linux-kernel - Re: [RFC][PATCH] signal: replace !likely with unlikely!

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Date:	Sat, 13 Sep 2014 06:28:18 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Richard Weinberger <richard@....at>,
	Li RongQing <roy.qing.li@...il.com>
CC:	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	fabf@...net.be, jkosina@...e.cz,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>, oleg@...hat.com,
	akpm@...ux-foundation.org
Subject: Re: [RFC][PATCH] signal: replace !likely with unlikely!

On 09/13/2014 03:04 AM, Richard Weinberger wrote:
> Am 13.09.2014 04:48, schrieb Li RongQing:
>> I did not test, how to test it?
>
> Compare the object files of both variants to find out whether both create
> semantically equivalent code and whether the un/likely have an effect.
> i.e. objdump -S -d kernel/signal.o
> I guess you can just remove the likely as gcc is smart enough to detect the "goto ret;"
> as an unlikely taken branch.
>
Or just write similar test code and observe the result.

When you are doing that, you might learn that if "(!likely(expression))"
is _not_ the same as "if (unlikely(!expression))".

Guenter

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