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

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Message-Id: <4AD8C33C-8B3F-4FE5-993B-C73F334B2507@linaro.org>
Date:   Sat, 25 Feb 2017 11:50:30 +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 Feb 2017, at 11:23, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> 
> 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()?

... 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.