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

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Message-ID: <1342739063.12353.82.camel@gandalf.stny.rr.com>
Date:	Thu, 19 Jul 2012 19:04:23 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 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.

-- Steve


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