Re: Internals details for

From: Xavier Leroy (
Date: Sat Dec 11 1999 - 19:09:08 MET

Date: Sat, 11 Dec 1999 19:09:08 +0100
From: Xavier Leroy <>
To: John Prevost <>,
Subject: Re: Internals details for
In-Reply-To: <>; from John Prevost on Fri, Dec 10, 1999 at 03:12:14AM -0500

> My main focus has been on adding small bits of code to
> asmcomp/ for the new functionality. Two of the new
> primitives (Pregionrefs and Pregionsets) need to check the protection
> bits recorded for the region before trying to access it, and somehow
> raise an exception if the access check fails.
> I'm currently using the following kludge to make bad region access
> checks fail:
> let region_checkaccess exp = function
> | Reg_Read ->
> Cifthenelse(
> Cop(Cand, [region_prot exp; Cconst_int 1]),
> Cconst_pointer 1,
> Cop(Ccheckbound, [Cconst_int 0; Cconst_int 0]))
> | Reg_Write ->
> Cifthenelse(
> Cop(Cand, [region_prot exp; Cconst_int 2]),
> Cconst_pointer 1,
> Cop(Ccheckbound, [Cconst_int 0; Cconst_int 0]))
> (* XXX Bounds check on 0 is a kludge to force exception *)
> Is there a better way for me to cause an exception to be thrown at
> this point? Do I need to fall back to a Cextcall to ask someone to
> throw an exception for me? Or could (and should) I actually use
> Craise with arcane knowledge that a certain exception maps to a
> certain integer value?

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"))

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)))

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.

- Xavier Leroy

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