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: Using the GNU GMP Library for Bignums in Emacs Date: Mon, 23 Apr 2018 07:36:39 -0700 Organization: UCLA Computer Science Department Message-ID: <12e4d927-c3fd-933c-6b7c-5e5086b45622@cs.ucla.edu> References: <29f933ac-a6bf-8742-66a7-0a9d6d3e5a88@disroot.org> <83bmecy6fx.fsf@gnu.org> <0d3175d8-d996-651e-b221-71978bde3a65@cs.ucla.edu> <51e619e0-ee38-eb97-6c1d-0925b675290a@disroot.org> <8e12135a-0fcc-7aa3-d000-731d2f26d918@disroot.org> <87lgde9v0b.fsf@linux-m68k.org> 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 1524494132 10644 195.159.176.226 (23 Apr 2018 14:35:32 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 23 Apr 2018 14:35:32 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 Cc: emacs-devel@gnu.org To: Andreas Schwab , Helmut Eller Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Mon Apr 23 16:35:28 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 1fAcYt-0002ZJ-7a for ged-emacs-devel@m.gmane.org; Mon, 23 Apr 2018 16:35:23 +0200 Original-Received: from localhost ([::1]:51991 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fAcay-0008JY-7E for ged-emacs-devel@m.gmane.org; Mon, 23 Apr 2018 10:37:32 -0400 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:51380) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fAcaI-0008IM-M7 for emacs-devel@gnu.org; Mon, 23 Apr 2018 10:36:51 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fAcaG-0003n1-3x for emacs-devel@gnu.org; Mon, 23 Apr 2018 10:36:50 -0400 Original-Received: from zimbra.cs.ucla.edu ([131.179.128.68]:41862) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fAcaF-0003m6-Tg for emacs-devel@gnu.org; Mon, 23 Apr 2018 10:36:48 -0400 Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 8F496161611; Mon, 23 Apr 2018 07:36:45 -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 FYtWQBLZ55CR; Mon, 23 Apr 2018 07:36:40 -0700 (PDT) Original-Received: from localhost (localhost [127.0.0.1]) by zimbra.cs.ucla.edu (Postfix) with ESMTP id 0847416161D; Mon, 23 Apr 2018 07:36:40 -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 vWasiIfMKboH; Mon, 23 Apr 2018 07:36:39 -0700 (PDT) Original-Received: from [192.168.1.9] (unknown [47.154.30.119]) by zimbra.cs.ucla.edu (Postfix) with ESMTPSA id CD3DA161611; Mon, 23 Apr 2018 07:36:39 -0700 (PDT) In-Reply-To: <87lgde9v0b.fsf@linux-m68k.org> 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:224808 Archived-At: Andreas Schwab wrote: > There is no way to detect overflow after the fact because overflow > invokes undefined behaviour. You need to either check the range > beforehand, or use special builtins offered by the compiler > (eg. __builtin_smul_overflow in GCC). All true, and Emacs lib/intprops.h has an INT_MULTIPLY_OVERFLOW macro tha= t=20 arranges for all that. On the x86-64 with GCC, it costs one additional=20 instruction (typically a conditional branch that is not taken) to check f= or=20 overflow in machine-word integer multiplication. Checking for fixnum over= flow=20 (as opposed to machine-word overflow) requires one more conditional branc= h after=20 some quick bit-twiddling.