[Rcpp-devel] copying or not when convering to arma

[Gabor Grothendieck]

On Thu, Jul 11, 2013 at 3:50 AM, Dirk Eddelbuettel <edd at debian.org> wrote:
>
> On 10 July 2013 at 11:58, Gabor Grothendieck wrote:
> | Suppose we have this program where we have started and ended with SEXP
> | and written out the as and wrap conversions to make them explicit.
> |
> | How many times is the data copied?
> | Does as copy the underlying data?
> | Does wrap?
> | Is it copied elsewhere?
> | Which file in the source code would one look to determine this?
> | If it does copy the data is there some way to tell as and wrap to just
> | point to it to avoid copying?
> |
> | #include <RcppArmadillo.h>
> | // [[Rcpp::depends(RcppArmadillo)]]
> | // [[Rcpp::export]]
> | SEXP test(SEXP v) {
> |    arma::vec va = Rcpp::as<arma::vec>(v);
> |    SEXP vs = Rcpp::wrap(va);
> |    return vs;
> | }
>
> You can reduce this to
>
> #include <RcppArmadillo.h>
> // [[Rcpp::depends(RcppArmadillo)]]
> // [[Rcpp::export]]
> arma::vec gabortest(arma::vec v) {
>    return v;
> }
>
> because as<> and wrap() can get called automagically given that
> RcppArmadillo provides them.
>
> You want to start with the few header files we add to Armadillo in
> RcppArmadillo (ie above the armadillo_bits/ directory which is verbatim
> Armadillo.
>
> In this case, a copy is made. How to avoid this has been discussed a dozen
> times here and is shown in FastLm.cpp as well.
>
> As for counts of objects:  I'd rather trust a good memory profiler than my
> word :)  But approximately, one each as we have to go from R/Rcpp to
> Armadillo (as use the easy way; for alternatives see above).  If you use just
> Rcpp no copies are being made.
>
> Cheers from Sydney,  Dirk
>
> | # run it from R
> | sourceCpp("test.cpp")
> | test(c(1.2, 1.3, 1.4))
> |
> |
> | --
> | Statistics & Software Consulting
> | GKX Group, GKX Associates Inc.
> | tel: 1-877-GKX-GROUP
> | email: ggrothendieck at gmail.com
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>
> --
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com

It might be nice if this were controllable by the user via something like:

// [[Rcpp::nocopy]]

or other syntax so that one could still use as and wrap (or approaches which
generate as and wrap automatically) while avoiding the copying.

--
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com