linux-kernel - Re: BUG: GCC-4.4.x changes the function frame on some functions

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Date:	Thu, 19 Nov 2009 14:06:54 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Richard Guenther <richard.guenther@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	feng.tang@...el.com, Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, jakub@...hat.com,
	gcc@....gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions

On Thu, 2009-11-19 at 19:47 +0100, Ingo Molnar wrote:
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > Admittedly, anybody who compiles with -pg probably doesn't care deeply 
> > about smaller and more efficient code, since the mcount call overhead 
> > tends to make the thing moot anyway, but it really looks like a 
> > win-win situation to just fix the mcount call sequence regardless.
> 
> Just a sidenote: due to dyn-ftrace, which patches out all mcounts during 
> bootup to be NOPs (and opt-in patches them in again if someone runs the 
> function tracer), the cost is not as large as one would have it with say 
> -pg based user-space profiling.
> 
> It's not completely zero-cost as the pure NOPs balloon the i$ footprint 
> a bit and GCC generates different code too in some cases. But it's 
> certainly good enough that it's generally pretty hard to prove overhead 
> via micro or macro benchmarks that the patched out mcounts call sites 
> are there.

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.

-- Steve


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