[Rd] speed up R_IsNA, R_IsNaN for vector input

[Jan Gorecki]

Dear R developers,

I spotted that R_isNA and R_IsNaN could be improved when applied on a
vector where we could take out small part of their logic, run it once,
and then reuse inside the loop.
I setup tiny plain-C experiment. Taking R_IsNA, R_IsNaN from R's
arithmetic.c, and building R_vIsNA and R_vIsNaN accordingly.
For double input of size 1e9 (having some NA and NaN) I observed
following timings:

R_IsNA    6.729s
R_vIsNA   4.386s

R_IsNaN   6.874s
R_vIsNaN  4.479s

ISNAN     4.392s

It looks like R_vIsN(A|aN) are close to ISNAN (which just wraps to
math.h::isnan).
Should I follow up with a patch?

The experiment is a single nan.c file of 127 lines (includes R C
funs). Large enough to not paste in the email. Here is the link:
https://gist.github.com/jangorecki/c140fed3a3672620c1e2af90a768d785

Run it as:

gcc nan.c -lm
./a.out R_vIsNA 8
./a.out R_IsNA 8
./a.out R_vIsNaN 8
./a.out R_IsNaN 8
./a.out ISNAN 8

Best regards,
Jan Gorecki