[PATCH v2 0/4] LD_DEAD_CODE_DATA_ELIMINATION fixes and enabling for powerpc

[PATCH v2 0/4] LD_DEAD_CODE_DATA_ELIMINATION fixes and enabling for powerpc

[Nicholas Piggin]

v2 changes only patch 4, updates the changelog with ppc32 numbers and
Tested-by tags, and also use TEXT_MAIN in the linker script as
Christophe suggested.

Thanks,
Nick

Nicholas Piggin (4):
  kbuild: Fix asm-generic/vmlinux.lds.h for
    LD_DEAD_CODE_DATA_ELIMINATION
  kbuild: LD_DEAD_CODE_DATA_ELIMINATION no
    -ffunction-sections/-fdata-sections for module build
  kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selectable if
    enabled
  powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

 Makefile                          |  4 +--
 arch/Kconfig                      | 15 ----------
 arch/powerpc/Kconfig              |  1 +
 arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++-------
 include/asm-generic/vmlinux.lds.h | 49 ++++++++++++++++++-------------
 init/Kconfig                      | 27 +++++++++++++++++
 6 files changed, 70 insertions(+), 48 deletions(-)

-- 
2.17.0

[PATCH v2 0/4] LD_DEAD_CODE_DATA_ELIMINATION fixes and enabling for powerpc

[Nicholas Piggin]

v2 changes only patch 4, updates the changelog with ppc32 numbers and
Tested-by tags, and also use TEXT_MAIN in the linker script as
Christophe suggested.

Thanks,
Nick

Nicholas Piggin (4):
  kbuild: Fix asm-generic/vmlinux.lds.h for
    LD_DEAD_CODE_DATA_ELIMINATION
  kbuild: LD_DEAD_CODE_DATA_ELIMINATION no
    -ffunction-sections/-fdata-sections for module build
  kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selectable if
    enabled
  powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

 Makefile                          |  4 +--
 arch/Kconfig                      | 15 ----------
 arch/powerpc/Kconfig              |  1 +
 arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++-------
 include/asm-generic/vmlinux.lds.h | 49 ++++++++++++++++++-------------
 init/Kconfig                      | 27 +++++++++++++++++
 6 files changed, 70 insertions(+), 48 deletions(-)

-- 
2.17.0

[PATCH v2 1/4] kbuild: Fix asm-generic/vmlinux.lds.h for LD_DEAD_CODE_DATA_ELIMINATION

[Nicholas Piggin]

KEEP more tables, and add the function/data section wildcard to more
section selections.

This is a little ad-hoc at the moment, but kernel code should be moved
to consistently use .text..x (note: double dots) for explicit sections
and all references to it in the linker script can be made with
TEXT_MAIN, and similarly for other sections.

For now, let's see if major architectures move to enabling this option
then we can do some refactoring passes. Otherwise if it remains unused
or superseded by LTO, this may not be required.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 include/asm-generic/vmlinux.lds.h | 49 ++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 20 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index af240573e482..9efb82a9a528 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -64,15 +64,24 @@
  * generates .data.identifier sections, which need to be pulled in with
  * .data. We don't want to pull in .data..other sections, which Linux
  * has defined. Same for text and bss.
+ *
+ * RODATA_MAIN is not used because existing code already defines .rodata.x
+ * sections to be brought in with rodata.
  */
 #ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
 #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
 #define DATA_MAIN .data .data.[0-9a-zA-Z_]*
+#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
+#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
 #define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
+#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
 #else
 #define TEXT_MAIN .text
 #define DATA_MAIN .data
+#define SDATA_MAIN .sdata
+#define RODATA_MAIN .rodata
 #define BSS_MAIN .bss
+#define SBSS_MAIN .sbss
 #endif
 
 /*
@@ -105,7 +114,7 @@
 #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()
@@ -113,7 +122,7 @@
 
 #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()
@@ -121,7 +130,7 @@
 
 #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()
@@ -238,8 +247,8 @@
 	*(DATA_MAIN)							\
 	*(.ref.data)							\
 	*(.data..shared_aligned) /* percpu related */			\
-	MEM_KEEP(init.data)						\
-	MEM_KEEP(exit.data)						\
+	MEM_KEEP(init.data*)						\
+	MEM_KEEP(exit.data*)						\
 	*(.data.unlikely)						\
 	VMLINUX_SYMBOL(__start_once) = .;				\
 	*(.data.once)							\
@@ -289,8 +298,8 @@
 	VMLINUX_SYMBOL(__start_init_task) = .;				\
 	VMLINUX_SYMBOL(init_thread_union) = .;				\
 	VMLINUX_SYMBOL(init_stack) = .;					\
-	*(.data..init_task)						\
-	*(.data..init_thread_info)					\
+	KEEP(*(.data..init_task))					\
+	KEEP(*(.data..init_thread_info))				\
 	. = VMLINUX_SYMBOL(__start_init_task) + THREAD_SIZE;		\
 	VMLINUX_SYMBOL(__end_init_task) = .;
 
@@ -487,8 +496,8 @@
 		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
 		*(.text..refcount)					\
 		*(.ref.text)						\
-	MEM_KEEP(init.text)						\
-	MEM_KEEP(exit.text)						\
+	MEM_KEEP(init.text*)						\
+	MEM_KEEP(exit.text*)						\
 
 
 /* sched.text is aling to function alignment to secure we have same
@@ -538,7 +547,7 @@
 		VMLINUX_SYMBOL(__softirqentry_text_end) = .;
 
 /* Section used for early init (in .S files) */
-#define HEAD_TEXT  *(.head.text)
+#define HEAD_TEXT  KEEP(*(.head.text))
 
