linux-kernel - Re: [PATCH] word-at-a-time: avoid undefined behaviour in zero

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Message-ID: <20140501090438.GA30166@arm.com>
Date:	Thu, 1 May 2014 10:04:39 +0100
From:	Will Deacon <will.deacon@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	Victor Kamensky <victor.kamensky@...aro.org>
Subject: Re: [PATCH] word-at-a-time: avoid undefined behaviour in
 zero_bytemask macro

Hi Peter,

On Wed, Apr 30, 2014 at 10:22:19PM +0100, H. Peter Anvin wrote:
> On 04/23/2014 09:52 AM, Will Deacon wrote:
> > diff --git a/include/asm-generic/word-at-a-time.h b/include/asm-generic/word-at-a-time.h
> > index d3909effd725..d96deb443f18 100644
> > --- a/include/asm-generic/word-at-a-time.h
> > +++ b/include/asm-generic/word-at-a-time.h
> > @@ -50,11 +50,7 @@ static inline bool has_zero(unsigned long val, unsigned long *data, const struct
> >  }
> >  
> >  #ifndef zero_bytemask
> > -#ifdef CONFIG_64BIT
> > -#define zero_bytemask(mask)	(~0ul << fls64(mask))
> > -#else
> > -#define zero_bytemask(mask)	(~0ul << fls(mask))
> > -#endif /* CONFIG_64BIT */
> > -#endif /* zero_bytemask */
> > +#define zero_bytemask(mask) (~0ul << __fls(mask) << 1)
> > +#endif
> >  
> >  #endif /* _ASM_WORD_AT_A_TIME_H */
> > 
> 
> Why not:
> 
> #define zero_bytemask(mask) (~1ul << __fls(mask))

Yup, that'll work too -- it produces an identical disassembly for arm64.

Will
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