From owner-sc22wg5@open-std.org Sat Feb 13 04:53:52 2010 Return-Path: X-Original-To: sc22wg5-dom8 Delivered-To: sc22wg5-dom8@www2.open-std.org Received: by www2.open-std.org (Postfix, from userid 521) id 93B5FC178DA; Sat, 13 Feb 2010 04:53:52 +0100 (CET) X-Original-To: sc22wg5@open-std.org Delivered-To: sc22wg5@open-std.org Received: from ppsw-0.csi.cam.ac.uk (ppsw-0.csi.cam.ac.uk [131.111.8.130]) by www2.open-std.org (Postfix) with ESMTP id AE91AC178D9 for ; Sat, 13 Feb 2010 04:53:49 +0100 (CET) X-Cam-AntiVirus: no malware found X-Cam-SpamDetails: not scanned X-Cam-ScannerInfo: http://www.cam.ac.uk/cs/email/scanner/ Received: from hermes-2.csi.cam.ac.uk ([131.111.8.54]:34171) by ppsw-0.csi.cam.ac.uk (smtp.hermes.cam.ac.uk [131.111.8.150]:25) with esmtpa (EXTERNAL:nmm1) id 1Ng94m-0003n4-1m (Exim 4.70) (return-path ); Sat, 13 Feb 2010 03:53:48 +0000 Received: from prayer by hermes-2.csi.cam.ac.uk (hermes.cam.ac.uk) with local (PRAYER:nmm1) id 1Ng94m-0000N6-HL (Exim 4.67) (return-path ); Sat, 13 Feb 2010 03:53:48 +0000 Received: from [216.241.51.98] by webmail.hermes.cam.ac.uk with HTTP (Prayer-1.3.2); 13 Feb 2010 03:53:48 +0000 Date: 13 Feb 2010 03:53:48 +0000 From: "N.M. Maclaren" To: fortran standards email list for J3 , WG5 Subject: IEEE 754 [was re letter ballot on interps] Message-ID: In-Reply-To: <20100212194545.AD859C3BA0D@www2.open-std.org> References: <20100201122928.54FB8C178E4@www2.open-std.org> <20100212194545.AD859C3BA0D@www2.open-std.org> X-Mailer: Prayer v1.3.2 Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset=ISO-8859-1 Sender: owner-sc22wg5@open-std.org Precedence: bulk On Feb 12 2010, Toon Moene wrote: > >F03/0078: This answer might come back to haunt us. People who want to > follow IEEE 754 strictly *need* the assurance that the > text of the program is followed verbatim, because, although > X(I) / A is mathematically equivalent to X(I) * (1/A), it > certainly isn't in IEEE 754 arithmetic (compilers do this to > turn division by a loop-invariant into a cheaper > multiplication). No, it is the converse answer that would come back to haunt us. If you want strictly deterministic floating-point results, then you should be using a portable assembler. Oh, and not writing seriously parallel code, either. I am not joking :-( If you want IEEE 754 determinism, then ask yourself which of the following should X**N generate (in general, they are all different): 1) The binary reduction code using IEEE 754, which is what almost every library does, but sometimes inline code doesn't. 2) The same result as X*X*X*... 3) The correctly rounded result of X^N. There is also a similar problem with (parallel) reductions and operation ordering. And, there, in general, solution (3) is intractable - but virtually nobody uses general reductions. One of the things that was in the original objectives of IEEE 754 was to tackle the reasons that no programming language had adopted it in a couple of decades. There were some reasonable proposals to change it in the general direction of where programming languages have been since the 1960s (sic). But they lost out in committee. Regards, Nick Maclaren.