From 1279730555550807704 X-Google-Thread: f78e5,eb811809d4f61b12 X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII Path: g2news2.google.com!news1.google.com!news3.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local01.nntp.dca.giganews.com!nntp.speakeasy.net!news.speakeasy.net.POSTED!not-for-mail NNTP-Posting-Date: Fri, 19 May 2006 06:50:07 -0500 Return-Path: X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Original-To: std-c++@mailman.ucar.edu Delivered-To: std-c++@mailman.ucar.edu Delivered-To: std-c++@ucar.edu From: "kanze" Newsgroups: comp.std.c++ Subject: Re: Using extended precision in floating point Organization: http://groups.google.com Message-ID: <1148033537.225653.254200@j55g2000cwa.googlegroups.com> References: <1147882581.972497.100340@u72g2000cwu.googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" X-Complaints-To: groups-abuse@google.com User-Agent: G2/0.2 X-HTTP-UserAgent: Mozilla/5.0 (X11; U; SunOS sun4u; en-US; rv:1.0.1) Gecko/20020920 Netscape/7.0,gzip(gfe),gzip(gfe) Complaints-To: groups-abuse@google.com Injection-Info: j55g2000cwa.googlegroups.com; posting-host=62.160.54.162; posting-account=qsfl8gwAAABZGaLp2a7FeTfDkJamzWYW X-Virus-Scanned: amavisd-new at ucar.edu X-Virus-Scanned: amavisd-new at cs.mu.OZ.AU Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mulga.cs.mu.OZ.AU id k4JAHTtF002953 Approved: Fergus Henderson , moderator of comp.std.c++ X-Virus-Scanned: amavisd-new at cs.mu.OZ.AU Date: Fri, 19 May 2006 06:40:47 CST Lines: 67 NNTP-Posting-Host: 65.182.171.162 X-Trace: sv3-u9IjqCC+vlw87WgeSYNlfgD0DVhjQAWD39UCmpYVFCJ9UqdRFatqYRQXkOOE67V5cCiXye5CYyHnhv8!ZopYgFOMCBRPFZHPEirHHX2YTKKewH29pTRwRFItm72xTQmMt3NjkQCkwPSu2b+kvPUD6pqsIHvh!8HsqpNYCdJbxQsFIK3eTI+E5JAIXkZSHgOlg5qkeVbk= X-Complaints-To: abuse@speakeasy.net X-DMCA-Complaints-To: abuse@speakeasy.net X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.32 Xref: g2news2.google.com comp.std.c++:2067 Jean-Marc Bourguet wrote: > "kanze" writes: > > I'd always been pretty sure of this, but on analysing some > > code which wasn't giving the expected results (they were too > > accurate for the algorithm being used !), I found that g++ > > (on a PC) used extended precision through out the loop, at > > least when optimization was requested. Is this an error in > > g++, or have I misunderstood something? > > (For those who are familiar with Intel assembler, the body of > > the loop is simply: > > .L9: > > fadd DWORD PTR [%ecx+%eax*4] > > inc %eax > > cmp %eax, %edx > > jb .L9 > > > > .) > I think this is a bug is gcc. But see: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323 > There are something like 70 duplicates. > One comment is in the bug log is: > Steven> ...to end this pointless discussion. > Steven> > Steven> Some people call this a bug in the x87 series. Other call it a bug in > Steven> gcc. See these mails at least for the reason why this could be considered > Steven> a bug in gcc: > Steven> http://gcc.gnu.org/ml/gcc/2003-08/msg01195.html > Steven> http://gcc.gnu.org/ml/gcc/2003-08/msg01234.html > Steven> http://gcc.gnu.org/ml/gcc/2003-08/msg01257.html > Steven> > Steven> Regardless of where one wishes to put the blame, this problem will _not_ be > Steven> fixed. Period. >From what I understand, adding the option -ffloat-store should cause conformant behavior. Maybe they should mention this in the documentation for options like -std=c++98. (IMHO, the current behavior of g++ is probably preferable in most cases. My question only concerned standards conformance.) I also think that the standard isn't as clear as it could be. I think that the intent is that even when extended precision is used in sub-expressions, the programmer can force normal precision by means of a cast or an assignment. The standard can certainly be interpreted this way, but I'm not sure that other interpretations are not possible. -- James Kanze GABI Software Conseils en informatique orient�e objet/ Beratung in objektorientierter Datenverarbeitung 9 place S�mard, 78210 St.-Cyr-l'�cole, France, +33 (0)1 30 23 00 34 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]