[Rcpp-devel] [Rd] R_PreserveObject, R_ReleaseObject : reference counting needed ?

[Simon Urbanek]

On Jan 2, 2010, at 5:07 AM, Romain Francois 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.
> 

I don't think so - the only documentation is the comment in the source.


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

Preserve/Release are for managing objects that are supposed to survive past the call and are not tied to any other R object. PROTECT/UNPROTECT are for temporary preservation within a call.

Although you're right that Preserve/Release is effectively implemented as a stack at the moment it is not stated explicitly anywhere (this goes all the way back to R 0.64 so chances are that only Ross can comment..). However, for practical purposes it would be potentially dangerous to have it work like a flag because you can simply never know whether the same object was not already registered by some other code.


> Reading the source (below, from memory.c) I think not, but some confirmation would help.
> 
> 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 more efficient because all it does is a pointer assignment -- Preserve has to allocate new node and fill it with all parts. On release the extra node is still floating in the GC pool etc.

Normally there is not really a question of choice - within a call you want to use PROTET/UNPROTECT and for anything else you simply cannot use it so you have to use Preserve/Release. As a side note Preserve/Release is merely a convenience call, it is often more efficient to simply assign the object to another object you have control of (which is all Preserve really does).

Cheers,
Simon