From: Felix Fietkau <nbd@openwrt.org>

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@openwrt.org>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
---

--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -89,10 +89,14 @@ all-$(CONFIG_SYS_SUPPORTS_ZBOOT)+= vmlin
 #
 cflags-y			+= -G 0 -mno-abicalls -fno-pic -pipe
 cflags-y			+= -msoft-float
-LDFLAGS_vmlinux			+= -G 0 -static -n -nostdlib
+LDFLAGS_vmlinux			+= -G 0 -static -n -nostdlib --gc-sections
 KBUILD_AFLAGS_MODULE		+= -mlong-calls
 KBUILD_CFLAGS_MODULE		+= -mlong-calls
 
+ifndef CONFIG_FUNCTION_TRACER
+KBUILD_CFLAGS_KERNEL		+= -ffunction-sections -fdata-sections
+endif
+
 cflags-y += -ffreestanding
 
 #
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -67,7 +67,7 @@ SECTIONS
 	/* Exception table for data bus errors */
 	__dbe_table : {
 		__start___dbe_table = .;
-		*(__dbe_table)
+		KEEP(*(__dbe_table))
 		__stop___dbe_table = .;
 	}
 
@@ -112,7 +112,7 @@ SECTIONS
 	. = ALIGN(4);
 	.mips.machines.init : AT(ADDR(.mips.machines.init) - LOAD_OFFSET) {
 		__mips_machines_start = .;
-		*(.mips.machines.init)
+		KEEP(*(.mips.machines.init))
 		__mips_machines_end = .;
 	}
 
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -87,7 +87,7 @@
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			VMLINUX_SYMBOL(__start_mcount_loc) = .; \
-			*(__mcount_loc)				\
+			KEEP(*(__mcount_loc))			\
 			VMLINUX_SYMBOL(__stop_mcount_loc) = .;
 #else
 #define MCOUNT_REC()
@@ -95,7 +95,7 @@
 
 #ifdef CONFIG_TRACE_BRANCH_PROFILING
 #define LIKELY_PROFILE()	VMLINUX_SYMBOL(__start_annotated_branch_profile) = .; \
-				*(_ftrace_annotated_branch)			      \
+				KEEP(*(_ftrace_annotated_branch))		      \
 				VMLINUX_SYMBOL(__stop_annotated_branch_profile) = .;
 #else
 #define LIKELY_PROFILE()
@@ -103,7 +103,7 @@
 
 #ifdef CONFIG_PROFILE_ALL_BRANCHES
 #define BRANCH_PROFILE()	VMLINUX_SYMBOL(__start_branch_profile) = .;   \
-				*(_ftrace_branch)			      \
+				KEEP(*(_ftrace_branch))			      \
 				VMLINUX_SYMBOL(__stop_branch_profile) = .;
 #else
 #define BRANCH_PROFILE()
@@ -112,7 +112,7 @@
 #ifdef CONFIG_EVENT_TRACING
 #define FTRACE_EVENTS()	. = ALIGN(8);					\
 			VMLINUX_SYMBOL(__start_ftrace_events) = .;	\
-			*(_ftrace_events)				\
+			KEEP(*(_ftrace_events))				\
 			VMLINUX_SYMBOL(__stop_ftrace_events) = .;
 #else
 #define FTRACE_EVENTS()
@@ -120,7 +120,7 @@
 