 #define HEAD_TEXT_SECTION							\
 	.head.text : AT(ADDR(.head.text) - LOAD_OFFSET) {		\
@@ -579,11 +588,11 @@
 /* init and exit section handling */
 #define INIT_DATA							\
 	KEEP(*(SORT(___kentry+*)))					\
-	*(.init.data)							\
-	MEM_DISCARD(init.data)						\
+	*(.init.data init.data.*)					\
+	MEM_DISCARD(init.data*)						\
 	KERNEL_CTORS()							\
 	MCOUNT_REC()							\
-	*(.init.rodata)							\
+	*(.init.rodata .init.rodata.*)					\
 	FTRACE_EVENTS()							\
 	TRACE_SYSCALLS()						\
 	KPROBE_BLACKLIST()						\
@@ -602,16 +611,16 @@
 	EARLYCON_TABLE()
 
 #define INIT_TEXT							\
-	*(.init.text)							\
+	*(.init.text .init.text.*)					\
 	*(.text.startup)						\
-	MEM_DISCARD(init.text)
+	MEM_DISCARD(init.text*)
 
 #define EXIT_DATA							\
-	*(.exit.data)							\
+	*(.exit.data .exit.data.*)					\
 	*(.fini_array)							\
 	*(.dtors)							\
-	MEM_DISCARD(exit.data)						\
-	MEM_DISCARD(exit.rodata)
+	MEM_DISCARD(exit.data*)						\
+	MEM_DISCARD(exit.rodata*)
 
 #define EXIT_TEXT							\
 	*(.exit.text)							\
@@ -629,7 +638,7 @@
 	. = ALIGN(sbss_align);						\
 	.sbss : AT(ADDR(.sbss) - LOAD_OFFSET) {				\
 		*(.dynsbss)						\
-		*(.sbss)						\
+		*(SBSS_MAIN)						\
 		*(.scommon)						\
 	}
 
@@ -754,7 +763,7 @@
 #define NOTES								\
 	.notes : AT(ADDR(.notes) - LOAD_OFFSET) {			\
 		VMLINUX_SYMBOL(__start_notes) = .;			\
-		*(.note.*)						\
+		KEEP(*(.note.*))					\
 		VMLINUX_SYMBOL(__stop_notes) = .;			\
 	}
 
-- 
2.17.0

[PATCH v2 1/4] kbuild: Fix asm-generic/vmlinux.lds.h for LD_DEAD_CODE_DATA_ELIMINATION

[Nicholas Piggin]

KEEP more tables, and add the function/data section wildcard to more
section selections.

This is a little ad-hoc at the moment, but kernel code should be moved
to consistently use .text..x (note: double dots) for explicit sections
and all references to it in the linker script can be made with
TEXT_MAIN, and similarly for other sections.

For now, let's see if major architectures move to enabling this option
then we can do some refactoring passes. Otherwise if it remains unused
or superseded by LTO, this may not be required.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 include/asm-generic/vmlinux.lds.h | 49 ++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 20 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index af240573e482..9efb82a9a528 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -64,15 +64,24 @@
  * generates .data.identifier sections, which need to be pulled in with
  * .data. We don't want to pull in .data..other sections, which Linux
  * has defined. Same for text and bss.
+ *
+ * RODATA_MAIN is not used because existing code already defines .rodata.x
+ * sections to be brought in with rodata.
  */
 #ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
 #define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
 #define DATA_MAIN .data .data.[0-9a-zA-Z_]*
+#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
+#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
 #define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
+#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
 #else
 #define TEXT_MAIN .text
 #define DATA_MAIN .data
+#define SDATA_MAIN .sdata
+#define RODATA_MAIN .rodata
 #define BSS_MAIN .bss
+#define SBSS_MAIN .sbss
 #endif
 
 /*
@@ -105,7 +114,7 @@
 #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()
@@ -113,7 +122,7 @@
 
 #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()
@@ -121,7 +130,7 @@
 
 #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()
@@ -238,8 +247,8 @@
 	*(DATA_MAIN)							\
 	*(.ref.data)							\
 	*(.data..shared_aligned) /* percpu related */			\
-	MEM_KEEP(init.data)						\
-	MEM_KEEP(exit.data)						\
+	MEM_KEEP(init.data*)						\
+	MEM_KEEP(exit.data*)						\
 	*(.data.unlikely)						\
 	VMLINUX_SYMBOL(__start_once) = .;				\
 	*(.data.once)							\
@@ -289,8 +298,8 @@
 	VMLINUX_SYMBOL(__start_init_task) = .;				\
 	VMLINUX_SYMBOL(init_thread_union) = .;				\
 	VMLINUX_SYMBOL(init_stack) = .;					\
-	*(.data..init_task)						\
-	*(.data..init_thread_info)					\
+	KEEP(*(.data..init_task))					\
+	KEEP(*(.data..init_thread_info))				\
 	. = VMLINUX_SYMBOL(__start_init_task) + THREAD_SIZE;		\
 	VMLINUX_SYMBOL(__end_init_task) = .;
 
@@ -487,8 +496,8 @@
 		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
 		*(.text..refcount)					\
 		*(.ref.text)						\
-	MEM_KEEP(init.text)						\
-	MEM_KEEP(exit.text)						\
+	MEM_KEEP(init.text*)						\
+	MEM_KEEP(exit.text*)						\
 
 
 /* sched.text is aling to function alignment to secure we have same
@@ -538,7 +547,7 @@
 		VMLINUX_SYMBOL(__softirqentry_text_end) = .;
 
 /* Section used for early init (in .S files) */
-#define HEAD_TEXT  *(.head.text)
+#define HEAD_TEXT  KEEP(*(.head.text))
 
 #define HEAD_TEXT_SECTION							\
 	.head.text : AT(ADDR(.head.text) - LOAD_OFFSET) {		\
@@ -579,11 +588,11 @@
 /* init and exit section handling */
 #define INIT_DATA							\
 	KEEP(*(SORT(___kentry+*)))					\
-	*(.init.data)							\
-	MEM_DISCARD(init.data)						\
+	*(.init.data init.data.*)					\
+	MEM_DISCARD(init.data*)						\
 	KERNEL_CTORS()							\
 	MCOUNT_REC()							\
-	*(.init.rodata)							\
+	*(.init.rodata .init.rodata.*)					\
 	FTRACE_EVENTS()							\
 	TRACE_SYSCALLS()						\
 	KPROBE_BLACKLIST()						\
@@ -602,16 +611,16 @@
 	EARLYCON_TABLE()
 
