Hi,

This demo script requires a compiler (eg gcc) that provides the _Decimal64 type:

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;

  d64 *= -1.DD;

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

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

  return 0;
}

EOC

foo();
[download]

That outputs:

!= 0
== -0
[download]

which tells me that -0 != 0 (and that is contrary to IEEE standards).

However, if I copy'n'past that C code across to a separate file and run it as a C program (using the very same compiler) I get correct output:

#include <stdio.h>

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

  d64 *= -1.DD;

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

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

  return 0;
}
[download]

AFAICS, all that has changed is that "foo" has been renamed to "main".
Yet, this now correctly outputs:

== 0
== -0
[download]

I'm seeing this behaviour on both Windows and Linux for a variety of perl versions and a variety of gcc compilers from 4.7.0 to 7.1.0 - so I'm hopeful that anyone else who has a gcc-4.x.x compiler (or later) will have no trouble reproducing the behaviour. Only other thing needed is Inline::C.

I've gazed upon the C code that Inline::C generates, and I've run the script through the C pre-processor, but I haven't managed to spot anything that helps me understand how this anomaly exists.
I've also spent some time trying various things that I hoped might trick Inline::C into behaving properly and I did find that this works:

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) {

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

  if(d64 != -0.DD) printf("!= -0\n");
  else printf("== -0\n");
}

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

  d64 *= -1.DD;

  _is_zero(d64);

  return 0;
}

EOC

foo();
[download]

All I've done there is to move the test that foo() was previously doing, into a separate sub. That produces correct output of:

== 0
== -0
[download]

Unfortunately the same approach doesn't work for the _Decimal128 type, where the same issue also arises.

There's nothing of much immediate importance with this ... but I can't stop scratching at it ;-)

Your insights are most welcome

Cheers,
Rob

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