Re: [Caml-list] Expanding the Float Array tag

[Markus Mottl <markus.mottl@gmail.com>]

On Mon, Sep 16, 2013 at 12:49 PM, Yotam Barnoy <yotambarnoy@gmail.com> wrote:
> Why do doubles need special handling though, even on a 32-bit system? My
> suggestion is that the Double_tag be changed to Flat_tag, meaning that all
> non-pointer objects can reside in this tag. The only issue I've found so far
> is that polymorphic <, <=, > and >= would not work. However, these operators
> should not be allowed on a vector anyway since there is no natural ordering
> scheme for vectors. If there are other issues, please let me know.

Here is a problem: If you marshal a float array on a 64-bit platform,
how is the 32-bit platform supposed to know about the "logical" and
"physical" size of the array?  On 64-bit platforms where everything is
the same size the distinction wouldn't matter, of course, but on
32-bit it does.  The header word can only provide one size (but tag
distinctions).

> I agree regarding the expansion of 246 constructors. This must have been
> kept for compatibility with 32 bit systems. I think what should happen in 32
> bit systems is that one constructor should be reserved for having >246
> constructors, in which case another word of memory could be utilized for the
> constructor code. In fact, you'd only need to use that extra word if the
> particular constructor exceeds 246.

Appending such a "long" constructor to the end of the block is
probably doable with acceptable complexity.  In that position it
should not require many changes elsewhere in the runtime system or
code generators.  On 64-bit platforms I'd still prefer seeing a change
of the header field layout for efficiency reasons even though this
would likely make a lot of "marshal" users cry and maybe require more
effort to implement.  But I wouldn't mind just using "extended" blocks
with the current header layout there, too, if this made everybody
happy.

Regards,
Markus

-- 
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com