R devel - Aug 2021 - Double to uint64_t on M1

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
	[[alternative HTML version deleted]]

Dipterix,

this has nothing to do with R. 2^63 is too large to be represented as singed
integer, so the behavior is undefined - to quote from the C99 specs (6.3.1.4):

"If the value of the integral part cannot be represented by the integer
type, the behavior is undefined."

Your subject doesn't match your question as the uint64_t conversion is
well-defined and the same on both platforms, but the conversion to int64_t in
undefined.

Cheers,
Simon

> On 12/08/2021, at 10:50 AM, Dipterix Wang <dipterix.wang at
gmail.com> wrote:
> 
> 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
> 	[[alternative HTML version deleted]]
> 
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