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>
---
 Makefile                          | 10 +++----
 arch/arm/Kconfig                  |  1 +
 arch/arm/boot/compressed/Makefile |  1 +
 arch/arm/kernel/vmlinux.lds.S     | 26 ++++++++--------
 arch/mips/Kconfig                 |  1 +
 arch/mips/kernel/vmlinux.lds.S    |  4 +--
 include/asm-generic/vmlinux.lds.h | 63 ++++++++++++++++++++-------------------
 7 files changed, 55 insertions(+), 51 deletions(-)

--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -112,6 +112,7 @@ 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
@@ -108,6 +108,7 @@ 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))
 
 # -fstack-protector-strong triggers protection checks in this code,
 # but it is being used too early to link to meaningful stack_chk logic.
--- 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/kernel/vmlinux.lds.h
+++ b/arch/arm/kernel/vmlinux.lds.h
@@ -22,7 +22,7 @@
 #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
 
 /*
@@ -41,7 +41,7 @@
 #define PROC_INFO							\
 		. = ALIGN(4);						\
 		__proc_info_begin = .;					\
-		*(.proc.info.init)					\
+		KEEP(*(.proc.info.init))				\
 		__proc_info_end = .;
 
 #define HYPERVISOR_TEXT							\
@@ -52,11 +52,11 @@
 #define IDMAP_TEXT							\
 		ALIGN_FUNCTION();					\
 		__idmap_text_start = .;					\
-		*(.idmap.text)						\
+		KEEP(*(.idmap.text))					\
 		__idmap_text_end = .;					\
 		. = ALIGN(PAGE_SIZE);					\
 		__hyp_idmap_text_start = .;				\
-		*(.hyp.idmap.text)					\
+		KEEP(*(.hyp.idmap.text))				\
 		__hyp_idmap_text_end = .;
 
 #define ARM_DISCARD							\
@@ -99,12 +99,12 @@
 	. = 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 = .;					\
 	}
 
@@ -116,7 +116,7 @@
 	__vectors_lma = .;						\
 	OVERLAY 0xffff0000 : NOCROSSREFS AT(__vectors_lma) {		\
 		.vectors {						\
-			*(.vectors)					\
+			KEEP(*(.vectors))				\
 		}							\
 		.vectors.bhb.loop8 {					\
 			*(.vectors.bhb.loop8)				\
@@ -134,7 +134,7 @@
 									\
 	__stubs_lma = .;						\
 	.stubs ADDR(.vectors) + 0x1000 : AT(__stubs_lma) {		\
-		*(.stubs)						\
+		KEEP(*(.stubs))						\
 	}								\
 	ARM_LMA(__stubs, .stubs);					\
 	. = __stubs_lma + SIZEOF(.stubs);				\