From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751683AbdBYLu5 (ORCPT ); Sat, 25 Feb 2017 06:50:57 -0500 Received: from mail-wr0-f169.google.com ([209.85.128.169]:32881 "EHLO mail-wr0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751360AbdBYLu4 (ORCPT ); Sat, 25 Feb 2017 06:50:56 -0500 Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (1.0) Subject: Re: gcc7 log2 compile issues in kernel/time/timekeeping.c From: Ard Biesheuvel X-Mailer: iPhone Mail (14D27) In-Reply-To: Date: Sat, 25 Feb 2017 11:50:30 +0000 Cc: Linus Torvalds , Will Deacon , Laura Abbott , John Stultz , Thomas Gleixner , Linux Kernel Mailing List Message-Id: <4AD8C33C-8B3F-4FE5-993B-C73F334B2507@linaro.org> References: <9c74d635-d0d1-0893-8093-ce20b0933fc7@redhat.com> <51e768e7-91af-86c5-3732-2e529364d376@redhat.com> <20170225081810.GA1364@x4> <20170225110928.GB1364@x4> To: Markus Trippelsdorf Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v1PBp4E4011028 > On 25 Feb 2017, at 11:23, Ard Biesheuvel wrote: > > On 25 February 2017 at 11:09, Markus Trippelsdorf > wrote: >> On 2017.02.25 at 09:11 +0000, Ard Biesheuvel wrote: >>>> On 25 February 2017 at 08:18, Markus Trippelsdorf wrote: >>>> >>>> Why not simply get rid of the ____ilog2_NaN thing altogether? >>>> >>> >>> That would remove the issue, sure. But we lose an opportunity to spot >>> incorrect code at compile time. >> >> In the case of kernel/time/timekeeping.c it is clearly a false positive. >> Was ever incorrect code spotted by ____ilog2_NaN in the past? >> >>> My concern is that it by not pushing back on changes to the semantics >>> of __builtin_constant_p() such as this one, we may start seeing other >>> issues where we can no longer use it, and we lose a very useful tool. >> >> We had a long discussion in: >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785 >> As you can see there is no real consensus. >> But ilog2 seems to be the only place where this ever popped up. >> (There were several distro-wide mass rebuilds with gcc-7 and no other >> __builtin_constant_p() issue was found yet.) >> > > Well, given that it is really dead code that is being emitted, and > that log2(0) is really undefined, perhaps we should simply replace > ilog2_NaN() with __builtin_unreachable()? ... or perhaps it is better to just pass the constant == 0 to the runtime implementation? The second ilog2_NaN is really unreachable, given that it deals with unsigned values >0 without a single bit set.