linux-kernel - Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size

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Date:	Fri, 16 Dec 2011 01:23:32 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frédéric Weisbecker 
	<fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Beulich <JBeulich@...e.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size

On 12/16/2011 12:53 AM, Ingo Molnar wrote:
> * Ingo Molnar <mingo@...e.hu> wrote:
>
>> [...]
>>
>> The call-chains are still intact for quality backtraces and 
>> for call-chain profiling (perf record -g), as the backtrace 
>> walker can deduct the full backtrace from the RIP of a leaf 
>> function and the parent chain.
> Hm, noticed one complication while looking at annotated assembly 
> code in perf top. Code doing function calls from within asm() is 
> incorrectly marked 'leaf' by GCC:
>
> ffffffff812b82d8 <arch_local_save_flags>:
> ffffffff812b82d8:       ff 14 25 00 d9 c1 81    callq  *0xffffffff81c1d900
> ffffffff812b82df:       c3                      retq   
>
> So all the paravirt details will have to be fixed, so that GCC 
> is able to see that there's a real function call done inside. 
> Jeremy, Konrad?

Um.  So the issue is that a function that contains only pvops looks like
it's a leaf to gcc and it does some leaf-function optimisation?

How can we tell gcc the asm contains a call, or otherwise suppress the
"leaf function" classification?

The alternative is to just make it a plain C-level indirect call, but
then we'd lose all the patching and callee-save optimisations.

Any suggestions?

Thanks,
    J

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