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Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in ho
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl |
Date: |
Sat, 02 Feb 2013 08:11:16 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 |
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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- [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl, Paolo Bonzini, 2013/02/01
- Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl, Eric Blake, 2013/02/01
- Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl, Andreas Färber, 2013/02/02
- Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl, Peter Maydell, 2013/02/02
- Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl,
Eric Blake <=
- Re: [Qemu-devel] [PATCH v3] bitops: unify bitops_ffsl with the one in host-utils.h, call it bitops_ctzl, Blue Swirl, 2013/02/03