From e3d8676f5722b7622685581e06e8f53e6138e3ab Mon Sep 17 00:00:00 2001
From: Felix Fietkau <nbd@nbd.name>
Date: Sat, 15 Jul 2017 23:42:36 +0200
Subject: use -ffunction-sections, -fdata-sections and --gc-sections

In combination with kernel symbol export stripping this significantly reduces
the kernel image size. Used on both ARM and MIPS architectures.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
---
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -113,6 +113,7 @@ config ARM
 	select HAVE_UID16
 	select HAVE_VIRT_CPU_ACCOUNTING_GEN
 	select IRQ_FORCED_THREADING
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select MODULES_USE_ELF_REL
 	select NEED_DMA_MAP_STATE
 	select OF_EARLY_FLATTREE if OF
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -98,6 +98,7 @@ $(foreach o, $(libfdt_objs) atags_to_fdt
 ifdef building_out_of_srctree
 $(shell rm -f $(addprefix $(obj)/, fdt_rw.c fdt_ro.c fdt_wip.c fdt.c))
 endif
+KBUILD_CFLAGS_KERNEL := $(patsubst -f%-sections,,$(KBUILD_CFLAGS_KERNEL))
 
 targets       := vmlinux vmlinux.lds piggy_data piggy.o \
 		 lib1funcs.o ashldi3.o bswapsdi2.o \
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -100,24 +100,24 @@ SECTIONS
 	}
 	.init.arch.info : {
 		__arch_info_begin = .;
-		*(.arch.info.init)
+		KEEP(*(.arch.info.init))
 		__arch_info_end = .;
 	}
 	.init.tagtable : {
 		__tagtable_begin = .;
-		*(.taglist.init)
+		KEEP(*(.taglist.init))
 		__tagtable_end = .;
 	}
 #ifdef CONFIG_SMP_ON_UP
 	.init.smpalt : {
 		__smpalt_begin = .;
-		*(.alt.smp.init)
+		KEEP(*(.alt.smp.init))
 		__smpalt_end = .;
 	}
 #endif
 	.init.pv_table : {
 		__pv_table_begin = .;
-		*(.pv_table)
+		KEEP(*(.pv_table))
 		__pv_table_end = .;
 	}
 
--- a/arch/arm/include/asm/vmlinux.lds.h
+++ b/arch/arm/include/asm/vmlinux.lds.h
@@ -23,19 +23,19 @@
 #define ARM_MMU_DISCARD(x)
 #else
 #define ARM_MMU_KEEP(x)
-#define ARM_MMU_DISCARD(x)	x
+#define ARM_MMU_DISCARD(x)	KEEP(x)
 #endif
 
 #define PROC_INFO							\
 		. = ALIGN(4);						\
 		__proc_info_begin = .;					\
-		*(.proc.info.init)					\
+		KEEP(*(.proc.info.init))				\
 		__proc_info_end = .;
 
 #define IDMAP_TEXT							\
 		ALIGN_FUNCTION();					\
 		__idmap_text_start = .;					\
-		*(.idmap.text)						\
+		KEEP(*(.idmap.text))					\
 		__idmap_text_end = .;					\
 
 #define ARM_DISCARD							\
@@ -96,12 +96,12 @@
 	. = ALIGN(8);							\
 	.ARM.unwind_idx : {						\
 		__start_unwind_idx = .;					\
-		*(.ARM.exidx*)						\
+		KEEP(*(.ARM.exidx*))					\
 		__stop_unwind_idx = .;					\
 	}								\
 	.ARM.unwind_tab : {						\
 		__start_unwind_tab = .;					\
-		*(.ARM.extab*)						\
+		KEEP(*(.ARM.extab*))					\
 		__stop_unwind_tab = .;					\
 	}
 
@@ -112,14 +112,14 @@
 #define ARM_VECTORS							\
 	__vectors_start = .;						\
 	.vectors 0xffff0000 : AT(__vectors_start) {			\
-		*(.vectors)						\
+		KEEP(*(.vectors))					\
 	}								\
 	. = __vectors_start + SIZEOF(.vectors);				\
 	__vectors_end = .;						\
 									\
 	__stubs_start = .;						\
 	.stubs ADDR(.vectors) + 0x1000 : AT(__stubs_start) {		\
-		*(.stubs)						\
+		KEEP(*(.stubs))						\
 	}								\
 	. = __stubs_start + SIZEOF(.stubs);				\
 	__stubs_end = .;						\