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Re: [PATCH -v4 4/9] tracing: add static function tracer support for MIPS

To: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH -v4 4/9] tracing: add static function tracer support for MIPS
From: Frederic Weisbecker <fweisbec@gmail.com>
Date: Thu, 22 Oct 2009 23:09:52 +0200
Cc: Adam Nemet <anemet@caviumnetworks.com>, David Daney <ddaney@caviumnetworks.com>, wuzhangjin@gmail.com, Richard Sandiford <rdsandiford@googlemail.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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On Thu, Oct 22, 2009 at 04:52:06PM -0400, Steven Rostedt wrote:
> On Thu, 2009-10-22 at 13:30 -0700, Adam Nemet wrote:
> > 
> > Speaking of performance, -pg also affects the instruction scheduling freedom
> > of the compiler in the prologue.  With profiling, we limit optimizations not
> > to move instructions in and out of the prologue.
> > 
> > Also note that for functions invoked via tail call you won't get an exit
> > event.  E.g. if bar is tail-called from foo:
> > 
> >   foo entered
> >   bar entered
> >   foo/bar exited
> > 
> > However, this is not MIPS-specific and you can always disable tail calls
> > with -fno-optimize-sibling-calls.


Ouch..

 
> The question is, would bar have a _mcount call? So far, we have not had
> any issues with this on either x86 nor PPC.


Nothing would prevent that I guess. I mean, we are doing a very specific
use of -pg, and common uses wouldn't require to disable the mcount call on
bar in this situation, so it's not something that -pg is supposed to care
about.


> /me knocks on wood.


Me too (but not so hard...)


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