From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Paul Eggert Newsgroups: gmane.emacs.devel Subject: Re: Floating-point constant folding in Emacs byte compiler Date: Thu, 22 Mar 2018 22:22:25 -0700 Organization: UCLA Computer Science Department Message-ID: References: <2ce39e5c-cd1b-65d6-b125-719caad67932@cs.ucla.edu> NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1521782436 14024 195.159.176.226 (23 Mar 2018 05:20:36 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 23 Mar 2018 05:20:36 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 To: Stefan Monnier , emacs-devel@gnu.org Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Fri Mar 23 06:20:32 2018 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ezF7u-0003Xk-2P for ged-emacs-devel@m.gmane.org; Fri, 23 Mar 2018 06:20:30 +0100 Original-Received: from localhost ([::1]:36096 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ezF9x-0003Qx-DE for ged-emacs-devel@m.gmane.org; Fri, 23 Mar 2018 01:22:37 -0400 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:47543) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ezF9q-0003Qe-Tp for emacs-devel@gnu.org; Fri, 23 Mar 2018 01:22:31 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ezF9n-0002xF-Oz for emacs-devel@gnu.org; Fri, 23 Mar 2018 01:22:30 -0400 Original-Received: from zimbra.cs.ucla.edu ([131.179.128.68]:53798) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ezF9n-0002x3-Iy for emacs-devel@gnu.org; Fri, 23 Mar 2018 01:22:27 -0400 Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 418B01614F9; Thu, 22 Mar 2018 22:22:26 -0700 (PDT) Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id n_i1CRiJuNsZ; Thu, 22 Mar 2018 22:22:25 -0700 (PDT) Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 7F6DC161706; Thu, 22 Mar 2018 22:22:25 -0700 (PDT) X-Virus-Scanned: amavisd-new at zimbra.cs.ucla.edu Original-Received: from zimbra.cs.ucla.edu ([127.0.0.1]) by localhost (zimbra.cs.ucla.edu [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id WXTU90dyInPv; Thu, 22 Mar 2018 22:22:25 -0700 (PDT) Original-Received: from [192.168.1.9] (unknown [47.154.30.119]) by zimbra.cs.ucla.edu (Postfix) with ESMTPSA id 6102A1614F9; Thu, 22 Mar 2018 22:22:25 -0700 (PDT) In-Reply-To: Content-Language: en-US X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [fuzzy] X-Received-From: 131.179.128.68 X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:223934 Archived-At: Stefan Monnier wrote: > I'm pretty sure those runtime differences are not on-purpose Although that's most likely true for the examples I gave, it's a problema= tic=20 assumption for code that intentionally inspects runtime behavior. I mention this because my recently-installed byte-opt patch fixed a probl= em of=20 this sort with lsh. byte-opt was evaluating (lsh -1 -1) at compile-time, = whereas=20 the intent of the (lsh -1 -1) was to calculate most-positive-fixnum at ru= n-time.=20 It's not hard to imagine similar sorts of code to inspect properties of t= he=20 machine's floating-point behavior, code that would yield the wrong value = if=20 compiled on a machine with different floating-point properties. Nowadays Emacs hosts almost invariably use IEEE floating point, and I sup= pose we=20 could hardwire that assumption into Emacs. Even so, IEEE floating-point i= s not=20 bit-for-bit identical on all platforms, so if we're constant-folding=20 floating-point expressions surely we should at least document that we're = doing=20 so, so that programmers know that they can't rely on constant expressions= =20 yielding a different value than what run-time evaluation would deliver.