Internal usage of Rcpp_fast_eval in Rcpp

[thirdwing]

This has been discussed on the Rcpp mailing list: http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2018-June/010029.html

Hi all,

I've followed with interest the development of the new evaluation API.
Now that it's finally merged, I was testing it. Perhaps I'm mistaken,
but shouldn't we expect a performance improvement in code such as the
following?

Rcpp::sourceCpp(code='
  #include <Rcpp.h>
  using namespace Rcpp;

  // [[Rcpp::export]]
  void old_api(Function func, int n) {
    for (int i=0; i<n; i++) func();
  }'
)

Rcpp::sourceCpp(code='
  #define RCPP_PROTECTED_EVAL
  #include <Rcpp.h>
  using namespace Rcpp;

  // [[Rcpp::export]]
  void new_api(Function func, int n) {
    for (int i=0; i<n; i++) func();
  }'
)

func <- function() 1
system.time(old_api(func, 1e5))
system.time(new_api(func, 1e5))

Regards,
Iñaki

[thirdwing]

This is because the Rcpp Function is still using the old Rcpp_eval: https://github.com/RcppCore/Rcpp/blob/master/inst/include/Rcpp/Function.h#L82

After changing to Rcpp_fast_eval in Rcpp Function, the timing result on my laptop is as follow:

> system.time(old_api(func, 1e5))
   user  system elapsed
  1.025   0.000   1.025
> system.time(new_api(func, 1e5))
   user  system elapsed
  0.025   0.000   0.025 

Should we port all the internal usage of Rcpp_eval in Rcpp to Rcpp_fast_eval for better performance? @kevinushey

[eddelbuettel]

We could try in a branch and run a reverse depends over it.

[Enchufa2]

I just tested this with simmer and I get an impressive x4 performance improvement in a reference test I have (!), so I'm very interested in these changes. Therefore, if no one is leading this already, I'd be more than glad to start a PR and check it against different R versions via rocker. I don't have the means to run such a heavy revdep check in a reasonable time though.

[eddelbuettel]

A PR would be great! I think we all would be happy to review / chip in.

I can do the rev.dep across all (first-level) dependencies; that is set up. It would be nice if you could help with a simpler R CMD check across older R releases; that is not something I have scripted yet here. And for what it is worth my main machine is still on R 3.4.4 so I can cover 'oldrel' ;-)