Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save_regs for i386 function calls

From: "H. Peter Anvin" <hpa@zytor.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	yrl.pp-manager.tt@hitachi.com
Subject: Re: [RFC][PATCH 2/4 v4] ftrace/x86: Add save_regs for i386 function calls
Date: Thu, 19 Jul 2012 16:07:15 -0700	[thread overview]
Message-ID: <50089323.6050203@zytor.com> (raw)
In-Reply-To: <1342739063.12353.82.camel@gandalf.stny.rr.com>

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