From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jonathan Nieder Subject: Re: [PATCH v2] do not depend on signed integer overflow Date: Mon, 4 Oct 2010 17:33:31 -0500 Message-ID: <20101004223331.GB9994@burratino> References: <1286227522-5468-1-git-send-email-kusmabite@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: git@vger.kernel.org To: Erik Faye-Lund X-From: git-owner@vger.kernel.org Tue Oct 05 00:36:58 2010 Return-path: Envelope-to: gcvg-git-2@lo.gmane.org Received: from vger.kernel.org ([209.132.180.67]) by lo.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1P2teT-0002PG-Pq for gcvg-git-2@lo.gmane.org; Tue, 05 Oct 2010 00:36:58 +0200 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756441Ab0JDWgx (ORCPT ); Mon, 4 Oct 2010 18:36:53 -0400 Received: from mail-wy0-f174.google.com ([74.125.82.174]:33490 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756079Ab0JDWgw (ORCPT ); Mon, 4 Oct 2010 18:36:52 -0400 Received: by wyb28 with SMTP id 28so5458155wyb.19 for ; Mon, 04 Oct 2010 15:36:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:received:received:date:from:to:cc:subject :message-id:references:mime-version:content-type:content-disposition :in-reply-to:user-agent; bh=71G47BV/jcRAXm9UVs+K9aREU8jWcrWj4IB+6Ta5Wlg=; b=e+8QmZEJK6od9FXsxf9BRJ2r7g2kilGJS3tzjVXF981v/kpZHVNSibfibXjfRNkVl2 Qom19iakvtGGmFsYSK3wQrO/JNwiI4UqxTD6Ztj5nwzvtR/Sy7NkqOdG8dmv3yuMLn1G KsHYuzux9HGpBdOGmPt5oav2P3h/p4EME5SY8= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; b=t36BH6o9K24lPXAwzDFMLM5M+/N/1EssSPuebSAV2PmRf5J6SURaLnb1K3loeoJ4Ma UWgigCW5uSuh3dujvBhm/xiI2kHf0WDRgkqaeNbo8VC4HddpPLbxSpVLLdLjV35m+jnm rLLuLrFjERot/AQOAS7eVlkl0USm/T/CU7eE8= Received: by 10.216.188.197 with SMTP id a47mr8338133wen.70.1286231811124; Mon, 04 Oct 2010 15:36:51 -0700 (PDT) Received: from burratino (adsl-68-255-106-176.dsl.chcgil.sbcglobal.net [68.255.106.176]) by mx.google.com with ESMTPS id w29sm3395449weq.18.2010.10.04.15.36.47 (version=SSLv3 cipher=RC4-MD5); Mon, 04 Oct 2010 15:36:48 -0700 (PDT) Content-Disposition: inline In-Reply-To: <1286227522-5468-1-git-send-email-kusmabite@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org Archived-At: Erik Faye-Lund wrote: > +++ b/git-compat-util.h > @@ -28,6 +28,15 @@ > #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0])) > #define bitsizeof(x) (CHAR_BIT * sizeof(x)) > > +/* > + * Signed integer overflow is undefined in C, so here's a helper macro > + * to detect if the sum of two integers will overflow. The bitsize to > + * overflow at is taken from the first parameter, which must be zero > + * or positive. > + */ > +#define signed_add_overflows(a, b) \ > + ((b) > ((INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a))) - (a))) Yes, I still like it. This could be made closer to self-documenting like so: #define maximum_signed_value_of_type(a) \ (INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a))) /* * Signed overflow is undefined in C, so here's a helper macro * to detect if the sum of two signed integers will overflow. * * Requires: a >= 0, typeof(a) equals typeof(b) */ #define signed_add_overflows(a, b) \ ((b) > maximum_signed_value_of_type(a) - (a))