 #ifdef CONFIG_TRACING
 #define TRACE_PRINTKS() VMLINUX_SYMBOL(__start___trace_bprintk_fmt) = .;      \
-			 *(__trace_printk_fmt) /* Trace_printk fmt' pointer */ \
+			 KEEP(*(__trace_printk_fmt)) /* Trace_printk fmt' pointer */ \
 			 VMLINUX_SYMBOL(__stop___trace_bprintk_fmt) = .;
 #define TRACEPOINT_STR() VMLINUX_SYMBOL(__start___tracepoint_str) = .;	\
 			 *(__tracepoint_str) /* Trace_printk fmt' pointer */ \
@@ -133,7 +133,7 @@
 #ifdef CONFIG_FTRACE_SYSCALLS
 #define TRACE_SYSCALLS() . = ALIGN(8);					\
 			 VMLINUX_SYMBOL(__start_syscalls_metadata) = .;	\
-			 *(__syscalls_metadata)				\
+			 KEEP(*(__syscalls_metadata))			\
 			 VMLINUX_SYMBOL(__stop_syscalls_metadata) = .;
 #else
 #define TRACE_SYSCALLS()
@@ -142,8 +142,8 @@
 #ifdef CONFIG_CLKSRC_OF
 #define CLKSRC_OF_TABLES() . = ALIGN(8);				\
 			   VMLINUX_SYMBOL(__clksrc_of_table) = .;	\
-			   *(__clksrc_of_table)				\
-			   *(__clksrc_of_table_end)
+			   KEEP(*(__clksrc_of_table))			\
+			   KEEP(*(__clksrc_of_table_end))
 #else
 #define CLKSRC_OF_TABLES()
 #endif
@@ -152,8 +152,8 @@
 #define IRQCHIP_OF_MATCH_TABLE()					\
 	. = ALIGN(8);							\
 	VMLINUX_SYMBOL(__irqchip_begin) = .;				\
-	*(__irqchip_of_table)		  				\
-	*(__irqchip_of_end)
+	KEEP(*(__irqchip_of_table))	  				\
+	KEEP(*(__irqchip_of_end))
 #else
 #define IRQCHIP_OF_MATCH_TABLE()
 #endif
@@ -161,8 +161,8 @@
 #ifdef CONFIG_COMMON_CLK
 #define CLK_OF_TABLES() . = ALIGN(8);				\
 			VMLINUX_SYMBOL(__clk_of_table) = .;	\
-			*(__clk_of_table)			\
-			*(__clk_of_table_end)
+			KEEP(*(__clk_of_table))			\
+			KEEP(*(__clk_of_table_end))
 #else
 #define CLK_OF_TABLES()
 #endif
@@ -170,7 +170,7 @@
 #define KERNEL_DTB()							\
 	STRUCT_ALIGN();							\
 	VMLINUX_SYMBOL(__dtb_start) = .;				\
-	*(.dtb.init.rodata)						\
+	KEEP(*(.dtb.init.rodata))					\
 	VMLINUX_SYMBOL(__dtb_end) = .;
 
 /* .data section */
@@ -186,16 +186,17 @@
 	/* implement dynamic printk debug */				\
 	. = ALIGN(8);                                                   \
 	VMLINUX_SYMBOL(__start___jump_table) = .;                       \
-	*(__jump_table)                                                 \
+	KEEP(*(__jump_table))                                           \
 	VMLINUX_SYMBOL(__stop___jump_table) = .;                        \
 	. = ALIGN(8);							\
 	VMLINUX_SYMBOL(__start___verbose) = .;                          \
-	*(__verbose)                                                    \
+	KEEP(*(__verbose))                                              \
 	VMLINUX_SYMBOL(__stop___verbose) = .;				\
 	LIKELY_PROFILE()		       				\
 	BRANCH_PROFILE()						\
 	TRACE_PRINTKS()							\
-	TRACEPOINT_STR()
+	TRACEPOINT_STR()                                                \
+	*(.data.[a-zA-Z_]*)
 
