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
Date: Sat, 25 Feb 2017 12:09:28 +0100 [thread overview]
Message-ID: <20170225110928.GB1364@x4> (raw)
In-Reply-To: <CAKv+Gu876LoXXiZnhOEksS+r=pqv4U2-OhwvWAiYjGuQ1OT_WA@mail.gmail.com>
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
next prev parent reply other threads:[~2017-02-25 11:10 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-23 18:43 gcc7 log2 compile issues in kernel/time/timekeeping.c Laura Abbott
2017-02-24 21:25 ` John Stultz
2017-02-24 21:45 ` Ard Biesheuvel
2017-02-24 23:33 ` Laura Abbott
2017-02-25 8:18 ` Markus Trippelsdorf
2017-02-25 9:11 ` Ard Biesheuvel
2017-02-25 11:09 ` Markus Trippelsdorf [this message]
2017-02-25 11:23 ` Ard Biesheuvel
2017-02-25 11:50 ` Ard Biesheuvel
2017-03-01 0:00 ` Laura Abbott
2017-03-01 17:39 ` Ard Biesheuvel
2017-03-02 10:11 ` Markus Trippelsdorf
2017-03-02 10:38 ` Ard Biesheuvel
2017-03-02 20:19 ` Linus Torvalds
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