 #define INIT_TEXT							\
-	*(.init.text)							\
+	*(.init.text .init.text.*)					\
 	*(.text.startup)						\
-	MEM_DISCARD(init.text)
+	MEM_DISCARD(init.text*)
 
 #define EXIT_DATA							\
-	*(.exit.data)							\
+	*(.exit.data .exit.data.*)					\
 	*(.fini_array)							\
 	*(.dtors)							\
-	MEM_DISCARD(exit.data)						\
-	MEM_DISCARD(exit.rodata)
+	MEM_DISCARD(exit.data*)						\
+	MEM_DISCARD(exit.rodata*)
 
 #define EXIT_TEXT							\
 	*(.exit.text)							\
@@ -629,7 +638,7 @@
 	. = ALIGN(sbss_align);						\
 	.sbss : AT(ADDR(.sbss) - LOAD_OFFSET) {				\
 		*(.dynsbss)						\
-		*(.sbss)						\
+		*(SBSS_MAIN)						\
 		*(.scommon)						\
 	}
 
@@ -754,7 +763,7 @@
 #define NOTES								\
 	.notes : AT(ADDR(.notes) - LOAD_OFFSET) {			\
 		VMLINUX_SYMBOL(__start_notes) = .;			\
-		*(.note.*)						\
+		KEEP(*(.note.*))					\
 		VMLINUX_SYMBOL(__stop_notes) = .;			\
 	}
 
-- 
2.17.0

[PATCH v2 2/4] kbuild: LD_DEAD_CODE_DATA_ELIMINATION no -ffunction-sections/-fdata-sections for module build

[Nicholas Piggin]

Modules do not tend to cope with -ffunction-sections, even though they
do not link with -gc-sections. It may be possible for unused symbols to
be trimmed from modules, but in general that would take much more work
in architecture module linker scripts.

For now, enable these only for kernel build.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 619a85ad716b..9dcbfb353f81 100644
--- a/Makefile
+++ b/Makefile
@@ -799,8 +799,8 @@ KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
 endif
 
 ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS	+= $(call cc-option,-ffunction-sections,)
-KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-ffunction-sections,)
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-fdata-sections,)
 endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included
-- 
2.17.0

[PATCH v2 2/4] kbuild: LD_DEAD_CODE_DATA_ELIMINATION no -ffunction-sections/-fdata-sections for module build

[Nicholas Piggin]

Modules do not tend to cope with -ffunction-sections, even though they
do not link with -gc-sections. It may be possible for unused symbols to
be trimmed from modules, but in general that would take much more work
in architecture module linker scripts.

For now, enable these only for kernel build.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 619a85ad716b..9dcbfb353f81 100644
--- a/Makefile
+++ b/Makefile
@@ -799,8 +799,8 @@ KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
 endif
 
 ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS	+= $(call cc-option,-ffunction-sections,)
-KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-ffunction-sections,)
+KBUILD_CFLAGS_KERNEL	+= $(call cc-option,-fdata-sections,)
 endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included
-- 
2.17.0

[PATCH v2 3/4] kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selectable if enabled

[Nicholas Piggin]

Architectures that are capable can select
HAVE_LD_DEAD_CODE_DATA_ELIMINATION to enable selection of that
option (as an EXPERT kernel option).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/Kconfig | 15 ---------------
 init/Kconfig | 27 +++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 8e0d665c8d53..3234b73d4626 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -596,21 +596,6 @@ config CC_STACKPROTECTOR_AUTO
 
 endchoice
 
-config LD_DEAD_CODE_DATA_ELIMINATION
-	bool
-	help
-	  Select this if the architecture wants to do dead code and
-	  data elimination with the linker by compiling with
-	  -ffunction-sections -fdata-sections and linking with
-	  --gc-sections.
-
-	  This requires that the arch annotates or otherwise protects
-	  its external entry points from being discarded. Linker scripts
-	  must also merge .text.*, .data.*, and .bss.* correctly into
-	  output sections. Care must be taken not to pull in unrelated
-	  sections (e.g., '.text.init'). Typically '.' in section names
-	  is used to distinguish them from label names / C identifiers.
-
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool
 	help
diff --git a/init/Kconfig b/init/Kconfig
index f013afc74b11..1706d963766b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1038,6 +1038,33 @@ config CC_OPTIMIZE_FOR_SIZE
 
 endchoice
 
+config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+	bool
+	help
+	  This requires that the arch annotates or otherwise protects
+	  its external entry points from being discarded. Linker scripts
+	  must also merge .text.*, .data.*, and .bss.* correctly into
+	  output sections. Care must be taken not to pull in unrelated
+	  sections (e.g., '.text.init'). Typically '.' in section names
+	  is used to distinguish them from label names / C identifiers.
+
+config LD_DEAD_CODE_DATA_ELIMINATION
+	bool "Dead code and data elimination (EXPERIMENTAL)"
+	depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+	depends on EXPERT
+	help
+	  Select this if the architecture wants to do dead code and
+	  data elimination with the linker by compiling with
+	  -ffunction-sections -fdata-sections, and linking with
+	  --gc-sections.
+
+	  This can reduce on disk and in-memory size of the kernel
+	  code and static data, particularly for small configs and
+	  on small systems. This has the possibility of introducing
+	  silently broken kernel if the required annotations are not
+	  present. This option is not well tested yet, so use at your
+	  own risk.
+
 config SYSCTL
 	bool
 
-- 
2.17.0

[PATCH v2 3/4] kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selectable if enabled

[Nicholas Piggin]

