linux-kernel - Re: [PATCH] bitops: avoid clang shift-count-overflow warnings

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Date:   Tue, 5 May 2020 17:32:52 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] bitops: avoid clang shift-count-overflow warnings

On Tue, May 5, 2020 at 4:08 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Tue, May 5, 2020 at 4:58 PM Arnd Bergmann <arnd@...db.de> wrote:
> > Clang normally does not warn about certain issues in inline functions when
> > it only happens in an eliminated code path. However if something else
> > goes wrong, it does tend to complain about the definition of hweight_long()
> > on 32-bit targets:
>
> Shouldn't it be fixed in CLang?
>
> > include/linux/bitops.h:75:41: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
> >         return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
> >                                                ^~~~~~~~~~~~
>
> sizeof(w) is compile-time constant. It can easily drop the second part
> without even looking at it.
>
> > Adding an explicit cast to __u64 avoids that warning and makes it easier
> > to read other output.
>
> Looks like papering over the real issue.

I'm not sure if there is anything to be done about it in clang, since it
always does syntactic analysis before dead-code elimination by design.

It is a bit odd though that it only prints the warning sometimes, but
I suspect this is also something that works as designed. Maybe someone
on the clang-built-linux list knows more about the background.

         Arnd