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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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