Architectures that are capable can select
HAVE_LD_DEAD_CODE_DATA_ELIMINATION to enable selection of that
option (as an EXPERT kernel option).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/Kconfig | 15 ---------------
 init/Kconfig | 27 +++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 8e0d665c8d53..3234b73d4626 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -596,21 +596,6 @@ config CC_STACKPROTECTOR_AUTO
 
 endchoice
 
-config LD_DEAD_CODE_DATA_ELIMINATION
-	bool
-	help
-	  Select this if the architecture wants to do dead code and
-	  data elimination with the linker by compiling with
-	  -ffunction-sections -fdata-sections and linking with
-	  --gc-sections.
-
-	  This requires that the arch annotates or otherwise protects
-	  its external entry points from being discarded. Linker scripts
-	  must also merge .text.*, .data.*, and .bss.* correctly into
-	  output sections. Care must be taken not to pull in unrelated
-	  sections (e.g., '.text.init'). Typically '.' in section names
-	  is used to distinguish them from label names / C identifiers.
-
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool
 	help
diff --git a/init/Kconfig b/init/Kconfig
index f013afc74b11..1706d963766b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1038,6 +1038,33 @@ config CC_OPTIMIZE_FOR_SIZE
 
 endchoice
 
+config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+	bool
+	help
+	  This requires that the arch annotates or otherwise protects
+	  its external entry points from being discarded. Linker scripts
+	  must also merge .text.*, .data.*, and .bss.* correctly into
+	  output sections. Care must be taken not to pull in unrelated
+	  sections (e.g., '.text.init'). Typically '.' in section names
+	  is used to distinguish them from label names / C identifiers.
+
+config LD_DEAD_CODE_DATA_ELIMINATION
+	bool "Dead code and data elimination (EXPERIMENTAL)"
+	depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION
+	depends on EXPERT
+	help
+	  Select this if the architecture wants to do dead code and
+	  data elimination with the linker by compiling with
+	  -ffunction-sections -fdata-sections, and linking with
+	  --gc-sections.
+
+	  This can reduce on disk and in-memory size of the kernel
+	  code and static data, particularly for small configs and
+	  on small systems. This has the possibility of introducing
+	  silently broken kernel if the required annotations are not
+	  present. This option is not well tested yet, so use at your
+	  own risk.
+
 config SYSCTL
 	bool
 
-- 
2.17.0

[PATCH v2 4/4] powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

[Nicholas Piggin]

This requires further changes to linker script to KEEP some tables
and wildcard compiler generated sections into the right place. This
includes pp32 modifications from Christophe Leroy.

When compiling powernv_defconfig with this option, the resulting
kernel is almost 400kB smaller (and still boots):

    text      data       bss        dec   filename
11827621   4810490   1341080   17979191   vmlinux
11752437   4598858   1338776   17690071   vmlinux.dcde

Mathieu's numbers for custom Mac Mini G4 config has almost 200kB
saving. It also had some increase in vmlinux size for as-yet
unknown reasons.

    text      data       bss        dec   filename
 7461457   2475122   1428064   11364643   vmlinux
 7386425   2364370   1425432   11176227   vmlinux.dcde

Tested-by: Christophe Leroy <christophe.leroy@c-s.fr> [8xx]
Tested-by: Mathieu Malaterre <malat@debian.org> [32-bit powermac]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/Kconfig              |  1 +
 arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++++++-----------
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c32a181a7cbb..ee6dbe2efc8b 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -205,6 +205,7 @@ config PPC
 	select HAVE_KPROBES
 	select HAVE_KPROBES_ON_FTRACE
 	select HAVE_KRETPROBES
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_LIVEPATCH			if HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_MEMBLOCK
 	select HAVE_MEMBLOCK_NODE_MAP
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index c8af90ff49f0..52a93cdd04bc 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -89,7 +89,7 @@ SECTIONS
 	 */
 	.text BLOCK(0) : AT(ADDR(.text) - LOAD_OFFSET) {
 #ifdef CONFIG_LD_HEAD_STUB_CATCH
-		*(.linker_stub_catch);
+		KEEP(*(.linker_stub_catch));
 		. = . ;
 #endif
 
@@ -98,7 +98,7 @@ SECTIONS
 		ALIGN_FUNCTION();
 #endif
 		/* careful! __ftr_alt_* sections need to be close to .text */
-		*(.text.hot .text .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
+		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
 		SCHED_TEXT
 		CPUIDLE_TEXT
 		LOCK_TEXT
@@ -170,10 +170,10 @@ SECTIONS
 	.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
 		INIT_DATA
 		__vtop_table_begin = .;
-		*(.vtop_fixup);
+		KEEP(*(.vtop_fixup));
 		__vtop_table_end = .;
 		__ptov_table_begin = .;
-		*(.ptov_fixup);
+		KEEP(*(.ptov_fixup));
 		__ptov_table_end = .;
 	}
 
@@ -194,26 +194,26 @@ SECTIONS
 	. = ALIGN(8);
 	__ftr_fixup : AT(ADDR(__ftr_fixup) - LOAD_OFFSET) {
 		__start___ftr_fixup = .;
-		*(__ftr_fixup)
+		KEEP(*(__ftr_fixup))
 		__stop___ftr_fixup = .;
 	}
 	. = ALIGN(8);
 	__mmu_ftr_fixup : AT(ADDR(__mmu_ftr_fixup) - LOAD_OFFSET) {
 		__start___mmu_ftr_fixup = .;
-		*(__mmu_ftr_fixup)
+		KEEP(*(__mmu_ftr_fixup))
 		__stop___mmu_ftr_fixup = .;
 	}
 	. = ALIGN(8);
 	__lwsync_fixup : AT(ADDR(__lwsync_fixup) - LOAD_OFFSET) {
 		__start___lwsync_fixup = .;
-		*(__lwsync_fixup)
+		KEEP(*(__lwsync_fixup))
 		__stop___lwsync_fixup = .;
 	}
 #ifdef CONFIG_PPC64
 	. = ALIGN(8);
 	__fw_ftr_fixup : AT(ADDR(__fw_ftr_fixup) - LOAD_OFFSET) {
 		__start___fw_ftr_fixup = .;
-		*(__fw_ftr_fixup)
+		KEEP(*(__fw_ftr_fixup))
 		__stop___fw_ftr_fixup = .;
 	}
 #endif
@@ -226,7 +226,7 @@ SECTIONS
 	. = ALIGN(8);
 	.machine.desc : AT(ADDR(.machine.desc) - LOAD_OFFSET) {
 		__machine_desc_start = . ;
-		*(.machine.desc)
+		KEEP(*(.machine.desc))
 		__machine_desc_end = . ;
 	}
 #ifdef CONFIG_RELOCATABLE
@@ -274,7 +274,7 @@ SECTIONS
 	.data : AT(ADDR(.data) - LOAD_OFFSET) {
 		DATA_DATA
 		*(.data.rel*)
-		*(.sdata)
+		*(SDATA_MAIN)
 		*(.sdata2)
 		*(.got.plt) *(.got)
 		*(.plt)
@@ -289,7 +289,7 @@ SECTIONS
 
