[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, 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.

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
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu