From -8498839956166725157 X-Google-Thread: f78e5,eb811809d4f61b12,start X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII Path: g2news2.google.com!news3.google.com!news4.google.com!news.glorb.com!news.alt.net!comp-std-cpp-robomod!not-for-mail From: "kanze" Newsgroups: comp.std.c++ Subject: Using extended precision in floating point Date: Wed, 17 May 2006 11:36:30 CST Organization: http://groups.google.com Lines: 67 Approved: Fergus Henderson , moderator of comp.std.c++ Message-ID: <1147882581.972497.100340@u72g2000cwu.googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 8bit X-Trace: posting.google.com 1147882587 25705 127.0.0.1 (17 May 2006 16:16:27 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Wed, 17 May 2006 16:16:27 +0000 (UTC) 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 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: u72g2000cwu.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 X-MIME-Autoconverted: from quoted-printable to 8bit by mulga.cs.mu.OZ.AU id k4HGGmtF018714 X-Virus-Scanned: amavisd-new at cs.mu.OZ.AU Xref: g2news2.google.com comp.std.c++:2016 As a result of a recent discussion in the French language newsgroup, I am now unsure as to when the compiler can use extra precision in floating point operations. �5/10 is clear with regards to expressions: "The values of the floating operands and the results of floading expressions may be represented in greater precision and range than required by the type; the types are not changed thereby." It seems clear enough here that in an expression like "(a + b) * c", the compiler can do the entire expression in long double, even if all of the variables are float -- and even if "a + b", processed as a float, was equal to a. However, there is a footnote (non-normative, I know) to the effect that "The cast and assignment operators must still perform their specific conversions as described in 5.4, 5.2.9 and 5.17." And �5.7/1 says quite clearly that "The result of the assignment operation is the value stored in the left operant after the assignment has taken place;" I would have normally interpreted this to mean that if I write something like "(a += b) * c", the value multiplied by c must be the actual value of the float, and not the extended precision value which the compiler was allowed to use when there was no assignment. Even more so, I would have interpreted this to mean that in a bit of code like: float total = 0.0 ; for ( size_t i = 0 ; i < size ; ++ i ) { total += array[ i ] ; } where array is float*, the compiler must ensure that the results are exactly the same as those that it would have obtained by storing the value to total each time in the loop, and rereading the stored value in the expression. 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 .) -- 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 ]