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Date: Fri, 30 Mar 2012 12:25:28 +0400
From: Andrey Chernov <ache@FreeBSD.ORG>
To: Dimitry Andric <dim@FreeBSD.ORG>
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Cc: svn-src-head@FreeBSD.ORG, svn-src-all@FreeBSD.ORG,
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Subject: Re: svn commit: r233684 - head/sys/x86/include
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On Thu, Mar 29, 2012 at 11:31:48PM +0000, Dimitry Andric wrote:
>   However, the arguments are not properly masked, which results in the
>   wrong value being calculated in some instances.  For example,
>   bswap32(0x12345678) returns 0x7c563412, and bswap64(0x123456789abcdef0)
>   returns 0xfcdefc9a7c563412.

Is sign extension considered in that place? Shifting any signed value to 
">>" direction (char, short, int, etc.) replicates sign bit, so cast to 
corresponding unsigned value must be done first, which may take less 
instructions, than masking (I am not sure about this part, just 
guessing). Casting in that case applies to the argument (x) not to result 
(x >> YY).

>  
>  #define	__bswap16_gen(x)	(__uint16_t)((x) << 8 | (x) >> 8)
>  #define	__bswap32_gen(x)		\
> -	(((__uint32_t)__bswap16(x) << 16) | __bswap16((x) >> 16))
> +	(((__uint32_t)__bswap16((x) & 0xffff) << 16) | __bswap16((x) >> 16))
>  #define	__bswap64_gen(x)		\
> -	(((__uint64_t)__bswap32(x) << 32) | __bswap32((x) >> 32))
> +	(((__uint64_t)__bswap32((x) & 0xffffffff) << 32) | __bswap32((x) >> 32))
>  
>  #ifdef __GNUCLIKE_BUILTIN_CONSTANT_P
>  #define	__bswap16(x)				\


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