X-Recipient: archive-cygwin AT delorie DOT com
X-SWARE-Spam-Status: No, hits=-1.7 required=5.0 	tests=AWL,BAYES_00,SPF_HELO_PASS,SPF_PASS
X-Spam-Check-By: sourceware.org
To: cygwin AT cygwin DOT com
From: Eric Backus <eric_backus AT agilent DOT com>
Subject:  Re: std::arg()   bug : not repetitive ?
Date: Wed, 2 Sep 2009 16:11:42 +0000 (UTC)
Lines: 46
Message-ID:  <loom.20090902T175821-355@post.gmane.org>
References:  <264058 DOT 8230 DOT qm AT web25502 DOT mail DOT ukl DOT yahoo DOT com> <4A9D6E81 DOT 3070904 AT gmail DOT com>
Mime-Version:  1.0
Content-Type:  text/plain; charset=us-ascii
Content-Transfer-Encoding:  7bit
User-Agent: Loom/3.14 (http://gmane.org/)
X-IsSubscribed: yes
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
List-Id: <cygwin.cygwin.com>
List-Subscribe: <mailto:cygwin-subscribe AT cygwin DOT com>
List-Archive: <http://sourceware.org/ml/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-help AT cygwin DOT com>, <http://sourceware.org/ml/#faqs>
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
Delivered-To: mailing list cygwin AT cygwin DOT com

Dave Korn <dave.korn.cygwin <at> googlemail.com> writes:

> 
> Marco Atzeri wrote:
> 
> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323#c2
> > 
> > $g++-4 -ffloat-store  comp_2.cc -O3 -o comp_4
> > 
> > ./comp_4
> > 0.785398
> > 0.785398
> > 0
> > 0
> 
>   Ah, good, thanks for the diagnosis.  IIUC this is basically fixed in GCC
> HEAD now and 4.5.0 shouldn't suffer the same problem.
> 
>     cheers,
>       DaveK
> 
> 

I guess I'm in the minority here, but I'll go out on a limb and say that the 
original behavior does NOT seem like a defect to me.  My reasoning:

1. Compilers are free to use and/or not use excess precision within 
expressions.  I believe the standard requires the excess precision to get 
thrown away when a value is stored into a variable, but that's not happenning 
here.  I think -ffloat-store tells gcc to strictly follow the standard, but 
(again, if my understanding is correct) since the values of arg(z1) and arg
(z2) are not stored into variables, it won't necessarily help.

Note that there is a performance penalty to strictly following the standard, 
as compared with freely using excess precision when convenient, so in many 
(most?) cases it is desirable to let the compiler go ahead and do that.  Also 
in many (most?) cases, excess precision only helps, so the default may be to 
let the compiler do what it wants for speed.

2. In general, you should not be comparing equality of floating-point values.  
This is true even if you get rid of the excess precision stuff above.  Sure, 
in this particular case, if you get rid of excess precision, the values will 
be exactly equal, but direct comparison is a bad habit to get into.

-- 
Eric Backus


--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple