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

To: rostedt@goodmis.org
Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS
From: David Daney <ddaney@caviumnetworks.com>
Date: Thu, 22 Oct 2009 09:16:25 -0700
Cc: wuzhangjin@gmail.com, linux-kernel@vger.kernel.org, linux-mips@linux-mips.org, Thomas Gleixner <tglx@linutronix.de>, Ralf Baechle <ralf@linux-mips.org>, Nicholas Mc Guire <der.herr@hofr.at>
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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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