linux-kernel - Re: [RFC PATCH 3/4] gcov: compile specific gcov implementation based on gcc version

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Message-ID: <20130828135459.GA2329@localhost.localdomain>
Date:	Wed, 28 Aug 2013 15:54:59 +0200
From:	Frantisek Hrbata <fhrbata@...hat.com>
To:	Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, jstancek@...hat.com,
	keescook@...omium.org, rusty@...tcorp.com.au,
	linux-arch@...r.kernel.org, arnd@...db.de, mgahagan@...hat.com,
	agospoda@...hat.com, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 3/4] gcov: compile specific gcov implementation based
 on gcc version

On Wed, Aug 28, 2013 at 03:46:05PM +0200, Peter Oberparleiter wrote:
> On 27.08.2013 15:34, Frantisek Hrbata wrote:
> > On Mon, Aug 26, 2013 at 04:14:07PM +0200, Peter Oberparleiter wrote:
> >> On 24.08.2013 21:44, Frantisek Hrbata wrote:
> >>> On Fri, Aug 23, 2013 at 05:21:12PM +0200, Peter Oberparleiter wrote:
> >>>> On 23.08.2013 17:15, Peter Oberparleiter wrote:
> >>>>> On 23.08.2013 10:39, Frantisek Hrbata wrote:
> >>>>>> Compile the correct gcov implementation file for a specific gcc version. In
> >>>>>> the future, if another file is added, the conditions will need to be somehow
> >>>>>> adjusted to if-elif-else case, but at this point the simple cc-ifversion should
> >>>>>> be enough.
> >>>>
> >>>> As promised, I'm also adding the patch that makes the format-specific part
> >>>> of gcov-kernel a loadable kernel module:
> >>>>
> >>>> ---
> >>>> kernel: gcov: make format-specific code loadable
> >>>>
> >>>> Turn the format-specific part of gcov-kernel into a loadable kernel
> >>>> module. This enables the use of gcov-kernel with kernel modules
> >>>> that were compiled with a version of GCC that produces a different
> >>>> gcov format when compared to the version of GCC that was used to
> >>>> compile the kernel.
> >>>
> >>> If I understand it correctly, this would mean that you will be able to use only
> >>> one implementation of gcov format at the time. Meaning you will be able to get
> >>> coverage data for module, but not for kernel if it was compiled with different
> >>> gcc(gcda format). This is probably ok if you work only on your module, but I'm
> >>> not sure this is generally the right approach. In this case I would probably
> >>> rather see some support for more gcov formats at the same time(e.g. set of
> >>> callback operations per gcov version). Again I'm probably missing something, but
> >>> I still cannot see reason why to add such feature. If you want gcov support just
> >>> compile your kernel and modules with the same gcc version(gcda format). But if
> >>> this is really needed maybe it would be better to consider some parallel support
> >>> for more gcov formats based on the gcov_info version.
> >>
> >> The callback approach has other drawbacks (see previous mail).
> > 
> > Agreed, I did not realized these problems for the first time when I was thinking
> > about the callback approach.
> > 	
> >>
> >>> Would it be possible to add support for the modified gcc 4.7 gcov format and
> >>> deal with this later? I can incorporate your changes: iter to use buffer,
> >>> .init_array for modules and possibility to explicitly select the gcda format.
> >>> In this case we will have at least the basic support in kernel. This is just me
> >>> thinking out loud.
> >>
> >> I think that's an approach I can live with. Maybe the need for a multi-version
> >> support will surface again later in a more refined form, but until then there
> >> should be no reason to delay base GCC 4.7 support any further.
> > 
> > Great. I can incorporate the changes you proposed and post V2. Or do you prefer
> > to post it by your self? Based on the info and patches you already provided I
> > guess you already have something ready. Simply what suits you best :).
> 
> If it's not too much hassle, I'd prefer that you post V2 of your patches. Also
> I guess it would make sense to put Andrew Morton on CC since the initial
> gcov-patches were picked up by him.

Ok, no problem. I will prepare v2. Again many thanks Peter for your help and
input. It's really appreciated.

> 
> -- 
> Peter Oberparleiter
> Linux on System z Development - IBM Germany
> 

-- 
Frantisek Hrbata
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