ARM: 9404/1: arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

The current arm32 architecture does not yet support the
HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. arm32 is widely used in
embedded scenarios, and enabling this feature would be beneficial for
reducing the size of the kernel image.

In order to make this work, we keep the necessary tables by annotating
them with KEEP, also it requires further changes to linker script to KEEP
some tables and wildcard compiler generated sections into the right place.
When using ld.lld for linking, KEEP is not recognized within the OVERLAY
command, and Ard proposed a concise method to solve this problem.

It boots normally with defconfig, vexpress_defconfig and tinyconfig.

The size comparison of zImage is as follows:
defconfig       vexpress_defconfig      tinyconfig
5137712         5138024                 424192          no dce
5032560         4997824                 298384          dce
2.0%            2.7%                    29.7%           shrink

When using smaller config file, there is a significant reduction in the
size of the zImage.

We also tested this patch on a commercially available single-board
computer, and the comparison is as follows:
a15eb_config
2161384         no dce
2092240         dce
3.2%            shrink

The zImage size has been reduced by approximately 3.2%, which is 70KB on
2.1M.

Signed-off-by: Yuntao Liu <[email protected]>
Tested-by: Arnd Bergmann <[email protected]>
Reviewed-by: Arnd Bergmann <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Signed-off-by: Russell King (Oracle) <[email protected]>

Author: Yuntao Liu

arch/arm/Kconfig

@@ -115,6 +115,7 @@ config ARM
 	select HAVE_KERNEL_XZ
 	select HAVE_KPROBES if !XIP_KERNEL && !CPU_ENDIAN_BE32 && !CPU_V7M
 	select HAVE_KRETPROBES if HAVE_KPROBES
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_MOD_ARCH_SPECIFIC
 	select HAVE_NMI
 	select HAVE_OPTPROBES if !THUMB2_KERNEL

arch/arm/boot/compressed/vmlinux.lds.S

@@ -125,7 +125,7 @@ SECTIONS
 
   . = BSS_START;
   __bss_start = .;
-  .bss			: { *(.bss) }
+  .bss			: { *(.bss .bss.*) }
   _end = .;
 
   . = ALIGN(8);		/* the stack must be 64-bit aligned */

arch/arm/include/asm/vmlinux.lds.h

@@ -42,7 +42,7 @@
 #define PROC_INFO							\
 		. = ALIGN(4);						\
 		__proc_info_begin = .;					\
-		*(.proc.info.init)					\
+		KEEP(*(.proc.info.init))				\
 		__proc_info_end = .;
 
 #define IDMAP_TEXT							\

arch/arm/kernel/entry-armv.S

@@ -1065,6 +1065,7 @@ vector_addrexcptn:
 	.globl	vector_fiq
 
 	.section .vectors, "ax", %progbits
+	.reloc  .text, R_ARM_NONE, .
 	W(b)	vector_rst
 	W(b)	vector_und
 ARM(	.reloc	., R_ARM_LDR_PC_G0, .L__vector_swi		)
@@ -1078,6 +1079,7 @@ THUMB(	.reloc	., R_ARM_THM_PC12, .L__vector_swi		)
 
 #ifdef CONFIG_HARDEN_BRANCH_HISTORY
 	.section .vectors.bhb.loop8, "ax", %progbits
+	.reloc  .text, R_ARM_NONE, .
 	W(b)	vector_rst
 	W(b)	vector_bhb_loop8_und
 ARM(	.reloc	., R_ARM_LDR_PC_G0, .L__vector_bhb_loop8_swi	)
@@ -1090,6 +1092,7 @@ THUMB(	.reloc	., R_ARM_THM_PC12, .L__vector_bhb_loop8_swi	)
 	W(b)	vector_bhb_loop8_fiq
 
 	.section .vectors.bhb.bpiall, "ax", %progbits
+	.reloc  .text, R_ARM_NONE, .
 	W(b)	vector_rst
 	W(b)	vector_bhb_bpiall_und
 ARM(	.reloc	., R_ARM_LDR_PC_G0, .L__vector_bhb_bpiall_swi	)

arch/arm/kernel/vmlinux-xip.lds.S

@@ -63,7 +63,7 @@ SECTIONS
 	. = ALIGN(4);
 	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
 		__start___ex_table = .;
-		ARM_MMU_KEEP(*(__ex_table))
+		ARM_MMU_KEEP(KEEP(*(__ex_table)))
 		__stop___ex_table = .;
 	}
 
@@ -83,7 +83,7 @@ SECTIONS
 	}
 	.init.arch.info : {
 		__arch_info_begin = .;
-		*(.arch.info.init)
+		KEEP(*(.arch.info.init))
 		__arch_info_end = .;
 	}
 	.init.tagtable : {

arch/arm/kernel/vmlinux.lds.S

@@ -74,7 +74,7 @@ SECTIONS
 	. = ALIGN(4);
 	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
 		__start___ex_table = .;
-		ARM_MMU_KEEP(*(__ex_table))
+		ARM_MMU_KEEP(KEEP(*(__ex_table)))
 		__stop___ex_table = .;
 	}
 
@@ -99,7 +99,7 @@ SECTIONS
 	}
 	.init.arch.info : {
 		__arch_info_begin = .;
-		*(.arch.info.init)
+		KEEP(*(.arch.info.init))
 		__arch_info_end = .;
 	}
 	.init.tagtable : {
@@ -116,7 +116,7 @@ SECTIONS
 #endif
 	.init.pv_table : {
 		__pv_table_begin = .;
-		*(.pv_table)
+		KEEP(*(.pv_table))
 		__pv_table_end = .;
 	}
 

drivers/firmware/efi/libstub/Makefile

@@ -56,6 +56,10 @@ KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_CFI), $(KBUILD_CFLAGS))
 # disable LTO
 KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))
 
+# The .data section would be renamed to .data.efistub, therefore, remove
+# `-fdata-sections` flag from KBUILD_CFLAGS_KERNEL
+KBUILD_CFLAGS_KERNEL := $(filter-out -fdata-sections, $(KBUILD_CFLAGS_KERNEL))
+
 lib-y				:= efi-stub-helper.o gop.o secureboot.o tpm.o \
 				   file.o mem.o random.o randomalloc.o pci.o \
 				   skip_spaces.o lib-cmdline.o lib-ctype.o \