Re: Internals details for

From: John Prevost (
Date: Sat Dec 18 1999 - 01:51:02 MET

To: Xavier Leroy <>
Subject: Re: Internals details for
From: John Prevost <>
Date: 17 Dec 1999 19:51:02 -0500
In-Reply-To: Xavier Leroy's message of "Sat, 11 Dec 1999 19:09:08 +0100"

Xavier Leroy <> writes:

> Using a "checkbound" is perhaps the simplest solution. Otherwise,
> some system-wide exceptions such as Invalid_argument are assigned
> global symbols and you don't need to guess their integer index inside
> their defining module: just emit the C-- code corresponding to
> (raise (symbol "Invalid_argument") (string "my message"))

I'll try this.

> If you're really into high-performance stuff, you could fold the
> permission check and the bounds check in one "checkbound" instruction.
> Just arrange the "write enable" flag to be (the Caml integer) 0 if
> write is allowed, and -1 if it is not. Then, generate something like
> (checkbound (or index (write_enable_flag region) (size region)))

I don't think this is necessary--especially since both reading and
writing are things that could fail (so I need more than one bit).
When you're going for extreme speed, you'll probably use the unsafe

(How does -unsafe work, by the way? Does it make the C-- "checkbound"
stuff work differently?)

> Although hacking is fun, you could get a more portable
> implementation by writing it in ML using unsafe string accesses.
> Those will happily work on any char *, not necessarily on well-formed
> Caml strings. Something like:
> external mmap : ... -> string
> type t = { data: string; length: int }
> let read_char reg idx =
> if idx < 0 || idx >= reg.length
> then raise (Invalid_argument "Region.read_char")
> else String.unsafe_get idx
> It will be a bit slower, but maybe not too much.

Hmm. What do you mean by "a more portable implementation"? One which
doesn't require compiler modifications, or one which works with
bytecode? I believe that with bytecode, the C functions are

As for unsafe string access: but doesn't the pointer point to an
O'Caml block, which includes a tag and length information?


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