Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.UUCP
Newsgroups: comp.lang.c
Subject: Re: Comments on ANSI public Oct 86 Public review draft.
Message-ID: <5708@brl-smoke.ARPA>
Date: Mon, 30-Mar-87 15:56:36 EST
Article-I.D.: brl-smok.5708
Posted: Mon Mar 30 15:56:36 1987
Date-Received: Wed, 1-Apr-87 01:37:23 EST
References: <4804@brl-adm.ARPA> <365@bms-at.UUCP> <704@mcgill-vision.UUCP> <5703@brl-smoke.ARPA> <341@zuring.mcvax.cwi.nl>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 35

In article <5703@brl-smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
> It occurs to me that we should not allow == with floating-point
> operands.  I'm serious!

So far the only valid use for floating == that anyone has suggested to me
is for branching to streamlined handling of cases where one variable is
precisely 0.0 (or perhaps 1.0).

It occurs to me that an example of why C floating == is not very useful
might be in order (apart from the numerical analysis problems):

	#include <stdio.h>

	int SameRatio( a, b, c, d )	/* return non-0 iff a/b == c/d */
		register double	a, b, c, d;
		{
		register double	ab = a / b;	/* `register' honored */
		register double cd = c / d;	/* `register' ignored */

		return ab == cd;
		}

	int main()
		{
		static double	a = 2.0, b = 3.0, c = 2.0, d = 3.0;

		(void)printf( "%s\n", SameRatio( a, b, c, d ) ? "ok" : "oops" );
		return 0;
		}

Now suppose that the machine has 5 available FP registers (other than the
scratch accumulators), that they include some guard bits, and that no real
optimization is done.  Since `cd' will be stored in memory, for the ab == cd
comparison it will be loaded into a scratch register with its guard bits set
to zero, unlike `ab', and very likely the comparison will fail.  "Oops."