Date: Thu, 3 Aug 2017 16:24:56 +0300
From: Yury Norov <ynorov@caviumnetworks.com>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: zijun_hu <zijun_hu@htc.com>, Andrew Morton <akpm@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, Mark Rutland <mark.rutland@arm.com>,
        Laura Abbott <labbott@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kernel@vger.kernel.org, Greg Hackmann <ghackmann@google.com>,
        Doug Anderson <dianders@chromium.org>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] bitops: Avoid integer overflow warning in GENMASK_ULL
Message-ID: <20170803132456.mwrt6yspkbqxcffw@yury-thinkpad>
References: <20170802225159.159536-1-mka@chromium.org>
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On Wed, Aug 02, 2017 at 03:51:58PM -0700, Matthias Kaehlcke wrote:
> GENMASK_ULL performs a left-shift of (~0ULL), which technically
> results in an integer overflow. clang raises a warning about
> this if the overflow occurs in a preprocessor expression. To
> avoid the overflow first perform a right-shift to clear the
> bits that are shifted out.
> 
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
>  include/linux/bitops.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index a83c822c35c2..21dfe63001e3 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -22,7 +22,7 @@
>  	(((~0UL) << (l)) & (~0UL >> (BITS_PER_LONG - 1 - (h))))

Once you touche GENMASK_ULL, why not to fix also GENMASK.

>  
>  #define GENMASK_ULL(h, l) \
> -	(((~0ULL) << (l)) & (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
> +	((((~0ULL) >> (l)) << (l)) & (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
>  
>  extern unsigned int __sw_hweight8(unsigned int w);
>  extern unsigned int __sw_hweight16(unsigned int w);
> -- 
> 2.14.0.rc1.383.gd1ce394fe2-goog
> 
> 
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