[Rd] nchar(x, type = "bytes") seems slower than it could be

[Hugh Parsonage]

While profiling some C code, I rolled my own nchar function which
appears to be much faster than base R's (25 times faster for a 10M
length vector).  Obviously base::nchar provides significantly more
features than my barebones function (C snippet below); however, for
argument type = "bytes" it seems that the R_nchar and do_nchar
functions do not actually do anything more than this function.

My suspicion is that I have overlooked some subtlety in the base R
code, or that my benchmarks are not representative.  Alternatively,
the action in `do_nchar` of preparing the potential error message
before being passed to `R_nchar` may be quite costly indeed.  Or the
function cannot be unswitched from the more complex width and chars
arguments by the compiler.

If I haven't missed something, would a patch be warranted?

SEXP Cnchar(SEXP x) {
  R_xlen_t N = xlength(x);
  SEXP ans = PROTECT(allocVector(INTSXP, N));
  int * restrict ansp = INTEGER(ans);

  // Ignoring NA to avoid the branch has a very small
  // impact on performance.
  for (R_xlen_t i = 0; i < N; ++i) {
    SEXP sxi = STRING_ELT(x, i);
    if (sxi == NA_STRING) {
      ansp[i] = NA_INTEGER;
      continue;
    }
    ansp[i] = length(sxi);
  }
  UNPROTECT(1);
  return ans;
}

x <- rep_len(c(as.character(c(5L, 1:1e6)), NA_character_, 1e6:15e5), 1e7)
Cnchar(x)
90ms
nchar(x, type = "bytes")
2500 ms