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Subject: Re: [PATCH] gcc mcount-nofp was Re: BUG: GCC-4.4.x changes the
 function frame on some functions
From: Steven Rostedt <rostedt@goodmis.org>
Reply-To: rostedt@goodmis.org
To: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>,
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Date: Fri, 20 Nov 2009 07:34:19 -0500
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On Fri, 2009-11-20 at 10:57 +0100, Andi Kleen wrote:
> Steven Rostedt <rostedt@goodmis.org> writes:
> >
> > And frame pointers do add a little overhead as well. Too bad the mcount
> > ABI wasn't something like this:
> >
> >
> > 	<function>:
> > 		call	mcount
> > 		[...]
> >
> > This way, the function address for mcount would have been (%esp) and the
> > parent address would be 4(%esp). Mcount would work without frame
> > pointers and this whole mess would also become moot.
> 
> I did a patch to do this in x86 gcc some time ago. The motivation
> was indeed the frame pointer overhead on Atom with tracing.
> 

Yes, I remember you talking about this but I don't remember how far it
went.



> Unfortunately it also requires glibc changes (I did those too). For
> compatibility and catching mistakes the new function was called
> __mcount_nofp.

Actually, could you change the name? I really hate the "mcount" name, it
is legacy and with a new feature, it should be abandoned. Something like
"__fentry__" would be nicer.

> 
> I haven't tried it with current gcc and last time I missed the 
> gcc feature merge window with this.
> 
> But perhaps you find it useful. Of course it would need more
> kernel changes to probe for the new option and handle it.
> 
> Here's the old patch. I haven't retested it with a current
> gcc version, but I think it still applies at least.
> 
> If there's interest I can polish it up and submit formally.

I would definitely be interested, and I would also be willing to test
it.

Thanks!

-- Steve



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