[Rcpp-devel] Forcing a shallow versus deep copy

[Gabor Grothendieck]

1. Just to be clear what we have been discussing here is not just how to
avoid copying but how to avoid copying while using as and wrap
or approaches that automatically generate as and wrap.  I was already
aware of how to avoid copying using Armadillo how to use Armadillo types
as arguments and return values to autogen as and wrap.  The problem is
not that but that these two things cannot be done at once - its either or.

2. Regarding the quesiton of performance impact there are two situations
which should be distinguished:

i. We call C++ from R and it does some processing and then returns and
we don't call it again. In that case its likely that copying or not won't
make a big difference or at least it won't if the actual C++ computation
time is large coimpared to the time spent in copying.

ii. We factor out the inner loop of the code and only recode that in C++
and repeatedly call it many times.  In that case the copying is multiplied
by the number of iterations and might very well have a significant impact.

On Thu, Jul 11, 2013 at 6:55 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
>
> Everybody has this existing example in their copy of Armadillo.
>
> I am running it here from SVN rather than the installed directory, but this
> should not make a difference. Machine is my not-overly-powerful thinkpad used
> for traveling:
>
> edd at don:~/svn/rcpp/pkg/RcppArmadillo/inst/examples$ r fastLm.r
> Loading required package: methods
>
> Attaching package: ‘Rcpp’
>
> The following object is masked from ‘package:inline’:
>
>     registerPlugin
>
>                        test replications relative elapsed user.self sys.self
> 2         fLmTwoCasts(X, y)         5000    1.000   0.184     0.204    0.164
> 1          fLmOneCast(X, y)         5000    1.011   0.186     0.200    0.172
> 4   fastLmPureDotCall(X, y)         5000    1.141   0.210     0.236    0.184
> 3          fastLmPure(X, y)         5000    2.027   0.373     0.412    0.332
> 6              lm.fit(X, y)         5000    2.685   0.494     0.528    0.456
> 5 fastLm(frm, data = trees)         5000   36.380   6.694     7.332    6.028
> 7     lm(frm, data = trees)         5000   42.734   7.863     8.628    7.068
> edd at don:~/svn/rcpp/pkg/RcppArmadillo/inst/examples$
>
> What we are talking about here is the difference between 'fLmTwoCasts' and
> 'fLmOneCasts'.  If you use larger objects, the different with be larger.  But
> the relative differences are tiny.
>
> It would be nice to make this more elegant, and I look forward to Romain's
> proposals, but methinks that we may well have bigger fish to fry.
>
> Dirk, still in Sydney
>
> --
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
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