 	.opd : AT(ADDR(.opd) - LOAD_OFFSET) {
 		__start_opd = .;
-		*(.opd)
+		KEEP(*(.opd))
 		__end_opd = .;
 	}
 
-- 
2.17.0

[PATCH v2 4/4] powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

[Nicholas Piggin]

This requires further changes to linker script to KEEP some tables
and wildcard compiler generated sections into the right place. This
includes pp32 modifications from Christophe Leroy.

When compiling powernv_defconfig with this option, the resulting
kernel is almost 400kB smaller (and still boots):

    text      data       bss        dec   filename
11827621   4810490   1341080   17979191   vmlinux
11752437   4598858   1338776   17690071   vmlinux.dcde

Mathieu's numbers for custom Mac Mini G4 config has almost 200kB
saving. It also had some increase in vmlinux size for as-yet
unknown reasons.

    text      data       bss        dec   filename
 7461457   2475122   1428064   11364643   vmlinux
 7386425   2364370   1425432   11176227   vmlinux.dcde

Tested-by: Christophe Leroy <christophe.leroy@c-s.fr> [8xx]
Tested-by: Mathieu Malaterre <malat@debian.org> [32-bit powermac]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/Kconfig              |  1 +
 arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++++++-----------
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c32a181a7cbb..ee6dbe2efc8b 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -205,6 +205,7 @@ config PPC
 	select HAVE_KPROBES
 	select HAVE_KPROBES_ON_FTRACE
 	select HAVE_KRETPROBES
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_LIVEPATCH			if HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_MEMBLOCK
 	select HAVE_MEMBLOCK_NODE_MAP
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index c8af90ff49f0..52a93cdd04bc 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -89,7 +89,7 @@ SECTIONS
 	 */
 	.text BLOCK(0) : AT(ADDR(.text) - LOAD_OFFSET) {
 #ifdef CONFIG_LD_HEAD_STUB_CATCH
-		*(.linker_stub_catch);
+		KEEP(*(.linker_stub_catch));
 		. = . ;
 #endif
 
@@ -98,7 +98,7 @@ SECTIONS
 		ALIGN_FUNCTION();
 #endif
 		/* careful! __ftr_alt_* sections need to be close to .text */
-		*(.text.hot .text .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
+		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
 		SCHED_TEXT
 		CPUIDLE_TEXT
 		LOCK_TEXT
@@ -170,10 +170,10 @@ SECTIONS
 	.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
 		INIT_DATA
 		__vtop_table_begin = .;
-		*(.vtop_fixup);
+		KEEP(*(.vtop_fixup));
 		__vtop_table_end = .;
 		__ptov_table_begin = .;
-		*(.ptov_fixup);
+		KEEP(*(.ptov_fixup));
 		__ptov_table_end = .;
 	}
 
@@ -194,26 +194,26 @@ SECTIONS
 	. = ALIGN(8);
 	__ftr_fixup : AT(ADDR(__ftr_fixup) - LOAD_OFFSET) {
 		__start___ftr_fixup = .;
-		*(__ftr_fixup)
+		KEEP(*(__ftr_fixup))
 		__stop___ftr_fixup = .;
 	}
 	. = ALIGN(8);
 	__mmu_ftr_fixup : AT(ADDR(__mmu_ftr_fixup) - LOAD_OFFSET) {
 		__start___mmu_ftr_fixup = .;
-		*(__mmu_ftr_fixup)
+		KEEP(*(__mmu_ftr_fixup))
 		__stop___mmu_ftr_fixup = .;
 	}
 	. = ALIGN(8);
 	__lwsync_fixup : AT(ADDR(__lwsync_fixup) - LOAD_OFFSET) {
 		__start___lwsync_fixup = .;
-		*(__lwsync_fixup)
+		KEEP(*(__lwsync_fixup))
 		__stop___lwsync_fixup = .;
 	}
 #ifdef CONFIG_PPC64
 	. = ALIGN(8);
 	__fw_ftr_fixup : AT(ADDR(__fw_ftr_fixup) - LOAD_OFFSET) {
 		__start___fw_ftr_fixup = .;
-		*(__fw_ftr_fixup)
+		KEEP(*(__fw_ftr_fixup))
 		__stop___fw_ftr_fixup = .;
 	}
 #endif
@@ -226,7 +226,7 @@ SECTIONS
 	. = ALIGN(8);
 	.machine.desc : AT(ADDR(.machine.desc) - LOAD_OFFSET) {
 		__machine_desc_start = . ;
-		*(.machine.desc)
+		KEEP(*(.machine.desc))
 		__machine_desc_end = . ;
 	}
 #ifdef CONFIG_RELOCATABLE
@@ -274,7 +274,7 @@ SECTIONS
 	.data : AT(ADDR(.data) - LOAD_OFFSET) {
 		DATA_DATA
 		*(.data.rel*)
-		*(.sdata)
+		*(SDATA_MAIN)
 		*(.sdata2)
 		*(.got.plt) *(.got)
 		*(.plt)
@@ -289,7 +289,7 @@ SECTIONS
 
 	.opd : AT(ADDR(.opd) - LOAD_OFFSET) {
 		__start_opd = .;
-		*(.opd)
+		KEEP(*(.opd))
 		__end_opd = .;
 	}
 
-- 
2.17.0

Re: [PATCH v2 0/4] LD_DEAD_CODE_DATA_ELIMINATION fixes and enabling for powerpc

[Masahiro Yamada]

2018-05-09 21:59 GMT+09:00 Nicholas Piggin <npiggin@gmail.com>:
> v2 changes only patch 4, updates the changelog with ppc32 numbers and
> Tested-by tags, and also use TEXT_MAIN in the linker script as
> Christophe suggested.


