On 6/15/2010 3:32 PM, Eliot Miranda wrote:
> Clearly with the 20/20 scheme its easy to steal a couple of bits for
> more GC flags by taking a bit from each 20 bit field.  Further, with
> Squeak, the inst size byte might be pointless (although contexts are
> still indexable objects with fixed fields) and so I doubt bits are at a
> premium.  Hence the real question is whether I should be whorish and go
> for the 16-bit compact fast class index access or not.

My $.02: Start with 16 bits and declare the free bits "reserved". When 
someone complains for the first time, review and decide how many more 
bits to devote. Rationale #1: Rather than taking away resources when you 
notice you need an extra header bit, start with the smaller number but 
keep some room for growth if it becomes necessary. Rationale #2: YAGNI 
(i.e., we really don't know if we'll need more header bits or if we need 
more room for classes).

Cheers,
   - Andreas