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

[Arnd Bergmann <arnd@arndb.de>]

On Tue, May 5, 2020 at 4:08 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
> On Tue, May 5, 2020 at 4:58 PM Arnd Bergmann <arnd@arndb.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