linux-kernel - Re: [for-linus][PATCH 01/10] tracing: Suppress sparse warnings triggered by is_signed

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Date:   Tue, 23 Aug 2022 09:06:50 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Bart Van Assche <bvanassche@....org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [for-linus][PATCH 01/10] tracing: Suppress sparse warnings
 triggered by is_signed_type()

On 22/08/2022 20.45, Linus Torvalds wrote:
> On Mon, Aug 22, 2022 at 11:20 AM Bart Van Assche <bvanassche@....org> wrote:
>>
>> I agree that it's better that sparse sees the same code as what is used to
>> build the kernel. However, I do not agree that the patch above is a solution.
>> Sparse reports a warning for the suggested definition above of is_signed_type()
>> because the new definition tries to use the less-than (<) operator to compare
>> two __bitwise types.
> 
> Argh. I forgot that part. It wasn't just the cast that warned, it was
> the compare too.
> 
> But we did have a sparse fix for it, didn't we? That fix required that
> the '< (type)1' cast be changed to '<= (type)0' iirc, and a patch to
> sparse, but it at least avoided the problem.

Heh. I originally wrote the comparison "< (t)1" instead of "< (t)0" to
avoid a -Wtype-limits warning when applied to unsigned types - yeah
yeah, the kernel isn't built with that, but it's a nice macro to
copy-paste to other projects, and sometimes people do explicitly enable
-Wtype-limits to manually go through some, and then it's nice to not
have tons of false positives from this macro.

But of course <1 is the same as <=0, and we can indeed spell it that way
without triggering Wtype-limits. So if that can help with also silencing
sparse, ack from me on that part.

Rasmus