linux-kernel - Re: gcc7 log2 compile issues in kernel/time/timekeeping.c

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Date:   Sat, 25 Feb 2017 11:23:48 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will.deacon@....com>,
        Laura Abbott <labbott@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: gcc7 log2 compile issues in kernel/time/timekeeping.c

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

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()?