Re: [Ocaml-lib-devel] a bug in BitSet?

[Janne H. <ja...@hy...>]

On Sun, 19 Dec 2004, Bardur Arantsson wrote:
> I'm no expert on OCaml optimization, but I believe there
> is a considerable performance benefit in using unsafe_get
> (bget), unsafe_set (bset), etc. because you can avoid
> bounds checking and always get proper inlining. Whether
> this level of optimization is really necessary for BitSet
> I'll leave as an exercise for the reader. :)

...

> I don't think the compiler is currently clever enough (or
> maybe the language semantics are not strict enough?) to do
> all the needed inlining, and the bounds checking would
> still slow you down.

I took the current BitSet module, modified bget and bset to just use 
String.[] normally and added the couple of necessary char_of_ints and 
int_of_chars.  I then went on a compiled the BitSet module with ocamlopt 
-unsafe (to turn off bounds checking).  The resulting assembly is pretty 
much the same as with the Obj.magic+unsafe_get trickery, except that 
ocamlopt seems to be unable to inline int_of_char calls (which was a 
surprise to me).

> I think compiling without bounds checking for "release"
> builds is unwise since it may obscure serious bugs which
> haven't been caught by the test suite. It's basically rude
> to anyone using the library to (possibly) obscure bugs
> within the library this way.

Yes.  However, it seems that many parts of the library *are* already using 
unsafe_gets.

But I guess people can go and compile with -unsafe themselves if they want 
the extra performance (if any).

ciao,
janne