[Rcpp-devel] RcppEigen: .conservativeResize() on 32-bit Windows

[Douglas Bates]

On Wed, Apr 4, 2012 at 8:53 AM, Andreas Alfons
<andreas.alfons at econ.kuleuven.be> wrote:
> Dear Rcpp devs, dear list,
>
> First of all, thanks a lot for all the effort you put into the Rcpp packages!
>
> I wrote a package using RcppEigen, where in one function I need to
> iteratively add and remove vector elements a lot. On 32-bit Windows 7,
> the program crashes the R session. It works fine on Ubuntu Oneiric, OS
> X Lion (32-bit and 64-bit) and 64-bit Windows 7.
>
> I could narrow the problem down to strange behavior of the function
> .conservativeResize(), which I use a lot to resize the vectors. Here
> is a minimal example:
>
>
> library("inline")
> test <- cxxfunction(signature(R_p = "integer", R_max = "integer"),
>    'int p = as<int>(R_p), max = as<int>(R_max);
>        // initialize vector with sequence from 0 to p-1
>        Eigen::VectorXi x(p);
>        for(int j = 0; j < p; j++) {
>            x(j) = j;
>        }
>        // iteratively remove vector elements
>        int k = 0;
>        while(k < max) {
>            p--;
>            x.segment(0, p) = x.tail(p).eval();  // move last elements up
>            x.conservativeResize(p);              // resize vector
>            k++;
>        }
>        return wrap(x);',
>    plugin = "RcppEigen")
>

Difficulties with this code would not originate in the R garbage
collection.  You are only passing two arguments as SEXPs and those are
immediately converted to local int's.  After that everything is done
in objects allocated and controlled by Eigen.

The fact that you are compiling the code for 32-bit R running in
64-bit Windows would suggest that somewhere along the line the
compiler may be confused about the size of objects.  It should be the
case that the base integer type is a 32-bit integer but I would not be
surprised if there are some gliches.  At times it is difficult to mix
and match the include files for the mingw system used to compile R for
Windows and the Windows include files, wherever they are.

As I don't have easy access to a Windows system with the R development
tools installed I don't think I can help debug this.

> test(10, 3)
>
>
> On 32-bit Windows 7, I get the output
> [1] 55862592        2        3        4        5        6        7
>
> The example is not reproducible all the time, sometimes I get the correct output
> [1] 3 4 5 6 7 8 9
>
> But restarting the R session once or twice worked for me to get some
> incorrect output (not always the same as above). Could the problem be
> that the R garbage collector deletes some memory there for some
> reason? A standalone C++ version of the example (without going through
> R) seems to work fine on 32-bit Windows 7.
>
> Resizing matrices seems to work correctly on 32-bit Windows 7, too, so
> maybe I this will provide a workaround for now.
>
> Thanks in advance for your help and sorry for the quite long mail!
>
> - Andreas
>
>
> --
> Andreas Alfons
> Faculty of Business and Economics, KU Leuven
> www.econ.kuleuven.be/andreas.alfons/public/
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