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>,
Stephane Eranian <eranian@google.com>,
"Yan, Zheng" <zheng.z.yan@intel.com>,
LKML <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Arnaldo Carvalho de Melo <acme@infradead.org>
Subject: Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support
Date: Wed, 9 Apr 2014 18:48:53 +0200 [thread overview]
Message-ID: <20140409164852.GA22728@two.firstfloor.org> (raw)
In-Reply-To: <20140409114857.GT11096@twins.programming.kicks-ass.net>
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.
next prev parent reply other threads:[~2014-04-09 16:48 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-18 6:07 [PATCH v3 00/14] perf, x86: Haswell LBR call stack support Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 01/14] perf, x86: Reduce lbr_sel_map size Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 02/14] perf, core: introduce pmu context switch callback Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 03/14] perf, x86: use context switch callback to flush LBR stack Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 04/14] perf, x86: Basic Haswell LBR call stack support Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 05/14] perf, core: pmu specific data for perf task context Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 06/14] perf, core: always switch pmu specific data during context switch Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 07/14] perf, x86: track number of events that use LBR callstack Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 08/14] perf, x86: allocate space for storing LBR stack Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 09/14] perf, x86: Save/resotre LBR stack during context switch Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 10/14] perf, core: simplify need branch stack check Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 11/14] perf, core: Pass perf_sample_data to perf_callchain() Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 12/14] perf, x86: use LBR call stack to get user callchain Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 13/14] perf, x86: enable LBR callstack when recording callchain Yan, Zheng
2014-02-18 6:07 ` [PATCH v3 14/14] perf, x86: Discard zero length call entries in LBR call stack Yan, Zheng
2014-02-23 19:47 ` [PATCH v3 00/14] perf, x86: Haswell LBR call stack support Stephane Eranian
2014-02-24 1:07 ` Yan, Zheng
2014-02-24 7:14 ` Peter Zijlstra
2014-02-26 2:39 ` Andy Lutomirski
2014-02-26 7:04 ` Stephane Eranian
2014-02-26 8:57 ` Yan, Zheng
2014-02-26 16:03 ` Andy Lutomirski
2014-02-26 18:55 ` Andi Kleen
2014-02-26 18:59 ` Andy Lutomirski
2014-02-26 19:19 ` David Ahern
2014-02-26 19:25 ` Andy Lutomirski
2014-02-26 20:14 ` David Ahern
2014-02-26 20:26 ` Andy Lutomirski
2014-04-09 11:48 ` Peter Zijlstra
2014-04-09 16:48 ` Andi Kleen [this message]
2014-04-09 17:40 ` Andi Kleen
2014-02-26 20:32 ` Peter Zijlstra
2014-02-26 20:53 ` Andi Kleen
2014-02-26 21:15 ` Peter Zijlstra
2014-02-26 21:33 ` Andi Kleen
2014-02-26 21:34 ` David Ahern
2014-02-26 21:42 ` Andi Kleen
2014-02-27 9:09 ` Stephane Eranian
2014-02-27 12:35 ` Ingo Molnar
2014-02-27 16:08 ` Andi Kleen
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