Replaced with v2.
Thanks!


> Thanks,
> Nick
>
> Nicholas Piggin (4):
>   kbuild: Fix asm-generic/vmlinux.lds.h for
>     LD_DEAD_CODE_DATA_ELIMINATION
>   kbuild: LD_DEAD_CODE_DATA_ELIMINATION no
>     -ffunction-sections/-fdata-sections for module build
>   kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selectable if
>     enabled
>   powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected
>
>  Makefile                          |  4 +--
>  arch/Kconfig                      | 15 ----------
>  arch/powerpc/Kconfig              |  1 +
>  arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++-------
>  include/asm-generic/vmlinux.lds.h | 49 ++++++++++++++++++-------------
>  init/Kconfig                      | 27 +++++++++++++++++
>  6 files changed, 70 insertions(+), 48 deletions(-)
>
> --
> 2.17.0
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



-- 
Best Regards
Masahiro Yamada

Re: [PATCH v2 0/4] LD_DEAD_CODE_DATA_ELIMINATION fixes and enabling for powerpc

[Masahiro Yamada]

2018-05-09 21:59 GMT+09:00 Nicholas Piggin <npiggin@gmail.com>:
> v2 changes only patch 4, updates the changelog with ppc32 numbers and
> Tested-by tags, and also use TEXT_MAIN in the linker script as
> Christophe suggested.


Replaced with v2.
Thanks!


> Thanks,
> Nick
>
> Nicholas Piggin (4):
>   kbuild: Fix asm-generic/vmlinux.lds.h for
>     LD_DEAD_CODE_DATA_ELIMINATION
>   kbuild: LD_DEAD_CODE_DATA_ELIMINATION no
>     -ffunction-sections/-fdata-sections for module build
>   kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selectable if
>     enabled
>   powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected
>
>  Makefile                          |  4 +--
>  arch/Kconfig                      | 15 ----------
>  arch/powerpc/Kconfig              |  1 +
>  arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++-------
>  include/asm-generic/vmlinux.lds.h | 49 ++++++++++++++++++-------------
>  init/Kconfig                      | 27 +++++++++++++++++
>  6 files changed, 70 insertions(+), 48 deletions(-)
>
> --
> 2.17.0
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



-- 
Best Regards
Masahiro Yamada

Re: [PATCH v2 4/4] powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

[Michael Ellerman]

Nicholas Piggin <npiggin@gmail.com> writes:

> This requires further changes to linker script to KEEP some tables
> and wildcard compiler generated sections into the right place. This
> includes pp32 modifications from Christophe Leroy.
>
> When compiling powernv_defconfig with this option, the resulting
> kernel is almost 400kB smaller (and still boots):
>
>     text      data       bss        dec   filename
> 11827621   4810490   1341080   17979191   vmlinux
> 11752437   4598858   1338776   17690071   vmlinux.dcde
>
> Mathieu's numbers for custom Mac Mini G4 config has almost 200kB
> saving. It also had some increase in vmlinux size for as-yet
> unknown reasons.
>
>     text      data       bss        dec   filename
>  7461457   2475122   1428064   11364643   vmlinux
>  7386425   2364370   1425432   11176227   vmlinux.dcde
>
> Tested-by: Christophe Leroy <christophe.leroy@c-s.fr> [8xx]
> Tested-by: Mathieu Malaterre <malat@debian.org> [32-bit powermac]
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/Kconfig              |  1 +
>  arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++++++-----------
>  2 files changed, 12 insertions(+), 11 deletions(-)

I'm happy for this to go in via the kbuild tree.

As long as it's in linux-next it will get tested by my CI which should
hopefully shake out any bugs.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>

cheers

Re: [PATCH v2 4/4] powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

[Michael Ellerman]

Nicholas Piggin <npiggin@gmail.com> writes:

> This requires further changes to linker script to KEEP some tables
> and wildcard compiler generated sections into the right place. This
> includes pp32 modifications from Christophe Leroy.
>
> When compiling powernv_defconfig with this option, the resulting
> kernel is almost 400kB smaller (and still boots):
>
>     text      data       bss        dec   filename
> 11827621   4810490   1341080   17979191   vmlinux
> 11752437   4598858   1338776   17690071   vmlinux.dcde
>
> Mathieu's numbers for custom Mac Mini G4 config has almost 200kB
> saving. It also had some increase in vmlinux size for as-yet
> unknown reasons.
>
>     text      data       bss        dec   filename
>  7461457   2475122   1428064   11364643   vmlinux
>  7386425   2364370   1425432   11176227   vmlinux.dcde
>
> Tested-by: Christophe Leroy <christophe.leroy@c-s.fr> [8xx]
> Tested-by: Mathieu Malaterre <malat@debian.org> [32-bit powermac]
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/Kconfig              |  1 +
>  arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++++++-----------
>  2 files changed, 12 insertions(+), 11 deletions(-)

I'm happy for this to go in via the kbuild tree.

As long as it's in linux-next it will get tested by my CI which should
hopefully shake out any bugs.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>

cheers

Re: [PATCH v2 4/4] powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

[Michael Ellerman]

Nicholas Piggin <npiggin@gmail.com> writes:

> This requires further changes to linker script to KEEP some tables
> and wildcard compiler generated sections into the right place. This
> includes pp32 modifications from Christophe Leroy.
>
> When compiling powernv_defconfig with this option, the resulting
> kernel is almost 400kB smaller (and still boots):
>
>     text      data       bss        dec   filename
> 11827621   4810490   1341080   17979191   vmlinux
> 11752437   4598858   1338776   17690071   vmlinux.dcde
>
> Mathieu's numbers for custom Mac Mini G4 config has almost 200kB
> saving. It also had some increase in vmlinux size for as-yet
> unknown reasons.
>
>     text      data       bss        dec   filename
>  7461457   2475122   1428064   11364643   vmlinux
>  7386425   2364370   1425432   11176227   vmlinux.dcde
>
> Tested-by: Christophe Leroy <christophe.leroy@c-s.fr> [8xx]
> Tested-by: Mathieu Malaterre <malat@debian.org> [32-bit powermac]
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/Kconfig              |  1 +
>  arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++++++-----------
>  2 files changed, 12 insertions(+), 11 deletions(-)

I'm happy for this to go in via the kbuild tree.

