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Date: Wed, 9 Apr 2014 18:48:53 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>, David Ahern <dsahern@gmail.com>,
        Andi Kleen <andi@firstfloor.org>,
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        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
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Subject: Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support
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On Wed, Apr 09, 2014 at 01:48:57PM +0200, Peter Zijlstra wrote:
> On Wed, Feb 26, 2014 at 12:26:43PM -0800, Andy Lutomirski wrote:
> > Speed.  FPO saves one register (a big deal on x86_32; not so important
> > on x86_64) but also saves a few cycles on function entry and exit,
> > which is a bigger deal for small functions.
> 
> So I though that LTO was supposed to get rid of a lot of the small
> function and inline them.

It does it when it can (no indirect), thinks it's profitable and won't
increase code size too much. 

> 
> I've also heard that in practise this is very 'hard', and thus we're
> still stuck with a gazillion small functions (mostly C++ people suffer
> from this).

They need devirtualization, which we cannot do currently in the kernel.

> 
> Can anybody give a concise explanation on why LTO doesn't rid us of
> these small functions or point to a web resource that describes the
> problem?

It depends on the code of course.
On one of my LTO builds I have ~10% less functions in System.map.

Actual results will vary of course on the config.

-Andi


-- 
ak@linux.intel.com -- Speaking for myself only.