[M3devel] fewer wrappers/more C? (or a wash?)

[hendrik at topoi.pooq.com]

On Sun, Apr 19, 2009 at 09:45:49PM -0500, Rodney M. Bates wrote:
> hendrik at topoi.pooq.com wrote:
> >On Fri, Apr 17, 2009 at 10:57:10PM +1000, Tony Hosking wrote:
> >  
> >>I am a little concerned about passing REFANY directly to C code as  
> >>there is no guarantee that REFANY and C pointers will always be  
> >>compatible.  ADDRESS can more safely be assumed compatible.
> >>    
> >
> >Indeed, I once read the X toolkit specs, and it was rife with small 
> >integers being packed into pointers.  Apparently the toolkit resolved 
> >it not by a tag bit, but by its magnitude.  There was some constant 
> >somewhere that identified which numbers were small enough to be 
> >considered not-pointers.
> >
> >This was a discrimination without a tag bit.  Similar concept to what 
> >we're planning for the future of REFANY, but different implementation.
> >
> >I don't know how they figured out which pointer values were safe to 
> >treat as integers.
> >  
> In my more elaborate "safe" proposal, at least, the language itself
> does not specify anything about what the bit-encodings of tagged
> types are.  It's an implementors' option, and the language
> can support whatever the implementors choose. 
> 
> This could be used to match the X toolkit's encoding.  However,
> using the lsb takes advantage of the fact that heap objects must
> always be aligned and thus the lsb is already always zero, when
> it's really a heap pointer.  That seems like by far the most efficient
> encoding.

The X toolkit was set up to be called from C, and the small integers 
were just parameters to a C function, or else pointers, whatever was 
more convenient.  They had a specific range of small integers (which was 
machine-dependent as far as I can tell), and the whole convention seems 
to have been set up for coding convenience.  I don't know how the limit 
on "small" inetgers was decided.

The LSB trick is what I would recommend for us.  I was just reinforcing 
the hopelessless of assuming we can make REFANY match C conventions.  I 
suspect there are too many different ones.

-- hendrik