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

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Date:   Tue, 5 May 2020 16:01:49 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] bitops: avoid clang shift-count-overflow warnings

On Tue, May 05, 2020 at 03:54:57PM +0200, Arnd Bergmann 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:
> 
> include/linux/bitops.h:75:41: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
>         return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
>                                                ^~~~~~~~~~~~
> include/asm-generic/bitops/const_hweight.h:29:49: note: expanded from macro 'hweight64'
>  define hweight64(w) (__builtin_constant_p(w) ? __const_hweight64(w) : __arch_hweight64(w))
>                                                 ^~~~~~~~~~~~~~~~~~~~
> include/asm-generic/bitops/const_hweight.h:21:76: note: expanded from macro '__const_hweight64'
>  define __const_hweight64(w) (__const_hweight32(w) + __const_hweight32((w) >> 32))
>                                                                            ^  ~~
> include/asm-generic/bitops/const_hweight.h:20:49: note: expanded from macro '__const_hweight32'
>  define __const_hweight32(w) (__const_hweight16(w) + __const_hweight16((w) >> 16))
>                                                 ^
> include/asm-generic/bitops/const_hweight.h:19:72: note: expanded from macro '__const_hweight16'
>  define __const_hweight16(w) (__const_hweight8(w)  + __const_hweight8((w)  >> 8 ))
>                                                                        ^
> include/asm-generic/bitops/const_hweight.h:12:9: note: expanded from macro '__const_hweight8'
>           (!!((w) & (1ULL << 2))) +     \
> 
> Adding an explicit cast to __u64 avoids that warning and makes it easier
> to read other output.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---

Acked-by: Christian Brauner <christian.brauner@...ntu.com>