On Wed, May 23, 2012 at 2:13 PM, David T. Lewis <lewis at mail.msen.com> wrote:

> On Wed, May 23, 2012 at 12:30:48PM -0700, Eliot Miranda wrote:
> > On Wed, May 23, 2012 at 12:04 PM, Andreas Raab <andreas.raab at gmx.de>
> wrote:
> >
> > > On 5/23/2012 20:51, Eliot Miranda wrote:
> > >
> > >> So given that the Interpreter VM now supports the mirror primitives,
> > >> does anyone object if I add the debugger changes to trunk that use the
> > >> mirror primitives and hence make accurate debugging of proxies
> possible?
> > >> The downside of doing this is that the debugger will be broken on
> > >> older VMs.
> > >>
> > >
> > > It should be possible to write the mirror prims relatively safely,
> e.g.:
> > >
> > > object: anObject instVarAt: anIndex
> > >        "Primitive. Answer a fixed variable in an object. The numbering
> of
> > > the
> > >        variables corresponds to the named instance variables. Fail if
> the
> > > index
> > >        is not an Integer or is not the index of a fixed variable.
> > > Essential for the
> > >        debugger. See  Object documentation whatIsAPrimitive."
> > >
> > >        <primitive: 73>
> > >        anIndex <= (self objectClass: anObject) instSize
> > >                ifTrue:["Assume mirror primitives are missing"
> > >                        ^anObject instVarAt: index]
> > >                ifFalse:["Access beyond fixed variables."
> > >                        ^self object: anObject basicAt: anIndex - (self
> > > objectClass: anObject) instSize].
> > >
> >
> > That's a good idea.  Alas it doesn't work because the primitives won't
> fail
> > on the older VMs.  Note that the same primitive is used for instVarAt:
> and
> > object:instVarAt: (and likewise for object:instVarAt:put: & objectClass:
> > etc).  The difference is that in the older VM the prims assume a fixed
> > argument count and only pop that many objects from the stack.  Hence
> there
> > is the potential of stack overflow in the older VMs.  In any case things
> > are likely to limp along rather than fail catastrophically.  I'm not sure
> > that that's a good thing or not ;)
>
> Would it be possible to perform one test at image startup time to determine
> if mirror primitive support is present, and handle things accordingly from
> then on?
>

Of course but it'll be clumsy.  It'll necessitate a layer of wrapper
methods, and for what purpose?  Better IMO to upgrade the VMs asap.



>
> Dave
>
>
>


-- 
best,
Eliot
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