Re: [PATCH] kbuild: Disable LD_DEAD_CODE_DATA_ELIMINATION with ftrace & GCC <= 4.7

[Masahiro Yamada <yamada.masahiro@socionext.com>]

On Fri, Jan 11, 2019 at 3:11 AM Paul Burton <paul.burton@mips.com> wrote:
>
> Hi Masahiro,
>
> On Thu, Jan 10, 2019 at 11:00:49AM +0900, Masahiro Yamada wrote:
> > On Thu, Jan 10, 2019 at 8:16 AM Paul Burton <paul.burton@mips.com> wrote:
> > > When building using GCC 4.7 or older, -ffunction-sections & the -pg flag
> > > used by ftrace are incompatible. This causes warnings or build failures
> > > (where -Werror applies) such as the following:
> > >
> > >   arch/mips/generic/init.c:
> > >     error: -ffunction-sections disabled; it makes profiling impossible
> > >
> > > This used to be taken into account by the ordering of calls to cc-option
> > > from within the top-level Makefile, which was introduced by commit
> > > 90ad4052e85c ("kbuild: avoid conflict between -ffunction-sections and
> > > -pg on gcc-4.7"). Unfortunately this was broken when the
> > > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION cc-option check was moved to
> > > Kconfig in commit e85d1d65cd8a ("kbuild: test dead code/data elimination
> > > support in Kconfig"), because the flags used by this check no longer
> > > include -pg.
> > >
> > > Fix this by not allowing CONFIG_LD_DEAD_CODE_DATA_ELIMINATION to be
> > > enabled at the same time as ftrace/CONFIG_FUNCTION_TRACER when building
> > > using GCC 4.7 or older.
> > >
> > > Signed-off-by: Paul Burton <paul.burton@mips.com>
> > > Fixes: e85d1d65cd8a ("kbuild: test dead code/data elimination support in Kconfig")
> > > Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> > > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> > > Cc: Nicholas Piggin <npiggin@gmail.com>
> > > Cc: stable@vger.kernel.org # v4.19+
> > > ---
> > >  init/Kconfig | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/init/Kconfig b/init/Kconfig
> > > index d47cb77a220e..c787f782148d 100644
> > > --- a/init/Kconfig
> > > +++ b/init/Kconfig
> > > @@ -1124,6 +1124,7 @@ config LD_DEAD_CODE_DATA_ELIMINATION
> > >         bool "Dead code and data elimination (EXPERIMENTAL)"
> > >         depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> > >         depends on EXPERT
> > > +       depends on !FUNCTION_TRACER || !CC_IS_GCC || GCC_VERSION >= 40800
> > >         depends on $(cc-option,-ffunction-sections -fdata-sections)
> > >         depends on $(ld-option,--gc-sections)
> > >         help
> >
> > Thanks for the fix.
> >
> > I prefer this explicit 'depends on'.
> >
> > Relying on the order of $(call cc-option, ...) in Makefile is fragile.
> >
> > We raise the compiler minimum version from time to time.
> > So, this 'depends on' will eventually go away in the future.
> >
> > BTW, which one do you think more readable?
> >
> > depends on !FUNCTION_TRACER || !CC_IS_GCC || GCC_VERSION >= 40800
> >
> >     OR
> >
> > depends on !(FUNCTION_TRACER && CC_IS_GCC && GCC_VERSION < 40800)
>
> Thanks - yes I agree it's nice that this is more explicit than the
> ordering we previously relied upon.
>
> I personally don't mind either of the 2 options above - let me know if
> you'd like me to submit a v2 using your second option.
>
> Thanks,
>     Paul



Personally, I slightly prefer this:
   depends on !(FUNCTION_TRACER && CC_IS_GCC && GCC_VERSION < 40800)


It is more consistent with your patch title:
   "Disable LD_DEAD_CODE_DATA_ELIMINATION with ftrace & GCC <= 4.7"


May I ask v2?



-- 
Best Regards
Masahiro Yamada