linux-kernel - Re: Re-tune x86 uaccess code for PREEMPT

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Message-ID: <5206C623.4020605@zytor.com>
Date:	Sat, 10 Aug 2013 16:00:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Mike Galbraith <bitbucket@...ine.de>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: Re-tune x86 uaccess code for PREEMPT_VOLUNTARY

On 08/10/2013 11:51 AM, Linus Torvalds wrote:
> Note that you still want the *test* to be done in C code, because together
> with "unlikely()" you'd likely do pretty close to optimal code
> generation, and hiding the decrement and test and conditional jump in
> asm you wouldn't get the proper instruction scheduling and branch
> following that gcc does.

How much instruction scheduling can gcc actually do given that there is
a barrier involved?  I guess some on the RISC architectures which need
load/subtract/store...

	-hpa


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