linux-kernel - Re: [PATCH] modpost.c: Add .text.unlikely to TEXT

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Date:	Wed, 1 May 2013 14:14:56 -0400
From:	Tom Rini <trini@...com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	<linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH] modpost.c: Add .text.unlikely to TEXT_SECTIONS

On 05/01/2013 07:18 AM, Tom Rini wrote:
> On 04/30/2013 10:19 PM, Rusty Russell wrote:
>> Tom Rini <trini@...com> writes:
>>> On 04/28/2013 10:59 PM, Rusty Russell wrote:
>>>> Tom Rini <trini@...com> writes:
>>>>
>>>>> Recent gcc's may place functions into the .text.unlikely section and we
>>>>> need to check this section as well for section mismatches now otherwise
>>>>> we may have false negatives for this test.
>>>>
>>>> Hmm, I don't think it's all that recent, is it?  I can find it back to
>>>> gcc 4.0.4:
>>>>
>>>> `-freorder-functions'
>>>>      Reorder functions in the object file in order to improve code
>>>>      locality.  This is implemented by using special subsections
>>>>      `.text.hot' for most frequently executed functions and
>>>>      `.text.unlikely' for unlikely executed functions.  Reordering is
>>>>      done by the linker so object file format must support named
>>>>      sections and linker must place them in a reasonable way.
>>>>
>>>>      Also profile feedback must be available in to make this option
>>>>      effective.  See `-fprofile-arcs' for details.
>>>>
>>>>      Enabled at levels `-O2', `-O3', `-Os'.
>>>>
>>>> The comment is the same in in gcc 4.7.
>>>>
>>>> So is your real issue that this section is generated with
>>>> -fprofile-arcs, or has something changed with gcc 4.8, or...?
>>>
>>> I've started seeing this with Linaro based 4.7 toolchains.  I can go
>>> back through their releases and see when it starts showing up there if
>>> it helps.  I didn't add .text.hot as I didn't have that section at all,
>>> fwiw.
>>
>> Weird, did you turn on CONFIG_GCOV_KERNEL?  AFAICT you shouldn't see
>> this section without that.
> 
> Nope, CONFIG_GCOV_KERNEL is off.  Must be related to whatever flags the
> Linaro folks set as default on -O2 (at least in their 2013.03 release),
> after reading over one of the .o.cmd files in the build.
> 
> Do you want me to re-word the commit message a bit or ?  Thanks!

I poked around, and every Linaro binary I can grab (2012.01 and gcc 4.6
to 2013.04 and gcc 4.8) has this behaviour.

-- 
Tom
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