My only suggestion is that you dump the values as bits and see where they differ

Definitely worth a try, but it appears not to have helped much.
So .... I changed the original Inline::C script to:

use strict;
use warnings;

use Inline C => Config =>
  BUILD_NOISY => 1,
  USING => 'ParseRegExp',
  #CLEAN_AFTER_BUILD => 0,
  ;

use Inline C => <<'EOC';

#include <stdio.h>

int foo(void) {
  _Decimal64 d64 = 0.DD, neg_zero, pos_zero = 0.DD;
  void *pd64 = &d64, *pneg_zero = &neg_zero, *ppos_zero = &pos_zero;
  int i;

  neg_zero = pos_zero * -1.DD;
  d64 *= -1.DD;

  if(d64 != pos_zero) printf("!= 0\n");
  else printf("== 0\n");

  if(d64 != neg_zero) printf("!= -0\n");
  else printf("== -0\n");

  for(i = 7; i >= 0; i--) 
    printf("%02X", ((unsigned char*)pd64)[i]);
  printf("\n");

  for(i = 7; i >= 0; i--) 
    printf("%02X", ((unsigned char*)pneg_zero)[i]);
  printf("\n");

  for(i = 7; i >= 0; i--) 
    printf("%02X", ((unsigned char*)ppos_zero)[i]);
  printf("\n");

  return 0;
}

EOC

foo();
[download]

It now outputs:

!= 0
== -0
B1C0000000000000
B1C0000000000000
31C0000000000000
[download]

"B1C000..." is a _Decimal64 -0, and "31C000..." a _Decimal64 0.
The first bit is the sign bit, the next 10 bits encode the exponent, and the 54-bit significand is an implicit 0 bit followed by the remaining 53 bits (all 0).
When I run that revised C code as a C program it outputs (correctly):

== 0
== -0
B1C0000000000000
B1C0000000000000
31C0000000000000
[download]

I also modified the Inline::C rendition that was getting it right to:

use strict;
use warnings;

use Inline C => Config =>
  BUILD_NOISY => 1,
  USING => 'ParseRegExp',
  #CLEAN_AFTER_BUILD => 0,
  ;

use Inline C => <<'EOC';

#include <stdio.h>

void _is_zero(_Decimal64 d64) {

  _Decimal64 neg_zero, pos_zero = 0.DD;
  void *pd64 = &d64, *pneg_zero = &neg_zero, *ppos_zero = &pos_zero;
  int i;

  neg_zero = pos_zero * -1.DD;

  if(d64 != pos_zero) printf("!= 0\n");
  else printf("== 0\n");

  if(d64 != neg_zero) printf("!= -0\n");
  else printf("== -0\n");

  for(i = 7; i >= 0; i--) 
    printf("%02X", ((unsigned char*)pd64)[i]);
  printf("\n");

  for(i = 7; i >= 0; i--) 
    printf("%02X", ((unsigned char*)pneg_zero)[i]);
  printf("\n");

  for(i = 7; i >= 0; i--) 
    printf("%02X", ((unsigned char*)ppos_zero)[i]);
  printf("\n");
}

int foo(void) {
  _Decimal64 d64 = 0.DD;

  d64 *= -1.DD;

  _is_zero(d64);

  return 0;
}

EOC

foo();
[download]

It still outputs (correctly):

== 0
== -0
B1C0000000000000
B1C0000000000000
31C0000000000000
[download]

It's as though the != operation (in the problem script) believes that the values it's comparing are unsigned.

Cheers,
Rob

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