linux-kernel - Re: [PATCH] RWSEM: Rewrite rwsem.c and rwsem-spinlock.c more simply.

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Message-ID: <20080325123633.GL2170@one.firstfloor.org>
Date:	Tue, 25 Mar 2008 13:36:33 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	"Robert P. J. Day" <rpjday@...shcourse.ca>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH] RWSEM: Rewrite rwsem.c and rwsem-spinlock.c more simply.

> i'm not sure what this means -- which of the transformations in that
> patch is considered unsafe?  here's a typical simplification:

It is not unsafe, just generates slight worse code.

current is inline assembler and the compiler doesn't know that it could
cache it in a register because it is not marked pure 
for various reasons. That is why current is often cached explicitely
in a local variable to tell the compiler that.

Before you run off and do that everywhere: it is also not a large
win, just a small one unless current is used very often.

-Andi
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