linux-kernel - Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save

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Message-ID: <50089323.6050203@zytor.com>
Date:	Thu, 19 Jul 2012 16:07:15 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	yrl.pp-manager.tt@...achi.com
Subject: Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save_regs for i386 function
 calls

On 07/19/2012 04:04 PM, Steven Rostedt wrote:
> On Thu, 2012-07-19 at 15:53 -0700, H. Peter Anvin wrote:
> 
>> lea is not typically faster than add, but in the case of Atom, it is
>> done in an earlier pipeline stage (AGU instead of ALU) which means lea
>> is faster if its inputs are already available as address expressions and
>> is consumed by address expressions; the goal is to avoid the ALU->AGU
>> forwarding latency.
> 
> Well, the question is, which is faster:
> 
> 	lea 8(%esp), %esp
> 	addl $8, %esp
> 
> Basically, all we want to do is add 8 to the stack pointer. And this is
> for the x86_32 version of whatever hardware is in use.
> 

What I'm telling you is that it depends on the context.

An address expression needs to be ready in the AGU; a piece of data
comes from the ALU.  Whenever something moves from the ALU to the AGU,
there is a penalty.  There is no penalty to move from the AGU to the
ALU, since the ALU is in a later stage.

I *believe* the stack adjustments push/pop are done in the AGU, but I
have to double-check.

	-hpa

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