[Vm-dev] #becomeUncompact not longer works in Cog/Stack VMs

[Bert Freudenberg]

On 08.05.2011, at 09:39, Mariano Martinez Peck wrote:

> On Sun, May 8, 2011 at 6:54 AM, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>>   Determining the class of an instance of a compact class is faster than determining the class of an instance of a non-compact class if the compact class index is known.  
> 
> This is what I don't understand. Even if the compact class index is known, why is it that faster than non-compact?   because in non-compact you need to fetch the class pointer from the object header?
> But... regardless that the compact class index is know, you also have to get the index (those 5 bits) from the OH.  So I don't see why there is so much difference in performance. 

The difference is that the index can be compared to a *constant*. If the index is 6 (or is it 5?) then this is a Float instance. No matter where in memory the Float class actually is. It's almost like a tag bit for classes, similar to the tag bit to distinguish immediate SmallIntegers. 

- Bert -