From owner-freebsd-amd64@FreeBSD.ORG  Wed Feb 29 16:03:59 2012
Return-Path: <owner-freebsd-amd64@FreeBSD.ORG>
Delivered-To: freebsd-amd64@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52])
	by hub.freebsd.org (Postfix) with ESMTP id A50D41065672
	for <freebsd-amd64@freebsd.org>; Wed, 29 Feb 2012 16:03:59 +0000 (UTC)
	(envelope-from sgk@troutmask.apl.washington.edu)
Received: from troutmask.apl.washington.edu (troutmask.apl.washington.edu
	[128.95.76.21]) by mx1.freebsd.org (Postfix) with ESMTP id 647378FC16
	for <freebsd-amd64@freebsd.org>; Wed, 29 Feb 2012 16:03:59 +0000 (UTC)
Received: from troutmask.apl.washington.edu (localhost.apl.washington.edu
	[127.0.0.1])
	by troutmask.apl.washington.edu (8.14.5/8.14.5) with ESMTP id
	q1TG3wr2032362; Wed, 29 Feb 2012 08:03:58 -0800 (PST)
	(envelope-from sgk@troutmask.apl.washington.edu)
Received: (from sgk@localhost)
	by troutmask.apl.washington.edu (8.14.5/8.14.5/Submit) id
	q1TG3wFe032361; Wed, 29 Feb 2012 08:03:58 -0800 (PST)
	(envelope-from sgk)
Date: Wed, 29 Feb 2012 08:03:58 -0800
From: Steve Kargl <sgk@troutmask.apl.washington.edu>
To: Bruce Evans <brde@optusnet.com.au>
Message-ID: <20120229160358.GA32337@troutmask.apl.washington.edu>
References: <4F3EA37F.9010207@speakeasy.org>
	<CAGE5yCpvF0-b1iKAVGbya=fUNaYbGyrpj1PHSQxw4BvycNMLDg@mail.gmail.com>
	<4F3EC0B4.6050107@speakeasy.org> <4F4DA398.6070703@speakeasy.org>
	<20120229161408.G2514@besplex.bde.org>
	<4F4DD942.8070106@speakeasy.org>
	<20120229191750.Y3167@besplex.bde.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20120229191750.Y3167@besplex.bde.org>
User-Agent: Mutt/1.4.2.3i
Cc: freebsd-amd64@freebsd.org
Subject: Re: Gcc46 and 128 Bit Floating Point
X-BeenThere: freebsd-amd64@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Porting FreeBSD to the AMD64 platform <freebsd-amd64.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-amd64>,
	<mailto:freebsd-amd64-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-amd64>
List-Post: <mailto:freebsd-amd64@freebsd.org>
List-Help: <mailto:freebsd-amd64-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-amd64>,
	<mailto:freebsd-amd64-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 29 Feb 2012 16:03:59 -0000

On Wed, Feb 29, 2012 at 07:23:36PM +1100, Bruce Evans wrote:
> On Tue, 28 Feb 2012, Thomas D. Dean wrote:
> 
> >On 02/28/12 22:03, Bruce Evans wrote:
> >>
> >>>#include <quadmath.h>
> >>>#include <stdio.h>
> >>>int main() {
> >>>char buf[128];
> >>>__float128 x = sqrtq(2.0Q);
> >>>quadmath_snprintf(buf, sizeof buf, "%.45Qf",x);
> >>>printf("sin(%s) = ",buf);
> >>>quadmath_snprintf(buf, sizeof buf, "%.45Qf",sinq(x));
> >>>printf("%s\n",buf);
> >>>return 0;
> >>>}
> >>>
> >>>gcc46 math.c -o math /usr/local/lib/gcc46/libquadmath.a /usr/lib/libm.a
> >
> >>objdump -d math | grep fsqrt
> > 4014fd:       d9 fa                   fsqrt
> > 407bb4:       d9 fa                   fsqrt
> >
> >Comes from the libs.
> 
> It's not unreasonable in the libraries.  A lower-precision sqrt gives
> a good place to start for a Newton approximation method.  I wouldn't
> have expected fsqrt to be a better place to start that a 64-bit sqrt
> using SSE though.  SSE also provides 32-bit sqrt and an even
> lower-precision but much faster reciprocal square root to start from.
> 

>From libquadmath/math/sqrtq.c

  if (x <= DBL_MAX && x >= DBL_MIN)
  {
    /* Use double result as starting point.  */
    y = sqrt ((double) x);

    /* Two Newton iterations.  */
    y -= 0.5q * (y - x / y);
    y -= 0.5q * (y - x / y);
    return y;
  }

This probably explains the presences of fsqrt.  There's
a similar block for LDBL_MAX and LDBL_MIN.

-- 
Steve