linux-kernel - Re: [PATCH v2 1/3] lib: find_*

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Date:	Thu, 05 Feb 2015 15:51:32 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Yury <yury.norov@...il.com>
Cc:	George Spelvin <linux@...izon.com>, akpm@...ux-foundation.org,
	chris@...is-wilson.co.uk, davem@...emloft.net, dborkman@...hat.com,
	hannes@...essinduktion.org, klimov.linux@...il.com,
	laijs@...fujitsu.com, linux-kernel@...r.kernel.org,
	msalter@...hat.com, takahiro.akashi@...aro.org, tgraf@...g.ch,
	valentinrothberg@...il.com, Yury Norov <y.norov@...sung.com>
Subject: Re: [PATCH v2 1/3] lib: find_*_bit reimplementation

On Thu, Feb 05 2015, Yury <yury.norov@...il.com> wrote:

> On 02.02.2015 15:56, Rasmus Villemoes wrote:
>> On Mon, Feb 02 2015, "George Spelvin" <linux@...izon.com> wrote:
>>
>>> Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
>>>> ... and this be part of _find_next_bit? Can find_next_bit not be simply
>>>> 'return _find_next_bit(addr, size, offset, 1);', and similarly for
>>>> find_next_zero_bit? Btw., passing true and false for the boolean
>>>> parameter may be a little clearer.
>>> Looking at the generated code, it would be better to replace the boolean
>>> parameter with 0ul or ~0ul and XOR with it.  The same number of registers,
>>> and saves a conditional branch.
>> Nice trick. When I compiled it, gcc inlined _find_next_bit into both its
>> callers, making the conditional go away completely. That was with gcc
>> 4.7. When I try with 5.0, I do see _find_next_bit being compiled
>> separately.
>>
>> With the proposed change, 4.7 also makes find_next{,_zero}_bit wrappers
>> for _find_next_bit, further reducing the total size, which is a good
>> thing. And, if some other version decides to still inline it, it
>> should then know how to optimize the xor with 0ul or ~0ul just as well
>> as when the conditional was folded away. 
>>
>> Yury, please also incorporate this in the next round.
>>
>> Rasmus
>>
> Ok.

Good.

> What are you thinking about joining _find_next_bit and
> _find_next_bit_le?

I don't think that should be done right now, if at all. The series is
pretty close to getting my Reviewed-by; I'd prefer not to start over.

Rasmus
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