[Rcpp-devel] Forcing a shallow versus deep copy

[baptiste auguie]

Hi,

That's great, thanks for considering this!

Following this discussion, I went to browse through my code looking for
wrap() and as() statements that could benefit from a speed-up of memory
reuse. Of course I didn't find any.
I switched to using Modules when they were introduced, the code being much
nicer to read, and these conversions only happen behind the scene.
My c++ functions thus only deal with native Armadillo / C++ objects, and I
leave it up to the modules to magically do the required conversions in and
out. It's a brilliant interface, very readable.

>From what I understand, however, the resulting code can often lose a factor
2-3 in speed, compared to the now much more verbose alternative of
explicitly converting and sharing the memory with this type of code:

arma::mat A(M.begin(), M.rows(), M.cols(), false);

>From this perspective, the possibility of setting copy_aux_mem to false in
as(), as used by modules, would be very welcome.

Best regards,

baptiste


On 11 July 2013 10:22, <romain at r-enthusiasts.com> wrote:

>
> Hello,
>
> This comes up every now and then, I think we can find a syntax to initiate
> an arma::mat that would allow what you want.
>
> It is not likely it will come via attributes. The idea is to keep them
> simple. The solutions I see below would eventually lead to clutter, and we
> are heading in the less clutter direction.
>
> I'll think about it and propose something.
>
> Romain
>
> Le 2013-07-11 14:32, Changi Han a écrit :
>
>  Hello,
>>
>> I think I (superficially) understand the difference between:
>>
>> // [[Rcpp::export]]
>> double sum1(Rcpp::NumericMatrix M) {
>>     arma::mat A(M.begin(), M.rows(), M.cols(), false);
>>      return sum(sum(A));
>> }
>>
>> // [[Rcpp::export]]
>> double sum2(arma::mat A) {
>>     return sum(sum(A));
>> }
>>
>> Partly out of laziness, partly because sum2 is more elegant, and
>> partly to avoid namespace pollution, I was wondering if there is a way
>> to "force" a "shallow" copy in sum2.
>>
>> If not, then may I submit a low priority feature request. An
>> attribute? Some thing like:
>>
>> // [[Rcpp::export]]
>> double sum2(arma::mat A) {
>>     // [[ Rcpp::shallow ( A ) ]]
>>      return sum(sum(A));
>> }
>>
>> Or (akin to C++11 generalized attributes)
>>
>> // [[Rcpp::export]] { [[ Rcpp::shallow ( A ) ]] }
>> double sum2(arma::mat A) {
>>     return sum(sum(A));
>>  }
>>
>> An alternative is to have an argument in sourceCpp that takes a
>> list/vector of objects that are to be shallow or deep copied.
>>
>> For example in sum1, if M is changed within the function before
>> casting to the arma::mat, then might be cleaner to add M to a
>> list/vector of objects to be deep copied rather than cloning M within
>> sum1: leads to one fewer variable name.
>>
>> Just a thought. I can certainly live with the additional step. As
>> always, thanks for all the Rcpp goodness.
>>
>> Cheers,
>> Changi Han
>>
>
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