 /*
  * Data section helpers
@@ -249,32 +250,32 @@
 	/* PCI quirks */						\
 	.pci_fixup        : AT(ADDR(.pci_fixup) - LOAD_OFFSET) {	\
 		VMLINUX_SYMBOL(__start_pci_fixups_early) = .;		\
-		*(.pci_fixup_early)					\
+		KEEP(*(.pci_fixup_early))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_early) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_header) = .;		\
-		*(.pci_fixup_header)					\
+		KEEP(*(.pci_fixup_header))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_header) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_final) = .;		\
-		*(.pci_fixup_final)					\
+		KEEP(*(.pci_fixup_final))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_final) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_enable) = .;		\
-		*(.pci_fixup_enable)					\
+		KEEP(*(.pci_fixup_enable))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_enable) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_resume) = .;		\
-		*(.pci_fixup_resume)					\
+		KEEP(*(.pci_fixup_resume))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_resume) = .;		\
 		VMLINUX_SYMBOL(__start_pci_fixups_resume_early) = .;	\
-		*(.pci_fixup_resume_early)				\
+		KEEP(*(.pci_fixup_resume_early))			\
 		VMLINUX_SYMBOL(__end_pci_fixups_resume_early) = .;	\
 		VMLINUX_SYMBOL(__start_pci_fixups_suspend) = .;		\
-		*(.pci_fixup_suspend)					\
+		KEEP(*(.pci_fixup_suspend))				\
 		VMLINUX_SYMBOL(__end_pci_fixups_suspend) = .;		\
 	}								\
 									\
 	/* Built-in firmware blobs */					\
 	.builtin_fw        : AT(ADDR(.builtin_fw) - LOAD_OFFSET) {	\
 		VMLINUX_SYMBOL(__start_builtin_fw) = .;			\
-		*(.builtin_fw)						\
+		KEEP(*(.builtin_fw))					\
 		VMLINUX_SYMBOL(__end_builtin_fw) = .;			\
 	}								\
 									\
@@ -283,49 +284,49 @@
 	/* Kernel symbol table: Normal symbols */			\
 	__ksymtab         : AT(ADDR(__ksymtab) - LOAD_OFFSET) {		\
 		VMLINUX_SYMBOL(__start___ksymtab) = .;			\
-		*(SORT(___ksymtab+*))					\
+		KEEP(*(SORT(___ksymtab+*)))				\
 		VMLINUX_SYMBOL(__stop___ksymtab) = .;			\
 	}								\
 									\
 	/* Kernel symbol table: GPL-only symbols */			\
 	__ksymtab_gpl     : AT(ADDR(__ksymtab_gpl) - LOAD_OFFSET) {	\
 		VMLINUX_SYMBOL(__start___ksymtab_gpl) = .;		\
-		*(SORT(___ksymtab_gpl+*))				\
+		KEEP(*(SORT(___ksymtab_gpl+*)))				\
 		VMLINUX_SYMBOL(__stop___ksymtab_gpl) = .;		\
 	}								\
 									\
 	/* Kernel symbol table: Normal unused symbols */		\
 	__ksymtab_unused  : AT(ADDR(__ksymtab_unused) - LOAD_OFFSET) {	\
 		VMLINUX_SYMBOL(__start___ksymtab_unused) = .;		\
-		*(SORT(___ksymtab_unused+*))				\
+		KEEP(*(SORT(___ksymtab_unused+*)))			\
 		VMLINUX_SYMBOL(__stop___ksymtab_unused) = .;		\
 	}								\
 									\
 	/* Kernel symbol table: GPL-only unused symbols */		\
 	__ksymtab_unused_gpl : AT(ADDR(__ksymtab_unused_gpl) - LOAD_OFFSET) { \
 		VMLINUX_SYMBOL(__start___ksymtab_unused_gpl) = .;	\
-		*(SORT(___ksymtab_unused_gpl+*))			\
