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From: fnf@mcdsun.UUCP (Fred Fish)
Newsgroups: comp.lang.c
Subject: Expressions compared to zero in *c*
Message-ID: <374@mcdsun.UUCP>
Date: Tue, 8-Sep-87 12:03:28 EDT
Article-I.D.: mcdsun.374
Posted: Tue Sep  8 12:03:28 1987
Date-Received: Thu, 10-Sep-87 00:47:37 EDT
Organization: Motorola Microcomputer Division, Tempe, Az.
Lines: 173


I am posting this for a coworker, please respond directly to him with
comments or questions.  Thanks.  His email address is

	hao!noao!mcdsun!nud!dragon!jon

-Fred

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Message-ID: <472@dragon.UUCP>
Date: 18 Aug 87 22:47:34 GMT
Reply-To: jon@dragon.UUCP (Jon Kettenhofen, x5656)
Organization: Motorola Microcomputer Division, Tempe, Az.
Lines: 159

Does  (a-b) < 0  imply   a < b   ?  Pick a and b for controversy.

Here's a program, the output of which may cause some uproar.
The answers I like most are the ones which ignore overflow.  Do you
believe that the ANSI standard should define this can of worms or
cop out with the usual "implementation defined"?
		jon kettenhofen, Motorola MCD, Tempe AZ
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/*
    Inconsistent comparison operator when overflow is forced.

    The problem in  these  cases  has  to  do  with  the  special  case  of
    comparison to zero.  The resultant status flags from the calculation of
    the (left-hand) expression are said (in reader.c) to be the same as the
    comparison  of  that result with zero.  However, if the zero is instead
    in a variable, we shouldn't get the same result  because  the  overflow
    status  will be reset (0) when the result of the expression is compared
    with the variable.  The VAX compiler code never looks at overflow.  jon
    870727

    Error status at present is arbitrary. jon 870728
*/
#define VAL1    0x81818181
#define VAL2    0x7f7f7f7f
main()
{
	int error = 0;
	char z;

	z ^= z;
	{
#define TYPA    char        /* char, short, int, long */
#define sTYPA   "(char)"      /* char, short, int, long */
		unsigned TYPA a, b;
		TYPA c = 0;


		a = VAL1;
		b = VAL2;
		if( (TYPA) a > (TYPA) b )
		{
			++error;
		}
		printf("%sa<%sb is %x\n", sTYPA, sTYPA,
						    ( (TYPA) a < (TYPA) b ) );
		if( (TYPA) ( a - b ) > (TYPA) 0 )
		{
			++error;
		}
		printf("%s(a-b)<%s0 is %x\n", sTYPA, sTYPA,
						 ( (TYPA)(a-b) < (TYPA) 0 ) );
		if( (TYPA) ( a - b ) > (TYPA) z )
		{
			++error;
		}
		printf( "%s(a-b)<%sz = %x\n", sTYPA, sTYPA,
						  ( (TYPA)(a-b) < (TYPA)z ) );
		if( (TYPA) ( a -= b ) > (TYPA) 0 )
		{
			++error;
		}
		a = VAL1;
		printf( "%s(a-=b)<%s0 = %x\n", sTYPA, sTYPA,
						 ( (TYPA)(a-=b) < (TYPA)0 ) );
		if( (TYPA) ( a -= b ) > (TYPA) z )
		{
			++error;
		}
		a = VAL1;
		printf( "%s(a-=b)<%sz = %x\n", sTYPA, sTYPA,
						 ( (TYPA)(a-=b) < (TYPA)z ) );
		if( (TYPA) ( c = a - b ) > (TYPA) 0 )
		{
			++error;
		}
		printf("%s(c=a-b)<%s0 is %x\n", sTYPA, sTYPA,
					       ( (TYPA)(c=a-b) < (TYPA) 0 ) );
		if( (TYPA) ( c = a - b ) > (TYPA) z )
		{
			++error;
		}
		printf("%s(c=a-b)<%sz is %x\n", sTYPA, sTYPA,
					       ( (TYPA)(c=a-b) < (TYPA) z ) );
	}
	{
		struct usl
		{
			int             sum     :   3;
			int             dummy   :  23;
			int             unbits  :   3;
			unsigned        tib932  :   3;
			unsigned        tiba32  :   3;
		} pgm;
#define a       pgm.tib932
#define b       pgm.tiba32
#define c       pgm.sum
#define z       pgm.unbits
#define TYPE    int            /* char, short, int, long */
#define sTYPE   "(3bits)"        /* char, short, int, long */
		a = 0x5;
		b = 0x3;
		z ^= z;
		if( (TYPE) a > (TYPE) b )
		{
			++error;
		}
		printf("%stib932<%stiba32 is %x\n", sTYPE, sTYPE,
						    ( (TYPE) a < (TYPE) b ) );
		if( (TYPE) ( a - b ) > (TYPE) 0 )
		{
			++error;
		}
		printf("%s(tib932-tiba32)<%s0 is %x\n", sTYPE, sTYPE,
						 ( (TYPE)(a-b) < (TYPE) 0 ) );
		if( (TYPE) ( a - b ) > (TYPE) z )
		{
			++error;
		}
		printf( "%s(tib932-tiba32)<%spgm.unbits = %x\n", sTYPE, sTYPE,
						  ( (TYPE)(a-b) < (TYPE)z ) );
		if( (TYPE) ( a -= b ) > (TYPE) 0 )
		{
			++error;
		}
		a = 0x5;
		printf( "%s(tib932-=tiba32)<%s0 = %x\n", sTYPE, sTYPE,
						 ( (TYPE)(a-=b) < (TYPE)0 ) );
		if( (TYPE) ( a -= b ) > (TYPE) z )
		{
			++error;
		}
		a = 0x5;
		printf( "%s(tib932-=tiba32)<%spgm.unbits = %x\n", sTYPE, sTYPE,
						 ( (TYPE)(a-=b) < (TYPE)z ) );
		if( (TYPE) ( c = a - b ) > (TYPE) 0 )
		{
			++error;
		}
		printf("%s(pgm.sum=tib932-tiba32)<%s0 is %x\n", sTYPE, sTYPE,
					       ( (TYPE)(c=a-b) < (TYPE) 0 ) );
		if( (TYPE) ( c = a - b ) > (TYPE) z )
		{
			++error;
		}
		printf("%s(pgm.sum=tib932-tiba32)<%spgm.unbits is %x\n", sTYPE, sTYPE,
					       ( (TYPD)(c=a-b) < (TYPD) z ) );
	}

	exit( error );
}




-- 
= Drug tests; just say *NO*!
= Fred Fish  Motorola Computer Division, 3013 S 52nd St, Tempe, Az 85282  USA
= seismo!noao!mcdsun!fnf    (602) 438-3614