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

Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS
From: David Daney <>
Date: Wed, 21 Oct 2009 10:07:44 -0700
Cc: Wu Zhangjin <>,,, Thomas Gleixner <>, Ralf Baechle <>, Nicholas Mc Guire <>
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Steven Rostedt wrote:
On Wed, 2009-10-21 at 09:37 -0700, David Daney wrote:

There is no deterministic way to identify MIPS function prologs. This is especially true for leaf functions, but also for functions with multiple return sites.

For certain GCC versions there may be a set of command line options that would give good results, but in general it is not possible. Attempts at fast backtrace generation using code inspection are not reliable and will invariably result in faults and panics when they fail.

Thanks for the update.

We can easily protect against panics, since we do fault protection
within the code (although currently it will panic on fault, but we can
fix that ;-). We can limit the search to a couple of 100 instructions,
as well as fail on first panic.

But are you sure that when compiled with -pg, that GCC does not give a
reliable prologue. Things are different when GCC is compiled with -pg,
it may indeed always have something that we can flag.

We could also add other tests, like the subtraction of the stack too.

I have not used -pg, so I don't know for sure, I think all it does is add the calls to _mcount. Someone could investigate -fno-omit-frame-pointer, with that you may be able to use:

   move    s8,sp

To identify function prologs, but it would still be ad hoc, as modern versions of GCC will reorder instructions in the prolog for better scheduling.

David Daney

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