Re: [Squeak-VMdev] Versiojn 4 changes

[Ian P. <ian...@in...>]

On 06 Apr 2004, at 23:50, Yoshiki Ohshima wrote:

> how about using "01" for OOP
> and "00" for SmallInteger?  Some processor's addressing mode let us
> access the word-aligned memory with such pointer, while "no-tag" for
> SmallInteger may save some bit-operations.

Just another data point: some Smalltalk implementations put the 
SmallInteger tag in the topmost bit.  This makes SI tag and overflow 
checks after arithmetic simpler: addition and subtraction work 
in-place, plus you can just look at the sign flag after the operation 
instead of "mask + test-zero" or "shift + xor + sign-test".

On architectures where you can set the sign flag during move this can 
also often eliminate any need to mask and test on the tag bit; after a 
move you can "trap" immediately on (non-)SI oops.

Cheers,
Ian