linux-kernel - Re: [PATCH] perf: fix wrong DEBUG configuration

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Message-ID: <20150520145506.GA15679@gmail.com>
Date:	Wed, 20 May 2015 16:55:06 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Martin Liška <mliska@...e.cz>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] perf: fix wrong DEBUG configuration


* Arnaldo Carvalho de Melo <acme@...nel.org> wrote:

> Em Wed, May 20, 2015 at 03:29:37PM +0200, Martin Liška escreveu:
> > On 05/20/2015 03:17 PM, Arnaldo Carvalho de Melo wrote:
> > >Em Wed, May 20, 2015 at 03:07:39PM +0200, Martin Liška escreveu:
> > >>Currently, GCC optimizes -O6 same as -O3 level. Right optimize debugging
> > >>experience is given by passing -Og to compiler.
> 
> > >Is this is this -Og available in old gcc versions? When was it
> > >introduced? Do you know?
>  
> > GCC 4.8.0 is the first version capable of the option: https://gcc.gnu.org/gcc-4.8/changes.html.
> > That can be problematic, which GCC version do you support in linux/perf?
> 
> So the rule has been: What are the kernel requirements for the 
> toolchain? tools/perf/ should build with that.

So we could use -Og if it works, like Kbuild does it:

        KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)

the 'cc-option' Make function does some magic of silently calling GCC 
with that option and observing the result.

See:

  scripts/Kbuild.include:cc-option = $(call try-run,\

et al.

Thanks,

	Ingo
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