Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux_link

On Sat, May 23, 2020 at 8:13 AM Masahiro Yamada [off-list ref] wrote:

Hi Nicholas,
(+CC: Sam Ravnborg)


On Sat, May 23, 2020 at 7:06 PM Nicholas Piggin [off-list ref] wrote:

quoted

Excerpts from Masahiro Yamada's message of May 23, 2020 3:44 am:

quoted

+ 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?

Sami