[Vm-dev] Questions related to immutability in Cog

Eliot Miranda eliot.miranda at gmail.com
Wed Nov 11 15:21:51 UTC 2015


Hi Tobias,

> On Nov 11, 2015, at 2:50 AM, Tobias Pape <Das.Linux at gmx.de> wrote:
> 
> 
> Hi all,
> 
> I just want to add my 2ct to this.
> I'd rather like to habe the immutability bit be one-shot,
> non-resettable, ie, an immutable object stays that way its entire lifetime.
> I know that conflicts with the idea of the lightweight read barrier, but
> I think this would be worthwhile for value classes :)
> 
> Can we find a compromise here?

The VM is agnostic about how the upper levels of the system uses it.  There's nothing to stop you overriding the method that sets the bit, so I think we can have both.  Just remember that the lightweight read barrier is /really/ useful for persistence schemes and that's v important to typical industrial applications.

> 
> Best regards
>    -Tobias
> 
>> On 10.11.2015, at 17:35, Ryan Macnak <rmacnak at gmail.com> wrote:
>> 
>> Hi Clement,
>> 
>> Per object immutability was implemented in the old, pre-Cog Newspeak VM (NewspeakInterpreter). It has not yet been implemented current Cog Newspeak VM (Stack/CoInterpreter + NewspeakVM=true), but the necessary bit is reserved in the object header. It was used as part of an orthogonal synchronization scheme to detect changed objects: mark synchronized objects as immutable, trap on change attempt, add to dirty list and mark as mutable again. A marked object is only shallowly immutable, and its instance variables may contain mutable objects.
>> 
>> Ryan
>> 
>> On Tue, Nov 10, 2015 at 6:17 AM, Clément Bera <bera.clement at gmail.com> wrote:
>> 
>> Hello everyone,
>> 
>> I tried to understand how immutability is implemented in the CogVM (NewSpeak flavor). I have some questions:
>> 
>> - Does it actually work and is it used ?
>> - Does an object being immutable means that you can't edit its variable fields only or that you can't either edit its instance variables ? Because I don't see any code checking for instance variable stores in the Newspeak interpreter.
>> - How does the immutability check works in machine code ? In the Newspeak interpreter there is an immutability check in #at:put: . In the at:put: generated by the JIT (for example CogObjectRepresentationFor32BitSpur) I don't see any immutability check. Did I miss some clever bit trick ? Does the Newspeak spur VMs use the #at:put: machine code version of the primitive ?
>> 
>> Thanks !
> 
> 


More information about the Vm-dev mailing list