Faster NA checking with punning

NA_punning.cpp

#include <Rcpp.h>
using namespace Rcpp;

union DoublePunner {
  double d;
  uint64_t p;
};

static DoublePunner NA_PUNNED = { NA_REAL };

// [[Rcpp::export]]
LogicalVector IsNA_(NumericVector x) {
  int n = x.size();
  LogicalVector output = no_init(n);
  for (int i=0; i < n; ++i) {
    DoublePunner xi = { x[i] };
    output[i] = xi.p == NA_PUNNED.p;
  }
  return output;
}

// [[Rcpp::export]]
LogicalVector memcmp_IsNA_(NumericVector x) {
  int n = x.size();
  LogicalVector output = no_init(n);
  for (int i=0; i < n; ++i) {
    output[i] = memcmp( 
      (char*) (&x[i]),
      (char*) (&NA_REAL),
      sizeof(double)
    ) == 0;
  }
  return output;
}

// [[Rcpp::export]]
LogicalVector R_IsNA_(NumericVector x) {
  int n = x.size();
  LogicalVector output = no_init(n);
  for (int i=0; i < n; ++i) {
    output[i] = R_IsNA(x[i]);
  }
  return output;
}

/*** R
library(microbenchmark)
noNA <- rnorm(1E6)
allNA <- rep(NA_real_, 1E6)
identical( IsNA_(noNA), R_IsNA_(noNA) )
identical( IsNA_(noNA), memcmp_IsNA_(noNA) )
microbenchmark(
  IsNA_(noNA),
  memcmp_IsNA_(noNA),
  R_IsNA_(noNA)
)
microbenchmark(
  IsNA_(allNA),
  memcmp_IsNA_(allNA),
  R_IsNA_(allNA)
)
*/

# > microbenchmark(
# +   IsNA_(noNA),
# +   memcmp_IsNA_(noNA),
# +   R_IsNA_(noNA)
# + )
# Unit: microseconds
#                expr      min       lq   median       uq      max neval
#         IsNA_(noNA)  884.262 1099.243 1174.779 1270.940 5260.795   100
#  memcmp_IsNA_(noNA)  933.734 1164.753 1187.450 1282.278 4796.834   100
#       R_IsNA_(noNA) 3198.864 3481.547 3507.371 3661.630 6710.078   100
# 
# > microbenchmark(
# +   IsNA_(allNA),
# +   memcmp_IsNA_(allNA),
# +   R_IsNA_(allNA)
# + )
# Unit: microseconds
#                 expr      min       lq   median       uq      max neval
#         IsNA_(allNA)  888.197 1131.228 1171.039 1259.298 4852.233   100
#  memcmp_IsNA_(allNA)  915.552 1140.643 1187.067 1254.516 4892.021   100
#       R_IsNA_(allNA) 2941.444 3200.485 3226.592 3415.858 7474.285   100

It would be interesting to split the tests in two. What is the peformance when :

• all values in x are NA
• no values in x are NA

Done, seems independent (more or less); more importantly, type punning still wins in each case.

One could get identical performance with probably more readable semantics simply using memcmp.