[Vm-dev] Gemstone 64-bit object format and pointers

[Eliot Miranda]

Hi Ben,

On Sat, Nov 2, 2013 at 5:57 PM, <btc at openinworld.com> wrote:

>
>
> I just watched [1] on Gemstone's object format and was interested to see
> how it uses 000 in the low order bits for direct in-memory object pointers.
>  I presume Spur does something similar but was curious how it specifically
> compares.
>

I developed the SmallDouble format for 64-bit VisualWorks with help from
Martin McClure :-).  The idea is essentially the same.  But in Spur theres'
both a 32-bit and a 64-bit version.  The 32-bit version currently doesn't
use the 2nd bit, confining its tags to bits 0 & 1.  Further, the 32-bit
system has 31-bit SmallIntegers, so if the lsb is 1 the pointer is a
SmallInteger, if the ls 2 bits are 10 it is a Character, otherwise it is a
pointer to an object header.

Bit in 64-bit Spur there will be 3 tag bits and indeed SmallInteger,
Character and SmallDouble are the tagged types.  Making nil true & false
tagged types IMO doesn't make much sense; it just complicates the
fetchClassOf code.  IMO it is better to have nil-true & false be normal
objects, albeit at locations where they'll never move.  So no tagged nil,
true & false.  Otherwise its the same basic set of ideas on object
representation.

HTH,
Eliot


Further, there's no object table in Spur, only flat pointers.


> [1] http://www.youtube.com/watch?v=BAFUafgEwvw
>
> Also, probably not a good idea but I wondered what pros/cons there might
> be to just adopting Gemstone's object format.
>
> cheers -ben
>



-- 
best,
Eliot
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