linux-kernel - Re: [PATCH v2 2/3] perf annotate: Introduce the new source code view

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Date:   Fri, 3 Mar 2017 14:09:20 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Taeung Song <treeze.taeung@...il.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Wang Nan <wangnan0@...wei.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, kernel-team@....com
Subject: Re: [PATCH v2 2/3] perf annotate: Introduce the new source code view

Hi Peter,

On Wed, Mar 01, 2017 at 04:07:46PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 01, 2017 at 11:56:39PM +0900, Namhyung Kim wrote:
> 
> > It's a kind of user experience issue.  We provide the asm-only and
> > asm+source annotation, and I think it'd be nice to add source-only
> > option.  And I remember that it was requested some time ago..
> 
> Thing is, an optimizing compiler -- that same beast that ensures your
> objdump -S output is such a garbled mess -- can generate code that
> becomes very hard to relate to the original source code.

I understand that.  Maybe it's not 100% accurate, but it still has
valuable information.  And I think the source-only view can give more
readable outputs using the info.  Also I guess many developers already
aware of the effect of optimizing compilers.

> 
> I'm really sceptical the source line only view is very useful; maybe if
> you build with -O0, but then, if you do that you're not bothered with
> performance.

Even with an optimizing compiler, it can be helpful to overview which
parts of the code are bottlenecks IMHO.  After that, one can see the
asm to identify the problem deeply, if needed.

Thanks,
Namhyung