[R] [External] Why is R making a copy-on-modification after using str?

[iuke-tier@ey m@iii@g oii uiow@@edu]

On Mon, 30 Nov 2020, luke-tierney using uiowa.edu wrote:

> On Mon, 30 Nov 2020, Georg Kindermann wrote:
>
>> Dear list members,
>> 
>> I was wondering why R is making a copy-on-modification after using str.
>> 
>> m <- matrix(1:12, 3)
>> tracemem(m)
>> #[1] "<0x559df861af28>"
>> dim(m) <- 4:3
>> m[1,1] <- 0L
>> m[] <- 12:1
>> str(m)
>> # int [1:4, 1:3] 12 11 10 9 8 7 6 5 4 3 ...
>> dim(m) <- 3:4  #Here after str a copy is made
>> #tracemem[0x559df861af28 -> 0x559df838e4a8]:
>> dim(m) <- 3:4
>> str(m)
>> # int [1:3, 1:4] 12 11 10 9 8 7 6 5 4 3 ...
>> dim(m) <- 3:4 #Here again after str a copy
>> #tracemem[0x559df838e4a8 -> 0x559df82c9d78]:
>
> As of R 4.0.0 it is in some cases possible to reduce reference counts
> internally and so avoid a copy in cases like this. It would be too
> costly to try to detect all cases where a count can be dropped, but it
> this case we can do better. It turns out that the internals of
> pos.to.env were unnecessarily creating an extra reference to the call
> environment (here in a call to exists()). This is fixed in r79528.
> Thanks.
>
>> Also I was wondering why a copy is made when having a Task Callback.
>> 
>> TCB <- addTaskCallback(function(...) TRUE)
>> m <- matrix(1:12, nrow = 3)
>> tracemem(m)
>> #[1] "<0x559dfa79def8>"
>> dim(m) <- 4:3  #Copy on modification
>> #tracemem[0x559dfa79def8 -> 0x559dfa8998e8]:
>> removeTaskCallback(TCB)
>> #[1] TRUE
>> dim(m) <- 4:3  #No copy
>> 
>
> This _may_ be related to references created in the process of
> protecting the return value from evaluation. If so, addressing the
> debug issue raised by Duncan may resolve this. If not, someone will
> have to take a closer look.

It turns out there were some issues with the way calls to the
callbacks were handled. This has been revised in R-devel in r79541.
This example will no longere need to duplicate in R-devel.

Thanks for the report.

luke

>
> Best,
>
> luke
>
>> I am using R version 4.0.3.
>> 
>> Kind regards,
>> Georg
>> 
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>
>

-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney using uiowa.edu
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