linux-kernel - Re: [PATCH] lib: Make _find_next

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Message-ID: <87si701maf.fsf@rasmusvillemoes.dk>
Date:	Sun, 30 Aug 2015 23:47:20 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Yury <yury.norov@...il.com>
Cc:	Alexey Klimov <klimov.linux@...il.com>,
	Cassidy Burden <cburden@...eaurora.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arm-msm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Mark Salter <msalter@...hat.com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	Thomas Graf <tgraf@...g.ch>,
	Valentin Rothberg <valentinrothberg@...il.com>,
	Chris Wilson <chris@...is-wilson.co.uk>, linux@...izon.com
Subject: Re: [PATCH] lib: Make _find_next_bit helper function inline

I've lost track of what's up and down in this, but now that I look at
this again let me throw in my two observations of stupid gcc behaviour:
For the current code, both debian's gcc (4.7) and 5.1 partially inlines
_find_next_bit, namely the "if (!nbits || start >= nbits)" test. I know it
does it to avoid a function call, but in this case the early return
condition is unlikely, so there's not much to gain. Moreover, it fails
to optimize the test to simply "if (start >= nbits)" - everything being
unsigned, these are obviously equivalent.

Rasmus
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