[Vm-dev] becomeForward: behavior

Tue Jan 25 11:44:12 UTC 2022

Hi Florin --

> it changes the identityHash of the argument, the non (obvious) target. 

I updated the comment in #becomeForward: ... hope it helps:

"Primitive. All variables in the entire system that used to point to the receiver now point to the argument. Fails if either argument is an immediate such as a SmallInteger, or if the receiver is read-only.
NOTE THAT the identityHash of the receiver IS NOT copied to the argument so that the argument should still be properly indexed in any existing hashed structures after the mutation. See #becomeForward:copyHash:."

Best,
Marcel
Am 24.01.2022 15:11:09 schrieb Florin Mateoc <florin.mateoc at gmail.com>:
Hi Clément and Marcel,

You are right, other use cases are supported, and there is probably no need
for a separate setIdentityHash: primitive.

I guess I would not have minded if the method currently called
#becomForward: would have been called #becomeForwardCopyHash: (plus a big,
fat comment). As it is, it is not strictly a one way become method, it is a
combination method instead, and its comment does not reflect that.

Best,
Florin

On Mon, Jan 24, 2022 at 3:00 AM Marcel Taeumel
wrote:

>
> 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
>
> Best,
> Marcel
>
> Am 24.01.2022 08:04:38 schrieb Clément Béra :
> 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
> 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
Clément and Marcel,

You are right, other use cases are supported, and there is probably no need for a separate setIdentityHash: primitive.

I guess I would not have minded if the method currently called #becomForward: would have been called #becomeForwardCopyHash: (plus a big, fat comment). As it is, it is not strictly a one way become method, it is a combination method instead, and its comment does not reflect that.

Best,
Florin

On Mon, Jan 24, 2022 at 3:00 AM Marcel Taeumel <marcel.taeumel at hpi.de [mailto:marcel.taeumel at hpi.de]> wrote:

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 [mailto: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.github.io/]

https://clementbera.wordpress.com/ [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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