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SubjectRe: Better way to force a rebuild
On Wed, 10 Dec 2008 21:02:42 +0100
Sam Ravnborg <sam@ravnborg.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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