[Numpy-discussion] Uint64 casting bug for MSVC builds

[Christoph Gohlke]

On 12/3/2014 8:44 AM, Matthew Brett wrote:
> Hi,
>
> I just noticed this using Christophe Gohlke's MKL builds of numpy:
>
>>>> import numpy as np
>>>> val = 2**63 + 2**62
>>>> np.float64(val)
> 1.3835058055282164e+19
>>>> np.float64(val).astype(np.uint64)
> 9223372036854775808
>
> In general it seems that floats get clipped at 2**63 when casting to
> uint64.   This appears to be a bug in MSVS express 2010 (the only
> version I tested):
>
> <test_cast.c>
> #include <stdio.h>
> #include <math.h>
>
> int main(int argc, char* argv[]) {
>      double fval = pow(2, 63) + pow(2, 11);
>      double fval2;
>      unsigned long long int ival = fval;
>      fval2 = ival;
>      printf("Float %f\n", fval);
>      printf("Integer %f\n", fval2);
>      printf("sizeof ulong %u\n", sizeof(unsigned long long int));
> }
> </test_cast.c>
>
> Z:\>test_cast.exe
> Float 9223372036854777900.000000
> Integer 9223372036854775800.000000
> sizeof ulong 8
>
> I realize there's nothing much numpy can do about this, just thought I'd
> let y'all know.
>
> Cheers,
>
> Matthew
>

This is a know issue with older (<= 2010) 32 bit msvc, which uses x87 
instead of SSE instructions. See also 
<https://github.com/scipy/scipy/blob/master/scipy/ndimage/tests/test_datatypes.py#L51>. 


Christoph