-formed string either, so any function
> > other than 'String.unsafe_get' is highly likely to fail.
> 
> Hi,
> this hairy version is as fast as the one with the external oneliner C,
> here are the elapsed times (from Sys.time) for 3_000_000 floats written 
> to /dev/null:
> 
>   0.51 sec. -- with extern oneliner C
>   0.50 sec. -- the hairy one wins, well done!

That's to be expected.  Obj.magic is just the %identity compiler
primitive, which the compiler optimizes away completely, so the only
overhead is (maybe) the function call and (maybe) the boxing up of the
float argument.

Rich.

-- 
Richard Jones
Red Hat