gcc interprets -0 incorrectly

gcc interprets -0 incorrectly

[Justin Pryzby]

/*
 * GCC interprets -0 as -INT_MIN==-INT_MAX - 1 (See <limits.h>).
 * It knows how to overflow from positive to negative,
 * and from negative to positive.
 *
 * However, I object to this interpretation,
 * even if it does allow an int to hold an extra number;
 * From algebra: it is clear that for all x=-x, 2x=0, x=0.
 * Also, for all x=0, 2x=0, x=-x.
 * That is, x=-x implies that x=0, AND x=0 implies that x=-x.
 *
 * Please CC: me in all replies, as I am not subscribed to the list.
 *
 * Justin
 */


#include <stdio.h>
#include <assert.h>

int main()
{
	int i=0;
	struct byte_struct {
		int a:1;
		int b:6;
		int c:1;
	} *byte;

	byte=(struct byte_struct *) (((char *) &i)+3);
	byte->c=1;

	// -2147483648
	printf("%d\n", i);

	// warning: this decimal constant is unsigned only in ISO C90.
	i=-2147483648;
	i--;
	printf("%d\n", i);

	i=2147483647;
	i++;
	printf("%d\n", i);

	return 0;
}

Re: gcc interprets -0 incorrectly

[Zack Weinberg]

On Fri, Dec 06, 2002 at 05:43:13PM -0500, Justin Pryzby wrote:
> /*
>  * GCC interprets -0 as -INT_MIN==-INT_MAX - 1 (See <limits.h>).
>  * It knows how to overflow from positive to negative,
>  * and from negative to positive.

No, both -0 and 0 are represented as 0x00000000 in 32-bit
twos-complement integer arithmetic.  Your code does not calculate -0.

> 	int i=0;
> 	struct byte_struct {
> 		int a:1;
> 		int b:6;
> 		int c:1;
> 	} *byte;
> 
> 	byte=(struct byte_struct *) (((char *) &i)+3);
> 	byte->c=1;

This has undefined behavior.  However, assuming it does what a literal
translation to assembly language would do, its effect is to set the
high bit of a four-byte little-endian integer to 1.  In other words,
it's equivalent to

 int i = 0x80000000U;

That has the numeric value -2147483648, which is indeed INT_MIN in
32-bit twos-complement arithmetic.  You seem to think that integer
arithmetic is sign-and-magnitude, which is not true for any
architecture supported by GCC.

Please read up on twos-complement arithmetic.

zw

Re: gcc interprets -0 incorrectly

[Neil Booth]

Justin Pryzby wrote:-

> 	// warning: this decimal constant is unsigned only in ISO C90.
> 	i=-2147483648;

Is this warning what's confusing you?

Neil.

Re: gcc interprets -0 incorrectly

[Segher Boessenkool]

Justin Pryzby wrote:
> 
>  * From algebra: it is clear that for all x=-x, 2x=0, x=0.

x=0 is not implied by 2x=0 in this ring (the ring of integers
modulo some power of two); this ring is not an integral domain,
and 2 is a zero divisor.

-MIN_INT == MIN_INT  is perfectly fine, mathematically.


Segher

Re: gcc interprets -0 incorrectly

[Gaute B Strokkenes]

On Thu, 19 Dec 2002, segher@koffie.nl wrote:
> Justin Pryzby wrote:
>> 
>>  * From algebra: it is clear that for all x=-x, 2x=0, x=0.
> 
> x=0 is not implied by 2x=0 in this ring (the ring of integers
> modulo some power of two); this ring is not an integral domain,
> and 2 is a zero divisor.
> 
> -MIN_INT == MIN_INT  is perfectly fine, mathematically.

OTOH a lot of interesting theory only applies when 2 != 0;
specifically because the ability to divide by 2 turns out to be
particularly useful.

One of my lecturers when I was a maths undergraduate once said that
some people liked to do stuff in fields with characteristic 2 as a
kind of special spice; but that people who wish to remain sane should
stay away from them...

-- 
Gaute Strokkenes                        http://www.srcf.ucam.org/~gs234/
A KAISER ROLL?!  What good is a Kaiser Roll without a little
 COLE SLAW on the SIDE?