[Rcpp-devel] use of auxiliary functions

[baptiste auguie]

"disguise" might be a more appropriate name? I like the idea.

Thanks,

baptiste



On 11 August 2010 21:59, Romain Francois <romain at r-enthusiasts.com> wrote:
> Hi,
>
> For one thing, the construct :
>
> arma::mat B = Rcpp::as<  arma::mat>( A ) ;
>
> probably makes copies along the way. I'm not sure what right now, but we
> might a different construct, something like:
>
> arma::mat B ;
> Rcpp::borrow<arma::mat>( B, A ) ;
>
> where borrow would have this signature perhaps
>
> template <typename T>
> void borrow( T& target, SEXP origin ) ;
>
>
> This would avoid some of these unnecessary copies. Not sure borrow is the
> right verb though.
>
> Also, the compiler might implicitely inline dum1 and dum2 because they are
> so simple, so the performance might be the same, I'm not sure about that
> though ...
>
> Romain
>
>
>
> Le 11/08/10 21:30, baptiste auguie a écrit :
>>
>> I think I understand the principle, however with my best effort I
>> cannot find a test case. Here I thought I'd be passing an arbitrarily
>> large arma matrix to some function, but again the timing is not
>> convincing (in fact, I had to stop the execution once with the dummy2
>> version),
>>
>> using namespace Rcpp ;
>> using namespace RcppArmadillo ;
>>
>> extern "C" {
>>
>>   arma::mat dum1(arma::mat B)
>>   {
>>     return (B*38);
>>   }
>>
>>   arma::mat dum2(const arma::mat&  B)
>>   {
>>     return (B*38);
>>   }
>>
>>   RCPP_FUNCTION_2(NumericMatrix, dummy1, NumericMatrix A, int N) {
>>     arma::mat B = Rcpp::as<  arma::mat>( A ) ;
>>     return wrap(dum1(repmat(B, N, N)));
>>   }
>>
>>   RCPP_FUNCTION_2(NumericMatrix, dummy2, NumericMatrix A, int N) {
>>     arma::mat B = Rcpp::as<  arma::mat>( A ) ;
>>     return wrap(dum2(repmat(B, N, N)));
>>   }
>>
>> }
>>
>> }
>>
>> Regards,
>>
>> baptiste
>>
>>
>> On 11 August 2010 19:21, Davor Cubranic<cubranic at stat.ubc.ca>  wrote:
>>>
>>> On August 11, 2010 02:48:23 am romain at r-enthusiasts.com wrote:
>>>>
>>>> Le 10 août 2010 à 22:10, baptiste auguie
>>>
>>> <baptiste.auguie at googlemail.com>  a écrit :
>>>>>
>>>>> OK, thanks. I have not been able to produce a minimal code that
>>>>> would exhibit an improved performance using this
>>>>> passing-by-reference idea.
>>>>
>>>> Hi,
>>>>
>>>> It would usually make a difference when copying the object that is
>>>> passed by value is "expensive" to copy. Rcpp objects are very cheap
>>>> to copy and both copies refer to the same actual data (the same
>>>> SEXP).
>>>>
>>>> For internal code my recommendation would be to always use pass by
>>>> reference and do an explicit call to clone when a real copy
>>>> (different SEXP) is needed.
>>>
>>> In other words, if you were passing large arma::mat's, then const
>>> references should make a real difference.
>>>
>>> Davor
>>>
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>
>
> --
> Romain Francois
> Professional R Enthusiast
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>