netdev - Re: [PATCH 1/1] mac80211: improve the contiguous mask checking

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Date:	Thu, 17 Dec 2015 16:04:28 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Zeng Zhaoxiu <zhaoxiu.zeng@...il.com>, davem@...emloft.net
Cc:	linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mac80211: improve the contiguous mask checking

On Thu, 2015-12-17 at 22:59 +0800, Zeng Zhaoxiu wrote:
> If the result of adding the first set bit to the mask is power of 2,
> the mask must be contiguous. "mask & -mask" can get the first set bit
> of mask gracefully.

> -		if (__ffs64(mask) + hweight64(mask) != fls64(mask))
> {
> +		inc = (mask & -mask);
> +		val = mask + inc;
> +		if ((val & (val - 1)) != 0) {
>  			/* not a contiguous mask ... not handled now! */

Hm. Ok, I can see how that would work, but it doesn't really seem like
much of an "improvement" to me? Surely I seem to need much more
thinking to understand this. There's no reason to optimise it either,
so why should we change it?

johannes
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