[Vm-dev] Re: [Pharo-project] BlockClosure>>ensure: implementation

[Eliot Miranda]

On Mon, Oct 22, 2012 at 2:17 AM, Clément Bera <clement.bera at inria.fr> wrote:

> Hello,
>
> I don't understand something on BlockClosure>>ensure:. Why does it use
> 'self valueNoContextSwitch'  and not 'self value' ? In which case is there
> an issue ?


Prior to my closure implementation BlockContext>>value[:value:*] was not a
context switch point.  So to preserve the threading semantics of ensure: we
implemented ensure: in terms of BlockClosure>>valueNoContextSwitch, which
is not a context-switch point, as opposed to BlockClosure>>value[:value:*].
 The context switch points in the VM are non-primitive sends (or failing
primitives, i.e. full method activation), backward branches, the Process
primitives suspend, resume, signal and wait (and CrtiticalSection/Mutex
enter and exit) and block evaluation.


>
> Thank you for any answers
>
> BlockClosure>>ensure: is implemented this way :
>
> ensure: aBlock
> "Evaluate a termination block after evaluating the receiver, regardless of
> whether the receiver's evaluation completes.  N.B.  This method is *not*
> implemented as a primitive.  Primitive 198 always fails.  The VM uses prim
> 198 in a context's method as the mark for an ensure:/ifCurtailed:
> activation."
>
> | complete returnValue |
> <primitive: 198>
> returnValue := *self valueNoContextSwitch*.
> complete ifNil:[
> complete := true.
> aBlock value.
> ].
> ^ returnValue
>
>


-- 
best,
Eliot
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