compiler support added for precise GC

[Walter Bright]

On 4/15/2012 7:29 PM, Walter Bright wrote:
> I realized after I posted this that the patterns:
>
> ptr .. int .. ptr .. int
> ptr .. int .. ptr
> ptr .. int .. ptr .. int .. int
>
> are the same as far as marking goes, and so should all generate the same
> function/data.

Another possibility is to just emit a bit mask:

0	no pointers
101	ptr .. int .. ptr
1011	ptr .. ptr .. int .. ptr
 >0xFFFF	a pointer to a function that does the marking

This has the advantage of almost never needing to resort to the indirect 
function call.