[PATCH] kbuild: reuse vmlinux.o in vmlinux_link

[Masahiro Yamada]

On Tue, Jun 16, 2020 at 6:47 AM Sami Tolvanen <samitolvanen at google.com> wrote:
>
> On Sat, May 23, 2020 at 8:13 AM Masahiro Yamada <masahiroy at kernel.org> wrote:
> >
> > Hi Nicholas,
> > (+CC: Sam Ravnborg)
> >
> >
> > On Sat, May 23, 2020 at 7:06 PM Nicholas Piggin <npiggin at gmail.com> wrote:
> > >
> > > Excerpts from Masahiro Yamada's message of May 23, 2020 3:44 am:
> > > > + Michael, and PPC ML.
> > > >
> > > > They may know something about the reason of failure.
> > >
> > > Because the linker can't put branch stubs within object code sections,
> > > so when you incrementally link them too large, the linker can't resolve
> > > branches into other object files.
> >
> >
> > Ah, you are right.
> >
> > So, this is a problem not only for PPC
> > but also for ARM (both 32 and 64 bit), etc.
> >
> > ARM needs to insert a veneer to jump far.
> >
> > Prior to thin archive, we could not compile
> > ARCH=arm allyesconfig because
> > drivers/built-in.o was too large.
> >
> > This patch gets us back to the too large
> > incremental object situation.
> >
> > With my quick compile-testing,
> > ARCH=arm allyesconfig
> > and ARCH=arm64 allyesconfig are broken.
>
> Thanks for looking into this! Clang doesn't appear to have this issue
> with LTO because it always enables both -ffunction-sections and
> -fdata-sections. I confirmed that -ffunction-sections also fixes arm64
> allyesconfig with this patch. While I'm fine with reusing vmlinux.o
> only with LTO, how would you feel about enabling -ffunction-sections
> in the kernel by default?


I am OK if it works.

Please do compile tests for some architectures.
(especially, ARCH=powerpc defconfig, and ARCH=arm(64) allyesconfig)


Thank you.




-- 
Best Regards
Masahiro Yamada