Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl

[Eric Blake <eblake@redhat.com>]

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On 02/02/2013 06:30 AM, Peter Maydell wrote:

>> - * Undefined if no bit exists, so code should check against 0 first.
>> + * Returns -1 if no bit exists.  Note that compared to the C library
>> + * routine ffsl, this one returns one less.
>>   */
> 
> Do any of our callers actually use the "-1 on 0 input" semantics?
> (I guess that means "your new code you added" since the previous
> callers all were happy with the undefined-on-zero semantics).

Yes, Paolo's code was replacing:

ffsl(var) - 1

with

bitops_ctzl(var)

where var==0 was a definite possibility, so we DO have code that depends
on this semantic of returning -1.

> It seems an odd choice, since I can see a justification for:
>  (a) "return number of bits in word" [every bit in the word is
>      a trailing zero in some sense]
>  (b) "undefined" [matches gcc builtin_ctz semantics]

For all non-zero values of var, ffsl(var) == bitops_ctzl(var)+1.
Extending the equivalency for var==0 makes the function usable in the
most places.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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