+		KEEP(*(SORT(___ksymtab_unused_gpl+*)))			\
 		VMLINUX_SYMBOL(__stop___ksymtab_unused_gpl) = .;	\
 	}								\
 									\
 	/* Kernel symbol table: GPL-future-only symbols */		\
 	__ksymtab_gpl_future : AT(ADDR(__ksymtab_gpl_future) - LOAD_OFFSET) { \
 		VMLINUX_SYMBOL(__start___ksymtab_gpl_future) = .;	\
-		*(SORT(___ksymtab_gpl_future+*))			\
+		KEEP(*(SORT(___ksymtab_gpl_future+*)))			\
 		VMLINUX_SYMBOL(__stop___ksymtab_gpl_future) = .;	\
 	}								\
 									\
 	/* Kernel symbol table: Normal symbols */			\
 	__kcrctab         : AT(ADDR(__kcrctab) - LOAD_OFFSET) {		\
 		VMLINUX_SYMBOL(__start___kcrctab) = .;			\
-		*(SORT(___kcrctab+*))					\
+		KEEP(*(SORT(___kcrctab+*)))				\
 		VMLINUX_SYMBOL(__stop___kcrctab) = .;			\
 	}								\
 									\
 	/* Kernel symbol table: GPL-only symbols */			\
 	__kcrctab_gpl     : AT(ADDR(__kcrctab_gpl) - LOAD_OFFSET) {	\
 		VMLINUX_SYMBOL(__start___kcrctab_gpl) = .;		\
-		*(SORT(___kcrctab_gpl+*))				\
+		KEEP(*(SORT(___kcrctab_gpl+*)))				\
 		VMLINUX_SYMBOL(__stop___kcrctab_gpl) = .;		\
 	}								\
 									\
@@ -339,14 +340,14 @@
 	/* Kernel symbol table: GPL-only unused symbols */		\
 	__kcrctab_unused_gpl : AT(ADDR(__kcrctab_unused_gpl) - LOAD_OFFSET) { \
 		VMLINUX_SYMBOL(__start___kcrctab_unused_gpl) = .;	\
-		*(SORT(___kcrctab_unused_gpl+*))			\
+		KEEP(*(SORT(___kcrctab_unused_gpl+*)))			\
 		VMLINUX_SYMBOL(__stop___kcrctab_unused_gpl) = .;	\
 	}								\
 									\
 	/* Kernel symbol table: GPL-future-only symbols */		\
 	__kcrctab_gpl_future : AT(ADDR(__kcrctab_gpl_future) - LOAD_OFFSET) { \
 		VMLINUX_SYMBOL(__start___kcrctab_gpl_future) = .;	\
-		*(SORT(___kcrctab_gpl_future+*))			\
+		KEEP(*(SORT(___kcrctab_gpl_future+*)))			\
 		VMLINUX_SYMBOL(__stop___kcrctab_gpl_future) = .;	\
 	}								\
 									\
@@ -365,14 +366,14 @@
 	/* Built-in module parameters. */				\
 	__param : AT(ADDR(__param) - LOAD_OFFSET) {			\
 		VMLINUX_SYMBOL(__start___param) = .;			\
-		*(__param)						\
+		KEEP(*(__param))					\
 		VMLINUX_SYMBOL(__stop___param) = .;			\
 	}								\
 									\
 	/* Built-in module versions. */					\
 	__modver : AT(ADDR(__modver) - LOAD_OFFSET) {			\
 		VMLINUX_SYMBOL(__start___modver) = .;			\
-		*(__modver)						\
+		KEEP(*(__modver))					\
 		VMLINUX_SYMBOL(__stop___modver) = .;			\
 		. = ALIGN((align));					\
 		VMLINUX_SYMBOL(__end_rodata) = .;			\
@@ -428,7 +429,7 @@
 #define ENTRY_TEXT							\
 		ALIGN_FUNCTION();					\
 		VMLINUX_SYMBOL(__entry_text_start) = .;			\
-		*(.entry.text)						\
+		KEEP(*(.entry.text))					\
 		VMLINUX_SYMBOL(__entry_text_end) = .;
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
@@ -456,7 +457,7 @@
 	. = ALIGN(align);						\
 	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {		\
 		VMLINUX_SYMBOL(__start___ex_table) = .;			\
-		*(__ex_table)						\
+		KEEP(*(__ex_table))						\
 		VMLINUX_SYMBOL(__stop___ex_table) = .;			\
 	}
 