As long as it's in linux-next it will get tested by my CI which should
hopefully shake out any bugs.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>

cheers

Re: [PATCH v2 4/4] powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

[Mathieu Malaterre]

On Wed, May 9, 2018 at 3:00 PM, Nicholas Piggin <npiggin@gmail.com> wrote:
> This requires further changes to linker script to KEEP some tables
> and wildcard compiler generated sections into the right place. This
> includes pp32 modifications from Christophe Leroy.
>
> When compiling powernv_defconfig with this option, the resulting
> kernel is almost 400kB smaller (and still boots):
>
>     text      data       bss        dec   filename
> 11827621   4810490   1341080   17979191   vmlinux
> 11752437   4598858   1338776   17690071   vmlinux.dcde
>
> Mathieu's numbers for custom Mac Mini G4 config has almost 200kB

Technically this is an oldconfig from debian original config file
(debian official powerpc kernel package)... tested on a mac mini g4.

> saving. It also had some increase in vmlinux size for as-yet
> unknown reasons.
>
>     text      data       bss        dec   filename
>  7461457   2475122   1428064   11364643   vmlinux
>  7386425   2364370   1425432   11176227   vmlinux.dcde
>
> Tested-by: Christophe Leroy <christophe.leroy@c-s.fr> [8xx]
> Tested-by: Mathieu Malaterre <malat@debian.org> [32-bit powermac]
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/Kconfig              |  1 +
>  arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++++++-----------
>  2 files changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index c32a181a7cbb..ee6dbe2efc8b 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -205,6 +205,7 @@ config PPC
>         select HAVE_KPROBES
>         select HAVE_KPROBES_ON_FTRACE
>         select HAVE_KRETPROBES
> +       select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>         select HAVE_LIVEPATCH                   if HAVE_DYNAMIC_FTRACE_WITH_REGS
>         select HAVE_MEMBLOCK
>         select HAVE_MEMBLOCK_NODE_MAP
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index c8af90ff49f0..52a93cdd04bc 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -89,7 +89,7 @@ SECTIONS
>          */
>         .text BLOCK(0) : AT(ADDR(.text) - LOAD_OFFSET) {
>  #ifdef CONFIG_LD_HEAD_STUB_CATCH
> -               *(.linker_stub_catch);
> +               KEEP(*(.linker_stub_catch));
>                 . = . ;
>  #endif
>
> @@ -98,7 +98,7 @@ SECTIONS
>                 ALIGN_FUNCTION();
>  #endif
>                 /* careful! __ftr_alt_* sections need to be close to .text */
> -               *(.text.hot .text .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
> +               *(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
>                 SCHED_TEXT
>                 CPUIDLE_TEXT
>                 LOCK_TEXT
> @@ -170,10 +170,10 @@ SECTIONS
>         .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
>                 INIT_DATA
>                 __vtop_table_begin = .;
> -               *(.vtop_fixup);
> +               KEEP(*(.vtop_fixup));
>                 __vtop_table_end = .;
>                 __ptov_table_begin = .;
> -               *(.ptov_fixup);
> +               KEEP(*(.ptov_fixup));
>                 __ptov_table_end = .;
>         }
>
> @@ -194,26 +194,26 @@ SECTIONS
>         . = ALIGN(8);
>         __ftr_fixup : AT(ADDR(__ftr_fixup) - LOAD_OFFSET) {
>                 __start___ftr_fixup = .;
> -               *(__ftr_fixup)
> +               KEEP(*(__ftr_fixup))
>                 __stop___ftr_fixup = .;
>         }
>         . = ALIGN(8);
>         __mmu_ftr_fixup : AT(ADDR(__mmu_ftr_fixup) - LOAD_OFFSET) {
>                 __start___mmu_ftr_fixup = .;
> -               *(__mmu_ftr_fixup)
> +               KEEP(*(__mmu_ftr_fixup))
>                 __stop___mmu_ftr_fixup = .;
>         }
>         . = ALIGN(8);
>         __lwsync_fixup : AT(ADDR(__lwsync_fixup) - LOAD_OFFSET) {
>                 __start___lwsync_fixup = .;
> -               *(__lwsync_fixup)
> +               KEEP(*(__lwsync_fixup))
>                 __stop___lwsync_fixup = .;
>         }
>  #ifdef CONFIG_PPC64
>         . = ALIGN(8);
>         __fw_ftr_fixup : AT(ADDR(__fw_ftr_fixup) - LOAD_OFFSET) {
>                 __start___fw_ftr_fixup = .;
> -               *(__fw_ftr_fixup)
> +               KEEP(*(__fw_ftr_fixup))
>                 __stop___fw_ftr_fixup = .;
>         }
>  #endif
> @@ -226,7 +226,7 @@ SECTIONS
>         . = ALIGN(8);
>         .machine.desc : AT(ADDR(.machine.desc) - LOAD_OFFSET) {
>                 __machine_desc_start = . ;
> -               *(.machine.desc)
> +               KEEP(*(.machine.desc))
>                 __machine_desc_end = . ;
>         }
>  #ifdef CONFIG_RELOCATABLE
> @@ -274,7 +274,7 @@ SECTIONS
>         .data : AT(ADDR(.data) - LOAD_OFFSET) {
>                 DATA_DATA
>                 *(.data.rel*)
> -               *(.sdata)
> +               *(SDATA_MAIN)
>                 *(.sdata2)
>                 *(.got.plt) *(.got)
>                 *(.plt)
> @@ -289,7 +289,7 @@ SECTIONS
>
>         .opd : AT(ADDR(.opd) - LOAD_OFFSET) {
>                 __start_opd = .;
> -               *(.opd)
> +               KEEP(*(.opd))
>                 __end_opd = .;
>         }
>
> --
> 2.17.0
>

