linux-kernel - Re: [PATCH] find_bit: Micro-optimise find_next_*

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Message-ID: <878tr65qbh.fsf@rasmusvillemoes.dk>
Date:   Fri, 23 Dec 2016 21:43:46 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Matthew Wilcox <mawilcox@...uxonhyperv.com>
Cc:     Yury Norov <yury.norov@...il.com>,
        George Spelvin <linux@...izon.com>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Matthew Wilcox <mawilcox@...rosoft.com>
Subject: Re: [PATCH] find_bit: Micro-optimise find_next_*_bit

On Fri, Dec 23 2016, Matthew Wilcox <mawilcox@...uxonhyperv.com> wrote:

> From: Matthew Wilcox <mawilcox@...rosoft.com>
>
> First, the first half of the test: (!nbits || start >= nbits) is
> trivially a subset of the second half, since nbits and start are both unsigned.

Yeah, I filed that as a missed optimization bug with gcc a year ago, but
it seems that even 6.3 still does two tests - clang 3.6 is a bit
smarter. Anyway,

Acked-by: Rasmus Villemoes <linux@...musvillemoes.dk>