@@ -472,8 +473,8 @@
 #ifdef CONFIG_CONSTRUCTORS
 #define KERNEL_CTORS()	. = ALIGN(8);			   \
 			VMLINUX_SYMBOL(__ctors_start) = .; \
-			*(.ctors)			   \
-			*(.init_array)			   \
+			KEEP(*(.ctors))			   \
+			KEEP(*(.init_array))		   \
 			VMLINUX_SYMBOL(__ctors_end) = .;
 #else
 #define KERNEL_CTORS()
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -18,11 +18,16 @@ ifeq ($(CONFIG_CPU_ENDIAN_BE8),y)
 LDFLAGS_vmlinux	+= --be8
 LDFLAGS_MODULE	+= --be8
 endif
+LDFLAGS_vmlinux += --gc-sections
 
 OBJCOPYFLAGS	:=-O binary -R .comment -S
 GZFLAGS		:=-9
 #KBUILD_CFLAGS	+=-pipe
 
+ifndef CONFIG_FUNCTION_TRACER
+KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
+endif
+
 # Never generate .eh_frame
 KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
 
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -12,13 +12,13 @@
 #define PROC_INFO							\
 	. = ALIGN(4);							\
 	VMLINUX_SYMBOL(__proc_info_begin) = .;				\
-	*(.proc.info.init)						\
+	KEEP(*(.proc.info.init))					\
 	VMLINUX_SYMBOL(__proc_info_end) = .;
 
 #define IDMAP_TEXT							\
 	ALIGN_FUNCTION();						\
 	VMLINUX_SYMBOL(__idmap_text_start) = .;				\
-	*(.idmap.text)							\
+	KEEP(*(.idmap.text))						\
 	VMLINUX_SYMBOL(__idmap_text_end) = .;				\
 	. = ALIGN(32);							\
 	VMLINUX_SYMBOL(__hyp_idmap_text_start) = .;			\
@@ -93,7 +93,7 @@ SECTIONS
 	.text : {			/* Real text segment		*/
 		_stext = .;		/* Text and read-only data	*/
 			__exception_text_start = .;
-			*(.exception.text)
+			KEEP(*(.exception.text))
 			__exception_text_end = .;
 			IRQENTRY_TEXT
 			TEXT_TEXT
@@ -118,7 +118,7 @@ SECTIONS
 	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
 		__start___ex_table = .;
 #ifdef CONFIG_MMU
-		*(__ex_table)
+		KEEP(*(__ex_table))
 #endif
 		__stop___ex_table = .;
 	}
@@ -130,12 +130,12 @@ SECTIONS
 	. = 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 = .;
 	}
 #endif
@@ -154,14 +154,14 @@ SECTIONS
 	 */
 	__vectors_start = .;
 	.vectors 0 : AT(__vectors_start) {
-		*(.vectors)
+		KEEP(*(.vectors))
 	}
 	. = __vectors_start + SIZEOF(.vectors);
 	__vectors_end = .;
 
 	__stubs_start = .;
 	.stubs 0x1000 : AT(__stubs_start) {
-		*(.stubs)
+		KEEP(*(.stubs))
 	}
 	. = __stubs_start + SIZEOF(.stubs);
 	__stubs_end = .;
@@ -175,24 +175,24 @@ 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 = .;
 	}
 	.init.data : {
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -120,6 +120,7 @@ ifeq ($(CONFIG_FUNCTION_TRACER),y)
 ORIG_CFLAGS := $(KBUILD_CFLAGS)
 KBUILD_CFLAGS = $(subst -pg, , $(ORIG_CFLAGS))
 endif
+KBUILD_CFLAGS_KERNEL := $(patsubst -f%-sections,,$(KBUILD_CFLAGS_KERNEL))
 
 ccflags-y := -fpic -mno-single-pic-base -fno-builtin -I$(obj)
 asflags-y := -DZIMAGE