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

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Message-ID: <20111216085459.GD4170@elte.hu>
Date:	Fri, 16 Dec 2011 09:54:59 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
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>,
	Jan Beulich <JBeulich@...e.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>
Subject: Re: [PATCH] x86: Use -m-omit-leaf-frame-pointer to shrink text size


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Fri, 16 Dec 2011 09:19:16 +0100 Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > This patch turns on -momit-leaf-frame-pointer on x86 builds and 
> > thus shrinks .text noticeably. On a defconfig-ish kernel:
> > 
> >    text	   data	    bss	    dec	    hex	filename
> >    9843902	1935808	3649536	15429246	 eb6e7e	vmlinux.before
> >    9813764	1935792	3649536	15399092	 eaf8b4	vmlinux.after
> > 
> > That's 0.3% off text size.
> > 
> > The actual win is larger than this percentage suggests: many 
> > small, hot helper functions such as find_next_bit(), 
> > do_raw_spin_lock() or most of the list_*() functions are leaf 
> > functions and are now shorter by 2 instructions.
> > 
> > Probably a good chunk of the framepointers related runtime 
> > overhead on common workloads is eliminated via this patch, as 
> > small leaf functions execute more often than larger parent 
> > functions.
> > 
> > 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.
> 
> The only problem I can think of (apart from tickling gcc bugs) is that
> it might break __builtin_return_address(n) for n>0 with frame pointers
> enabled?  The only code I can find which does this is
> drivers/isdn/hardware/mISDN/ and ftrace.

Well, AFAICS it won't really 'break' it but behave as if the 
leaf function got inlined into the parent function. I think we 
can live with that.

Thanks,

	Ingo
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