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>
To: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Richard Guenther <richard.guenther@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	feng.tang@intel.com, Fr??d??ric Weisbecker <fweisbec@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	jakub@redhat.com, gcc@gcc.gnu.org
Subject: Re: [PATCH] gcc mcount-nofp was Re: BUG: GCC-4.4.x changes the function frame on some functions
Date: Fri, 20 Nov 2009 07:34:19 -0500	[thread overview]
Message-ID: <1258720459.22249.1018.camel@gandalf.stny.rr.com> (raw)
In-Reply-To: <87iqd54iz5.fsf_-_@basil.nowhere.org>

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