[Vm-dev] becomeForward: behavior

Marcel Taeumel marcel.taeumel at hpi.de
Mon Jan 24 07:59:59 UTC 2022


Hi Florin, hi Clément --

yes, that's what Squeak's #becomeForward:copyHash: is for.

See this discussion, where the interference between #becomeForward:, copy hash, and ModificationForbidden is explained:
http://lists.squeakfoundation.org/pipermail/squeak-dev/2020-April/208596.html [http://lists.squeakfoundation.org/pipermail/squeak-dev/2020-April/208596.html]


Best,
Marcel
Am 24.01.2022 08:04:38 schrieb Clément Béra <bera.clement at gmail.com>:
Hi Florin,

I believe there are 2 primitives for 2 different use-cases:
- primitiveArrayBecomeOneWayNoCopyHash 248
- primitiveArrayBecomeOneWayCopyHash 249

The difference between both is which hash is preserved. I think for your
use-case you should use the other primitive.

On Mon, Jan 24, 2022 at 5:04 AM Florin Mateoc
wrote:

>
> Hi,
>
> I am a bit surprised by the #becomeForward: behavior in Squeak. This is a
> one way become, where the target of the operation is the receiver, which
> sheds its identity/existence. Nobody points to it after the primitive
> execution, so it is discarded. This understanding also conforms to the
> method comment.
> As such, I remember a pattern of usage in VisualAge Smalltalk, where one
> way become was used as a cheap cleanup/avoidance of memory leaks, by doing
> oneWayBecome: nil. It's not that I advocate for it, but this works in
> Squeak too, except in Squeak #becomeForward: does an additional thing to
> the pointers redirection, it changes the identityHash of the argument, the
> non (obvious) target. While I understand this may be useful in certain
> situations, I think it is a dangerous conflation of activities. A new
> primitive that sets the identity hash could be used (VA has it) explicitly
> instead when such behavior is desired.
> As it is, if I do "Object new becomeForward: nil", it succeeds and it
> changes nil's identityHash.
>
> Sorry if this has been debated before,
>
> Cheers,
> Florin
>


--
Clément Béra
https://clementbera.github.io/
https://clementbera.wordpress.com/

Hi Florin,

I believe there are 2 primitives for 2 different use-cases:
- primitiveArrayBecomeOneWayNoCopyHash 248
- primitiveArrayBecomeOneWayCopyHash 249

The difference between both is which hash is preserved. I think for your use-case you should use the other primitive.

On Mon, Jan 24, 2022 at 5:04 AM Florin Mateoc <florin.mateoc at gmail.com [mailto:florin.mateoc at gmail.com]> wrote:

Hi,

I am a bit surprised by the #becomeForward: behavior in Squeak. This is a one way become, where the target of the operation is the receiver, which sheds its identity/existence. Nobody points to it after the primitive execution, so it is discarded. This understanding also conforms to the method comment.

As such, I remember a pattern of usage in VisualAge Smalltalk, where one way become was used as a cheap cleanup/avoidance of memory leaks, by doing oneWayBecome: nil. It's not that I advocate for it, but this works in Squeak too, except in Squeak #becomeForward: does an additional thing to the pointers redirection, it changes the identityHash of the argument, the non (obvious) target. While I understand this may be useful in certain situations, I think it is a dangerous conflation of activities. A new primitive that sets the identity hash could be used (VA has it) explicitly
instead

when such behavior is desired.
As it is, if I do "Object new becomeForward: nil", it succeeds and it changes nil's identityHash.

Sorry if this has been debated before,

Cheers,
Florin




--

Clément Béra
https://clementbera.github.io/ [https://clementbera.github.io/]
https://clementbera.wordpress.com/ [https://clementbera.wordpress.com/]
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