[Rd] Double to uint64_t on M1

[Dipterix Wang]

Hi, 

I was trying to convert REALSXP to int64_t in C, then found that converting 2^63 is inconsistent across platforms:


On M1 ARM osx, 2^63 (double) bit converting to `int64_t` becomes 9223372036854775807
On x86_64 ubuntu server, 2^63 (double) bit converting to `int64_t` is -9223372036854775808

I was wondering if this is desired behavior to R?

Here's the code to replicate the results above.

print_bit <- Rcpp::cppFunction(r"(
SEXP print_bit(SEXP obj){

  int64_t tmp1 = *REAL0(obj);
  printf("%lld ", tmp1);

  return(R_NilValue);
}
)")

print_bit(2^63)

Thanks,
- Dipterix
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