kbuild: allow archs to select link dead code/data elimination

Introduce LD_DEAD_CODE_DATA_ELIMINATION option for architectures to
select to build with -ffunction-sections, -fdata-sections, and link
with --gc-sections. It requires some work (documented) to ensure all
unreferenced entrypoints are live, and requires toolchain and build
verification, so it is made a per-arch option for now.

On a random powerpc64le build, this yelds a significant size saving,
it boots and runs fine, but there is a lot I haven't tested as yet, so
these savings may be reduced if there are bugs in the link.

    text      data        bss        dec   filename
11169741   1180744    1923176	14273661   vmlinux
10445269   1004127    1919707	13369103   vmlinux.dce

~700K text, ~170K data, 6% removed from kernel image size.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 70de1448c571..6d40f3a3a8cc 100644
--- a/Makefile
+++ b/Makefile
@@ -622,6 +622,11 @@ include arch/$(SRCARCH)/Makefile
 KBUILD_CFLAGS   += $(call cc-option,-fno-delete-null-pointer-checks,)
 KBUILD_CFLAGS   += $(call cc-disable-warning,maybe-uninitialized,)
 
+ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+KBUILD_CFLAGS   += $(call cc-option,-ffunction-sections,)
+KBUILD_CFLAGS   += $(call cc-option,-fdata-sections,)
+endif
+
 ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS   += -Os
 else
@@ -809,6 +814,10 @@ LDFLAGS_BUILD_ID = $(patsubst -Wl$(comma)%,%,\
 KBUILD_LDFLAGS_MODULE += $(LDFLAGS_BUILD_ID)
 LDFLAGS_vmlinux += $(LDFLAGS_BUILD_ID)
 
+ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+LDFLAGS_vmlinux += $(call ld-option, --gc-sections,)
+endif
+
 ifeq ($(CONFIG_STRIP_ASM_SYMS),y)
 LDFLAGS_vmlinux += $(call ld-option, -X,)
 endif