From owner-freebsd-standards@FreeBSD.ORG  Thu Dec  6 09:08:49 2007
Return-Path: <owner-freebsd-standards@FreeBSD.ORG>
Delivered-To: freebsd-standards@FreeBSD.ORG
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 5C21B16A417
	for <freebsd-standards@FreeBSD.ORG>;
	Thu,  6 Dec 2007 09:08:49 +0000 (UTC) (envelope-from das@FreeBSD.ORG)
Received: from VARK.MIT.EDU (VARK.MIT.EDU [18.95.3.179])
	by mx1.freebsd.org (Postfix) with ESMTP id 1F5C413C45B
	for <freebsd-standards@FreeBSD.ORG>;
	Thu,  6 Dec 2007 09:08:48 +0000 (UTC) (envelope-from das@FreeBSD.ORG)
Received: from VARK.MIT.EDU (localhost [127.0.0.1])
	by VARK.MIT.EDU (8.14.2/8.14.1) with ESMTP id lB698YuB095678;
	Thu, 6 Dec 2007 04:08:34 -0500 (EST) (envelope-from das@FreeBSD.ORG)
Received: (from das@localhost)
	by VARK.MIT.EDU (8.14.2/8.14.1/Submit) id lB698XrJ095677;
	Thu, 6 Dec 2007 04:08:34 -0500 (EST) (envelope-from das@FreeBSD.ORG)
Date: Thu, 6 Dec 2007 04:08:33 -0500
From: David Schultz <das@FreeBSD.ORG>
To: Steve Kargl <sgk@troutmask.apl.washington.edu>
Message-ID: <20071206090833.GA95428@VARK.MIT.EDU>
Mail-Followup-To: Steve Kargl <sgk@troutmask.apl.washington.edu>,
	freebsd-standards@FreeBSD.ORG
References: <20071012180959.GA36345@troutmask.apl.washington.edu>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20071012180959.GA36345@troutmask.apl.washington.edu>
Cc: freebsd-standards@FreeBSD.ORG
Subject: Re: [PATCH] hypotl, cabsl, and code removal in cabs
X-BeenThere: freebsd-standards@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Standards compliance <freebsd-standards.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-standards>, 
	<mailto:freebsd-standards-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-standards>
List-Post: <mailto:freebsd-standards@freebsd.org>
List-Help: <mailto:freebsd-standards-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-standards>, 
	<mailto:freebsd-standards-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 06 Dec 2007 09:08:49 -0000

I like the approach. One concern, though, is that IEEE 754
requires sqrt to always be correctly rounded, and one might
reasonably expect the same from hypot. If we believe that, then
the intermediate quantity a*a + b*b needs to be computed exactly;
if it is rounded before we take the square root, the double
rounding may cause us to round the wrong way. This basically
requires computing things with twice as many bits of precision.
Arguably, though, your code is correct in the most important
ways (avoiding underflow/overflow).

Also, umm, I've been busy and unable to pay attention for a while,
so forgive me if I'm missing something, but isn't it the case that
we don't have a sqrtl(), except for the gcc builtin on some
architectures?