openwrt/target/linux/generic/hack-4.19/220-gc_sections.patch

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/Makefile
+++ b/Makefile
@@ -294,6 +294,11 @@ else
 scripts/Kbuild.include: ;
 include scripts/Kbuild.include
 
+ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
+LDFLAGS_vmlinux += --gc-sections
+endif
+
 # Read KERNELRELEASE from include/config/kernel.release (if it exists)
 KERNELRELEASE = $(shell cat include/config/kernel.release 2> /dev/null)
 KERNELVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)
@@ -781,11 +786,6 @@ ifdef CONFIG_DEBUG_SECTION_MISMATCH
 KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
 endif
 
-ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
-LDFLAGS_vmlinux += --gc-sections
-endif
-
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -98,6 +98,7 @@ 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 NO_BOOTMEM
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -106,6 +106,7 @@ 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
@@ -22,13 +22,13 @@
 #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 HYPERVISOR_TEXT							\
@@ -39,11 +39,11 @@
 #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							\
@@ -86,12 +86,12 @@
 	. = 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 = .;					\
 	}
 
@@ -102,14 +102,14 @@
 #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 = .;						\
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -43,6 +43,7 @@ config MIPS
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if CPU_SUPPORTS_HUGEPAGES && 64BIT
 	select HAVE_CBPF_JIT if (!64BIT && !CPU_MICROMIPS)
 	select HAVE_EBPF_JIT if (64BIT && !CPU_MICROMIPS)
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_CONTEXT_TRACKING
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_C_RECORDMCOUNT
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -72,7 +72,7 @@ SECTIONS
 	/* Exception table for data bus errors */
 	__dbe_table : {
 		__start___dbe_table = .;
-		*(__dbe_table)
+		KEEP(*(__dbe_table))
 		__stop___dbe_table = .;
 	}
 
@@ -123,7 +123,7 @@ 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 = .;
 	}