linux-kernel - Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support

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Message-ID: <20140409114857.GT11096@twins.programming.kicks-ass.net>
Date:	Wed, 9 Apr 2014 13:48:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	David Ahern <dsahern@...il.com>, Andi Kleen <andi@...stfloor.org>,
	Stephane Eranian <eranian@...gle.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support

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.

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).

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?
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