linux-kernel - Re: [PATCH v7 04/17] tracing: add static function tracer support for MIPS

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Date:	Wed, 11 Nov 2009 21:51:53 +0800
From:	Wu Zhangjin <wuzhangjin@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	David Daney <ddaney@...iumnetworks.com>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, zhangfx@...ote.com, zhouqg@...il.com,
	Ralf Baechle <ralf@...ux-mips.org>, rostedt@...dmis.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Nicholas Mc Guire <der.herr@...r.at>,
	Richard Sandiford <rdsandiford@...glemail.com>,
	Patrik Kluba <kpajko79@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Simek <monstr@...str.eu>
Subject: Re: [PATCH v7 04/17] tracing: add static function tracer support
 for MIPS

Hi,

On Wed, 2009-11-11 at 13:15 +0000, Maciej W. Rozycki wrote:
> On Wed, 11 Nov 2009, Wu Zhangjin wrote:
> 
> > > -mlong-calls really degrades performance.  I have seen things like 6% 
> > > drop in network packet forwarding rates with -mlong-calls.
> > > 
> > 
> > so much drop? seems only two instructions added for it: lui, addi. from
> > this view point, I think the -fno-omit-frame-pointer(add, sd, move...)
> > will also bring with much drop.
> 
>  No, register jumps cannot be predicted -- this is where the performance 
> goes on any serious processor -- the two extra instructions are nothing 
> compared to that.  OTOH frame pointer calculations are pure arithmetic, so 
> you only lose time incurred by the instructions themselves.

Yes, I only mean the -mlong-calls and the original -mno-long-calls with
-pg.

The orignal one looks like this:

move ra, at
jal _mcount

The new one with -mlong-calls looks like this:

lui v1, HI_16BIT_OF_MCOUNT
addiu v1, v1, LOW_16BIT_OF_MCOUNT
move ra, at
jalr v1

both of them have a "jump" instruciton, so, only two lui, addiu added
for -mlong-calls ;)

what about the difference between that "jal _mcount"  and "jalr v1"?

Regards,
	Wu Zhangjin

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