[Rd] R-devel Digest, Vol 83, Issue 2

[Laurent Gautier]

[Disclaimer: what is below reflects my understanding from reading the R 
source, others will correct where deemed necessary]

On 1/2/10 12:00 PM, r-devel-request at r-project.org wrote:
>
> Hello,
>
> We are currently making lots of changes to Rcpp (see the open Rcpp
> mailing list if interested [1] in the details).
>
> We are now using [2] R_PreserveObject and R_ReleaseObject to manage
> garbage collection instead of the PROTECT/UNPROTECT dance. This seems to
> work well, but I was wondering if there was documentation about it.

The most precise technical documentation is in memory.c
PROTECT is an alias for Rf_protect, itself an alias for
SEXP protect(SEXP s);
and uses a stack (R_PPStack) to store protected objects.

> In particular, if we preserve the same SEXP twice (or more), should we
> implement some sort of reference counting ?

This depends on the requirements for your system.

For example, in rpy2 I added a reference counting layer(*) because I 
wanted to allow several Python objects to share the same underlying R 
object, but that's not currently(*) counting how many times an object 
should be freed.
(*: imperfect, but currently doing a very decent job - details upon 
request).

That kind of feature could be provided by R's C-level API, since this 
could be seen of general use as well as give an opportunity to improve 
the performances of the R_PreservedObject/R_ReleaseObject duo whenever a 
lot of objects are protected and/or external code is 
protecting/releasing objects through a FIFO proxy.


> Reading the source (below, from memory.c) I think not, but some
> confirmation would help.

I understand the code in memory.c like an object preserved twice needs 
to be freed twice: R_PreciousList is just a (linked) list, and 
"R_PreserveObject(object)" adds the object to the beginning of the list 
while "R_ReleaseObject(object)" removes the first "object" found from 
the list.



> void R_PreserveObject(SEXP object)
> {
>       R_PreciousList = CONS(object, R_PreciousList);
> }
>
> static SEXP RecursiveRelease(SEXP object, SEXP list)
> {
>       if (!isNull(list)) {
> 	if (object == CAR(list))
> 	    return CDR(list);
> 	else
> 	    CDR(list) = RecursiveRelease(object, CDR(list));
>       }
>       return list;
> }
>
> void R_ReleaseObject(SEXP object)
> {
>       R_PreciousList =  RecursiveRelease(object, R_PreciousList);
> }
>
>
> I'd also be interested if there is some ideas on the relative efficiency
> of the preserve/release mechanism compared to PROTECT/UNPROTECT.

PROTECT/UNPROTECT is trading granularity for speed. It is a stack with 
only tow operations possible:
- push 1 object into the stack
- pull (unprotect) N last objects from the stack


HTH,


L.




> Thanks,
>
> Romain
>
> [1]http://lists.r-forge.r-project.org/pipermail/rcpp-devel/
> [2]
> http://r-forge.r-project.org/plugins/scmsvn/viewcvs.php/pkg/src/RObject.cpp?rev=255&root=rcpp&view=markup
>
> -- Romain Francois Professional R Enthusiast +33(0) 6 28 91 30 30
> http://romainfrancois.blog.free.fr |- http://tr.im/IW9B : C++ exceptions
> at the R level |- http://tr.im/IlMh : CPP package: exposing C++ objects
> `- http://tr.im/HlX9 : new package : bibtex