[Vm-dev] Questions related to immutability in Cog

[Tobias Pape]

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?

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 !