linux-kernel - Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS

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Message-ID: <4AE08559.40806@caviumnetworks.com>
Date:	Thu, 22 Oct 2009 09:16:25 -0700
From:	David Daney <ddaney@...iumnetworks.com>
To:	rostedt@...dmis.org
CC:	wuzhangjin@...il.com, linux-kernel@...r.kernel.org,
	linux-mips@...ux-mips.org, Thomas Gleixner <tglx@...utronix.de>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Nicholas Mc Guire <der.herr@...r.at>
Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for
 MIPS

Steven Rostedt wrote:
> On Thu, 2009-10-22 at 08:59 -0700, David Daney wrote:
> 
>> This is what I was talking about up-thread.  Leaf functions may have no 
>> function prolog.  If you do code scanning you will fail.  While scanning 
>> backwards, there is no way to know when you have entered a new function. 
>>   Looking for function return sequences 'jr ra' doesn't work as there 
>> may be functions with multiple return sites, functions that never 
>> return, or arbitrary data before the function.  I think you have to 
>> force a frame pointer to be established if you want this to work.
> 
> Functions that run off into another function?? I guess the compiler
> could do that, but with -pg enable, I would think is broken.
> 

Use of GCC-4.5's __builtin_unreachable() can lead to this, as well as 
functions that call noreturn functions.


Note to self: Time to resend the __builtin_unreachable() patch set again...

David Daney
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