On Wed, Jan 13, 2010 at 12:45 PM, Igor Stasenko <siguctua@gmail.com> wrote:
2010/1/13 Martin McClure <martin@hand2mouse.com>:
Eliot Miranda wrote:
A *much* better way to implement this is to support immutability in the VM (I know, I know, but all the code is available in the Newspeak VM), mark objects one wants to mark as dirty as immutable, catch the NoModificationError exception, and proceed after having made the object mutable and marked it dirty. No creating hidden classes, no trying to get change class to work for compact classes, etc. Just a simple VM-implemented write barrier that can also be used for a host of other things (object-database mapping, debugging, immutable literals etc).
Since object dirtying is at the core of the product I work with, and I've worked extensively with both methods of implementing write barriers...
I very strongly agree with Eliot. *So* much nicer a way to do this.
It could be more efficient, in respect that you don't need to create shadow classes. But triggering exception leads to stack unwinding to find a handler, which inevitably leads to deoptimizing all contexts..
Uh, not so. In VW and Cog examining a context does _not_ deoptimize a context. A context will be "deoptimized" (actually converted to a vanilla heap context) only if you write to other than its stack contents or sender. i.e. a context's frame is only discarded if - one assigns to any of its instance variables other than sender - the stack zone runs out of room for new frames and a stack page must be vacated, causing all frames on that page to be converted to stack contexts So exception handling does *not* usually involve converting contexts. It would be very slow if it did.
and if stack depth is high (between point of writing attempt and hook, where magma will handle exception), this will be much slower than WriteBarrier implementation, described by Cris, which checking the value in-place, without the need of touching exception machinery.
I disagree strongly. Martin's experience with Gemstone (and Gemstone's experience) covers over twenty years and the VM-supported immutability implementations (first in VisualAge IIRC) of Gemstone are far more performant and less problematic than the code-rewriting implementations similar to Magma. Martin really knows what he's talking about.
Just 2 cents.
Regards,
-Martin
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-- Best regards, Igor Stasenko AKA sig.
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