[Rd] reference counting problem in .Primitive's?

[William Dunlap]

> -----Original Message-----
> From: luke at stat.uiowa.edu [mailto:luke at stat.uiowa.edu] 
> Sent: Thursday, April 23, 2009 11:06 AM
> To: William Dunlap
> Cc: r-devel at r-project.org
> Subject: Re: [Rd] reference counting problem in .Primitive's?
> 
> On Thu, 23 Apr 2009, William Dunlap wrote:
> 
> > I think the following rather wierd expressions show a problem in how
> > some of the .Primitive functions evaluate their arguments.  
> I haven't
> > yet thought of a way that a nonabusive user might run into 
> this problem.
> > In each case the first argument, x, is modified in the course of
> > evaluating the second argument and then modified x gets used
> > as the first argument:
> >
> >> x<-as.integer(1:5); y <- x + { x[3]<-33L ; 1L } ; y
> > [1]  2  3 34  5  6
> >> x<-2^(0:4) ; y <- log(x, { x[3]<-64 ; 2 }) ; y
> > [1] 0 1 6 3 4
> >
> > The reason I think it looks like a sharing problem (and not an order
> > of evaluation problem) is that if your modification to x 
> causes it to
> > use a new block of memory then the unmodified version of x gets
> > used as the first argument.  E.g.,
> >
> >> x<-as.integer(1:5) ; y <- x + { x[3]<-33.3; 1L} ; y
> > [1] 2 3 4 5 6
> >
> > I haven't yet thought of a way that a nonabusive user might run
> > into this problem.

An hour after writing this one of our support folks sent me some
user-written code that contained something very close to this idiom;
the second argument to ":" is an altered version of the first argument:

   lengths<-5:1 ; start<-1
   for(i in seq(along=lengths)) {
        thisSeq <- start:((start <- start + lengths[i])-1)
        print(thisSeq)
   }
   [1] 1 2 3 4 5
   [1] 6 7 8 9
   [1] 10 11 12
   [1] 13 14
   [1] 15

That works.  However, if that user had also used 'start[] <- ' instead
of 'start <- ' then they would have run into this bug:

  lengths<-5:1 ; start<-1
  for(i in seq(along=lengths)) {
        thisSeq <- start:((start[] <- start + lengths[i])-1)
        print(thisSeq)
  }
  [1] 1 2 3 4 5
  [1] 10  9
  [1] 13 12
  [1] 15 14
  [1] 16 15

If they use start[] or start[1] consistently in the call to ":" then
they
don't hit the bug.

> 
> You are probably right.  I have not yet looked at the code but am
> virtually certain it does not try to temporarily bump up the NAMED
> values on argument values.  Doing so would cure this but probably at
> serious cost to performance, as NAMED values of 2 cannot be brought
> down again and so cause copying on next modify. (Might be worth
> running some tests on that though to see what the cost would be).

So, if NAMED were not limited to 0,1,or 2 this sort of thing might be
avoided with less pain?

> I'm not sure if it is written anywhere that argunments of primitives
> (BUILTINS in articular as those are always strict; SPECIALS can be
> non-strict but log is strict) are evaluated in any particular order.
> All these examples are consistent with _some_ evaluation order, but
> not the same one.  It might be possible to show that the results
> obtained in these situations will always be consistent with some
> evaluation order, in which case documenting that order of evaluation
> is unspecified would be good enough form me.  It may also be possible
> that an order that does compound expressions first and then symbols
> would also solve the issue (I don't think I would want to do this in
> the interpreter though because of the performance overhead.)
> 
> luke
> 
> 
> >
> > Bill Dunlap
> > TIBCO Software Inc - Spotfire Division
> > wdunlap tibco.com
> >
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
> 
> -- 
> Luke Tierney
> Chair, Statistics and Actuarial Science
> 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 at stat.uiowa.edu
> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu
>