From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751470AbdBYLKa (ORCPT <rfc822;w@1wt.eu>);
        Sat, 25 Feb 2017 06:10:30 -0500
Received: from ud10.udmedia.de ([194.117.254.50]:59338 "EHLO
        mail.ud10.udmedia.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751251AbdBYLK3 (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Sat, 25 Feb 2017 06:10:29 -0500
Date: Sat, 25 Feb 2017 12:09:28 +0100
From: Markus Trippelsdorf <markus@trippelsdorf.de>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>, Laura Abbott <labbott@redhat.com>,
        John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: gcc7 log2 compile issues in kernel/time/timekeeping.c
Message-ID: <20170225110928.GB1364@x4>
References: <9c74d635-d0d1-0893-8093-ce20b0933fc7@redhat.com>
 <CALAqxLXd6YSDn7GnV76dwuo48oDufYXwFpgAFwvFzKqaTyA2BA@mail.gmail.com>
 <CAKv+Gu_PuY1aHjyTtoVuUhFZbt-vL_LVC7frN+rtxm8PX0C2Cg@mail.gmail.com>
 <51e768e7-91af-86c5-3732-2e529364d376@redhat.com>
 <20170225081810.GA1364@x4>
 <CAKv+Gu876LoXXiZnhOEksS+r=pqv4U2-OhwvWAiYjGuQ1OT_WA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAKv+Gu876LoXXiZnhOEksS+r=pqv4U2-OhwvWAiYjGuQ1OT_WA@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 2017.02.25 at 09:11 +0000, Ard Biesheuvel wrote:
> On 25 February 2017 at 08:18, Markus Trippelsdorf <markus@trippelsdorf.de> 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.)

-- 
Markus