Re: [PATCH v2 4/4] powerpc: Allow LD_DEAD_CODE_DATA_ELIMINATION to be selected

[Mathieu Malaterre]

On Wed, May 9, 2018 at 3:00 PM, Nicholas Piggin <npiggin@gmail.com> wrote:
> This requires further changes to linker script to KEEP some tables
> and wildcard compiler generated sections into the right place. This
> includes pp32 modifications from Christophe Leroy.
>
> When compiling powernv_defconfig with this option, the resulting
> kernel is almost 400kB smaller (and still boots):
>
>     text      data       bss        dec   filename
> 11827621   4810490   1341080   17979191   vmlinux
> 11752437   4598858   1338776   17690071   vmlinux.dcde
>
> Mathieu's numbers for custom Mac Mini G4 config has almost 200kB

Technically this is an oldconfig from debian original config file
(debian official powerpc kernel package)... tested on a mac mini g4.

> saving. It also had some increase in vmlinux size for as-yet
> unknown reasons.
>
>     text      data       bss        dec   filename
>  7461457   2475122   1428064   11364643   vmlinux
>  7386425   2364370   1425432   11176227   vmlinux.dcde
>
> Tested-by: Christophe Leroy <christophe.leroy@c-s.fr> [8xx]
> Tested-by: Mathieu Malaterre <malat@debian.org> [32-bit powermac]
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/Kconfig              |  1 +
>  arch/powerpc/kernel/vmlinux.lds.S | 22 +++++++++++-----------
>  2 files changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index c32a181a7cbb..ee6dbe2efc8b 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -205,6 +205,7 @@ config PPC
>         select HAVE_KPROBES
>         select HAVE_KPROBES_ON_FTRACE
>         select HAVE_KRETPROBES
> +       select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>         select HAVE_LIVEPATCH                   if HAVE_DYNAMIC_FTRACE_WITH_REGS
>         select HAVE_MEMBLOCK
>         select HAVE_MEMBLOCK_NODE_MAP
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index c8af90ff49f0..52a93cdd04bc 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -89,7 +89,7 @@ SECTIONS
>          */
>         .text BLOCK(0) : AT(ADDR(.text) - LOAD_OFFSET) {
>  #ifdef CONFIG_LD_HEAD_STUB_CATCH
> -               *(.linker_stub_catch);
> +               KEEP(*(.linker_stub_catch));
>                 . = . ;
>  #endif
>
> @@ -98,7 +98,7 @@ SECTIONS
>                 ALIGN_FUNCTION();
>  #endif
>                 /* careful! __ftr_alt_* sections need to be close to .text */
> -               *(.text.hot .text .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
> +               *(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
>                 SCHED_TEXT
>                 CPUIDLE_TEXT
>                 LOCK_TEXT
> @@ -170,10 +170,10 @@ SECTIONS
>         .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
>                 INIT_DATA
>                 __vtop_table_begin = .;
> -               *(.vtop_fixup);
> +               KEEP(*(.vtop_fixup));
>                 __vtop_table_end = .;
>                 __ptov_table_begin = .;
> -               *(.ptov_fixup);
> +               KEEP(*(.ptov_fixup));
>                 __ptov_table_end = .;
>         }
>
> @@ -194,26 +194,26 @@ SECTIONS
>         . = ALIGN(8);
>         __ftr_fixup : AT(ADDR(__ftr_fixup) - LOAD_OFFSET) {
>                 __start___ftr_fixup = .;
> -               *(__ftr_fixup)
> +               KEEP(*(__ftr_fixup))
>                 __stop___ftr_fixup = .;
>         }
>         . = ALIGN(8);
>         __mmu_ftr_fixup : AT(ADDR(__mmu_ftr_fixup) - LOAD_OFFSET) {
>                 __start___mmu_ftr_fixup = .;
> -               *(__mmu_ftr_fixup)
> +               KEEP(*(__mmu_ftr_fixup))
>                 __stop___mmu_ftr_fixup = .;
>         }
>         . = ALIGN(8);
>         __lwsync_fixup : AT(ADDR(__lwsync_fixup) - LOAD_OFFSET) {
>                 __start___lwsync_fixup = .;
> -               *(__lwsync_fixup)
> +               KEEP(*(__lwsync_fixup))
>                 __stop___lwsync_fixup = .;
>         }
>  #ifdef CONFIG_PPC64
>         . = ALIGN(8);
>         __fw_ftr_fixup : AT(ADDR(__fw_ftr_fixup) - LOAD_OFFSET) {
>                 __start___fw_ftr_fixup = .;
> -               *(__fw_ftr_fixup)
> +               KEEP(*(__fw_ftr_fixup))
>                 __stop___fw_ftr_fixup = .;
>         }
>  #endif
> @@ -226,7 +226,7 @@ SECTIONS
>         . = ALIGN(8);
>         .machine.desc : AT(ADDR(.machine.desc) - LOAD_OFFSET) {
>                 __machine_desc_start = . ;
> -               *(.machine.desc)
> +               KEEP(*(.machine.desc))
>                 __machine_desc_end = . ;
>         }
>  #ifdef CONFIG_RELOCATABLE
> @@ -274,7 +274,7 @@ SECTIONS
>         .data : AT(ADDR(.data) - LOAD_OFFSET) {
>                 DATA_DATA
>                 *(.data.rel*)
> -               *(.sdata)
> +               *(SDATA_MAIN)
>                 *(.sdata2)
>                 *(.got.plt) *(.got)
>                 *(.plt)
> @@ -289,7 +289,7 @@ SECTIONS
>
>         .opd : AT(ADDR(.opd) - LOAD_OFFSET) {
>                 __start_opd = .;
> -               *(.opd)
> +               KEEP(*(.opd))
>                 __end_opd = .;
>         }
>
> --
> 2.17.0
>