remove 'clever' NA checks

[kevinushey]

This PR removes the 'clever' NA checking code I implemented some time ago (way back in 2014). I recall benchmarking these as a bit faster than R's own NA checking routines, but I either I was mistaken or compilers have matured such that the performance difference no longer exists:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
LogicalVector is_na(NumericVector x)
{
    std::size_t n = x.size();
    LogicalVector output = no_init(n);
    for (std::size_t i = 0; i < n; i++)
        output[i] = Rcpp::internal::Rcpp_IsNA(x[i]);
    return output;
}

/*** R
library(microbenchmark)

rnd <- rnorm(1E6)
rnd[sample(1:1E6, 1E5)] <- NA
identical(is.na(rnd), is_na(rnd))
microbenchmark::microbenchmark(
    R = is.na(rnd),
    Rcpp = is_na(rnd)
)
*/

I see, using the current Rcpp Rcpp_IsNA implementation,

> microbenchmark::microbenchmark(
+     R = is.na(rnd),
+     Rcpp = is_na(rnd)
+ )
Unit: microseconds
 expr     min      lq     mean   median       uq      max neval cld
    R 551.458 649.605 1397.701  997.121 1179.275 5433.238   100   a
 Rcpp 626.599 767.000 1412.142 1031.284 1219.745 5157.141   100   a

So let's just get rid of the old Rcpp code and delegate back to R's own NA checkers.

[eddelbuettel]

Thumbs up for simpler is better.

I see the removal, but do we need a fill-in to delegate the old access points back to something else? Or do I need more coffee as I am missing something obvious?

[kevinushey]

I don't think we need to fill in anything -- the old parts removed were implementation details living in an internal namespace; any client packages that were using those functions were doing so shouldn't have (and I don't think any packages were).

The APIs Rcpp_IsNA and Rcpp_IsNaN still exist; they just delegate directly to their R counterparts, so nothing has changed on that front.