linux-kernel - Re: Better way to force a rebuild

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Message-Id: <20081210121048.6186eaf9.akpm@linux-foundation.org>
Date:	Wed, 10 Dec 2008 12:10:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	rostedt@...dmis.org, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, zippel@...ux-m68k.org, mingo@...e.hu
Subject: Re: Better way to force a rebuild

On Wed, 10 Dec 2008 21:02:42 +0100
Sam Ravnborg <sam@...nborg.org> wrote:

> On Wed, Dec 10, 2008 at 02:48:35PM -0500, Steven Rostedt wrote:
> > 
> > Hi,
> > 
> > In include/linux/kernel.h I currently have the following lines at the 
> > bottom of the file:
> > 
> > /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
> > #ifdef CONFIG_FTRACE_MCOUNT_RECORD
> > # define REBUILD_DUE_TO_FTRACE_MCOUNT_RECORD
> > #endif
> > 
> > 
> > This is only there to force a rebuild of all files when 
> > CONFIG_FTRACE_MCOUNT_RECORD is modified. Since the modification of that 
> > config causes the build to act different, we need to rebuild all C 
> > objects. I added the #ifdef in kernel.h as a hack to force the rebuild by 
> > the config dependencies used in kbuild.
> > 
> > My question is, is there a better way to force a full rebuild on a 
> > modification of a config?
> 
> Why is that hack required in the first place?
> We rebuild if:
> - target file is missing (not the case here)
> - any prerequisite files are newer than target (not the case here)
> - any CONFIG_* options used by any prerequisite has changed (not the case here)
>   [It is due to your hack]
> - gcc changed (not the case here)
> - options are added/deleted to gcc (this should be the case for relevant files)
> 
> 
> Looking a bit deeper into this.
> I see that we have in Makefile.build:
> 
> define rule_cc_o_c
>         $(call echo-cmd,checksrc) $(cmd_checksrc)                         \
>         $(call echo-cmd,cc_o_c) $(cmd_cc_o_c);                            \
>         $(cmd_modversions)                                                \
>         $(cmd_record_mcount)   
> 
> So you want everyting to be rebuild if the command above changes.
> That should be doable - and it is indeed a bug that we do not do so.
> The integration of record_mcount needs to be improved.

Yes, changing the state of CONFIG_CC_OPTIMIZE_FOR_SIZE does exactly
what Steven wants.

But damn me it's hard to work out where and how this is implemented :(
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