New subject: [Pharo-project] Integrating Changes in 1.4 that require a new VM

22 Sep 2011

      On Thu, Sep 22, 2011 at 11:28 AM, Igor Stasenko siguctua@gmail.com wrote:
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On 22 September 2011 20:06, Eliot Miranda eliot.miranda@gmail.com wrote:
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Hi Igor,
On Thu, Sep 22, 2011 at 10:53 AM, Igor Stasenko siguctua@gmail.com
wrote:
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On 22 September 2011 19:16, Eliot Miranda eliot.miranda@gmail.com
wrote:
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(apologies for the duplicate reply; someone needs to sort out their
threading for the benefit of the community ;) )
On Thu, Sep 22, 2011 at 2:36 AM, Marcus Denker <
marcus.denker@inria.fr>
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wrote:
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Hi,
There are two changesets waiting for integrating in 1.4 that have
serious
consequences:

Ephemerons. The VM level changes are in the Cog VMs build on

Jenkins,
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but have not
 been integrated in the VMMaker codebase.
   http://code.google.com/p/pharo/issues/detail?id=4265

I would *really* like to back out these changes.  The Ephemeron
implementation is very much a prototype, requiring a hack to determine
whether an object is an ephemeron (the presence of a  marker class in
the
first inst var) that I'm not at all happy with.  There is a neater
implementation available via using an unused instSpec which IMO has
significant advantages (much simpler & faster, instSpec is valid at
all
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times, including during compaction, less overhead, doesn't require a
marker
class), and is the route I'm taking with the new
GC/object-representation
I'm working on now.  Note that other than determining whether an
object
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is
an ephemeron (instSpec/format vs inst var test) the rest of Igor's
code
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remains the same.  I'd like to avoid too much VM forking.  Would you
all
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consider putting these changes on hold for now?
If so, I'll make the effort to produce prototype changes (in the area
of
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ClassBuilder and class definition; no VM code necessary as yet) to
allow
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defining Ephemerons via the int spec route by next week at the latest.
i agree that in my implementation this is a weak point. But its hard
to do anything without
making changes to object format to identify these special objects.
The main story behind this is can we afford to change the internals of
VM without being beaten hard
by "backwards compatibility" party? :)
I don't think we get stuck in this at all.  The instSpec/format field has
an
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unused value (5 i believe) and this can easily be used for Ephemerons.
All
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that is needed is a little image work on these methods:
    Behavior>>typeOfClass
        needs to answer e.g. #ephemeron for ephemeron classes
    ClassBuilder>>computeFormat:instSize:forSuper:ccIndex:
        needs to accept e.g. #ephemeron for type and pass variable: false
and weak: true for ephemerons to format:variable:words:pointers:weak:.
    ClassBuilder>>format:variable:words:pointers:weak:
        needs to respond to variable: false and weak: true by computing
the
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ephemeron instSpec.
Class>>weakSubclass:instanceVariableNames:classVariableNames:poolDictionaries:category:
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ClassBuilder>>superclass:weakSubclass:instanceVariableNames:classVariableNames:poolDictionaries:category:
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    need siblings, e.g.

ephemeronSubclass:instanceVariableNames:classVariableNames:poolDictionaries:category
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superclass:ephemeronSubclass:instanceVariableNames:classVariableNames:poolDictionaries:category:
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Right?  This is easy.  Then in the VM there are a few places where
pointer
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indexability (formats 3 and 4) need to be firmed up to exclude 5, but
nothing difficult.  We talked about this in email last week.
Do you think this will require boosting an image format version number?
That would make sense.  Boosting it so that the new VMs can run older images
and newer, ephemeron images, but that ephemeron images won't open on older
VMs.  Yes, this makes perfect sense.
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I think it is, because clearly, an images which expect ephemerons to
function cannot work with older VMs properly without
support of ephemerons.
And will lead us to numerous reports "cannot open your f*king image"
with my VM :)
Internally, discussing with Marcus and Stef we came to agreement, that
for each Pharo release we should ship
own version of VM (signed appropriately), so then there will be less
confusion. We also thinking that VM versioning
in future should follow the image versioning, again to make things
simpler and to avoid confusion.
--
Best regards,
Igor Stasenko.
-- 
best,
Eliot

Re: [Pharo-project] Integrating Changes in 1.4 that require a new VM