linux-kernel - Re: [GIT PULL] perf fixes

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Message-ID: <4FE60A26.30109@linux.intel.com>
Date:	Sat, 23 Jun 2012 11:25:42 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] perf fixes

On 06/22/2012 05:51 PM, Arjan van de Ven wrote:
> 
> if it's truely an official nop, it will take decoder bandwidth obviously
> (which can decode 3 to 4 instructions per cycle, depending on the
> brand/model of CPU and the total size in bytes of these instructions).
> likewise, at the end of the out of order pipeline, NOPs may take a
> retirement slot (again 3 to 4 instructions per cycle)
> 
> icache is there as well, and if the NOP actually changes cpu flags (some
> of the less fortunate ones do) that can create a false data dependency.
> 

If it changes CPU flags it's most definitely not a NOP.

Either way, the 0F 1F ops are official, and listed in the SDM.

	-hpa

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