From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5E59AC433DF for ; Wed, 3 Jun 2020 19:27:58 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 305DE2067B for ; Wed, 3 Jun 2020 19:27:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="JoLj3EFc" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 305DE2067B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:55384 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jgZ3N-0007gT-6S for qemu-devel@archiver.kernel.org; Wed, 03 Jun 2020 15:27:57 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:35536) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jgZ2Q-0007Ao-Ds for qemu-devel@nongnu.org; Wed, 03 Jun 2020 15:26:58 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:20611 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jgZ2P-00076d-GE for qemu-devel@nongnu.org; Wed, 03 Jun 2020 15:26:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1591212416; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Z3rsp1lITzHuJ3hon2Fq1CyznwPEk23zrr+ZQMRUygU=; b=JoLj3EFcV0lPuE0H3gNOXe0QGaKube0RB0msQTQlE0tg5x0bfkL4/N7W9NARHQ4rilWehf pRcKGet/TrUr1O4cPTsr+eMHlGReudkFX8+taUIkL4rw01gSlmQNK2gZGPcylL1y4sUVka 9UaH+sIs1ULTYxYldnDDN5o6P+IE4kc= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-107-jWfkU_qONUmxXK063zNXRA-1; Wed, 03 Jun 2020 15:26:51 -0400 X-MC-Unique: jWfkU_qONUmxXK063zNXRA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 313D4185695A; Wed, 3 Jun 2020 19:26:50 +0000 (UTC) Received: from [10.3.113.22] (ovpn-113-22.phx2.redhat.com [10.3.113.22]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 857A25C240; Wed, 3 Jun 2020 19:26:46 +0000 (UTC) Subject: Re: [PATCH v3] osdep: Make MIN/MAX evaluate arguments only once To: Richard Henderson , qemu-devel@nongnu.org References: <20200603013603.2400199-1-eblake@redhat.com> <03a13d47-fe7d-88e5-b92e-3c396591f9c7@linaro.org> <2e894b8d-a510-0689-83dc-f6c8da16410e@linaro.org> From: Eric Blake Organization: Red Hat, Inc. Message-ID: Date: Wed, 3 Jun 2020 14:26:46 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <2e894b8d-a510-0689-83dc-f6c8da16410e@linaro.org> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=207.211.31.120; envelope-from=eblake@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/03 15:26:56 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , "open list:Block layer core" , crosthwaite.peter@gmail.com, quintela@redhat.com, f4bug@amsat.org, dgilbert@redhat.com, kraxel@redhat.com, dirty.ice.hu@gmail.com, pbonzini@redhat.com, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/3/20 11:32 AM, Richard Henderson wrote: >>> I'd prefer we generate a compile-time error than a runtime trap (or nothing, >>> depending on compiler flags controlling __builtin_unreachable). >> >> What we have DOES produce a compile-time error.  If either expression to >> MIN_CONST() is not actually const, the fact that __builtin_unreachable() >> returns void causes a compilation failure because a value is expected. > > Ah! Well, that's good and certainly sufficient for my needs. > > I do now wonder if it wouldn't be clearer to use "(void)0" > instead of __builtin_unreachable, and add a note to the comment just above. Yes, I just tested; using "((void)0)" in place of "__builtin_unreachable()" has the same effect (no change to valid use, and still a compiler error on misuse). gcc: /home/eblake/qemu/qemu-img.c: In function ‘is_allocated_sectors’: /home/eblake/qemu/qemu-img.c:1225:15: error: void value not ignored as it ought to be 1225 | i = MIN_CONST(i, n); | ^ clang: /home/eblake/qemu/qemu-img.c:1225:15: error: assigning to 'int' from incompatible type 'void' i = MIN_CONST(i, n); ^ ~~~~~~~~~~~~~~~ Of course, a comment explaining the intent can't hurt either. I'll wait to see if this gathers any other comments before spinning a v4 with that change. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org