As soon as the value is loaded into an FP register, it gets silently converted to a QNan

I was thinking that when we do: 
$packed = pack 'd', '7ff0000000000001';
$d = unpack 'h16', $packed;

[download]
there would be no loading of the double into an FP register.
But I guess that thinking must be wrong as $d is definitely set to '7ff8000000000001', on my Windows box at least.

On my Linux box (also little-endian), however, there's no such problem and I can assign '7ff0000000000001' just fine.
Is this difference a Windows v Linux thing ? ... or is it to be explained in terms of different hardware ?

Do you know of any way that one can assign the value '7ff0000000000001' on Windows ? (I haven't found a way, yet.)

Here's a nice Perl demo (utilising the union you provided) of the Windows/Linux discrepancy: 
use warnings;
use strict;

use Inline C => Config =>
  BUILD_NOISY => 1;

use Inline C => <<'EOC';

#ifdef _WIN32
typedef __int64 BIGGUN;
#else
/* "long" is 64-bit on my linux box */
typedef long BIGGUN;
#endif

typedef union {
    double  d;
    BIGGUN i;
} BITS64;

void bytes(double d) {
  int i;
  void * p = &d;

  for(i = 7; i >=0; i--)
    printf("%02x", ((unsigned char*)p)[i]);

  printf("\n");
}

double get_snan () {
  BITS64 x;

  x.i = 0x7ff0000000000001;

  bytes(x.d);

  return x.d;
}

EOC

print doubleToHex(get_snan());

sub doubleToHex { scalar reverse unpack 'h16', pack 'd', $_[0] }

[download]
On Windows it outputs:
7ff0000000000001
7ff8000000000001

On Ubuntu it outputs:
7ff0000000000001
7ff0000000000001

Cheers,
Rob

In reply to Re^3: unintentional conversion of signaling NaN to quiet NaN by syphilis
in thread unintentional conversion of signaling NaN to quiet NaN by pryrt

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