[PATCH v3 0/1] Enable Dead Code Elimination

[PATCH v3 0/1] Enable Dead Code Elimination

[Kele Zhang]

This patch enables `--gc-sections`. Dead Code Elimination (DCE) can
reduce binary size and the attack surface without introducing side
effects, and it has been used in the Linux kernel for many years.

After testing, enabling DCE does not affect the normal functionality of
OpenSBI.

---
v1: https://lists.infradead.org/pipermail/opensbi/2024-August/007286.html
v2: https://lists.infradead.org/pipermail/opensbi/2024-August/007301.html
---
Changes in v2:
- Move --gc-sections to ELFFLAGS
- Remove redundant KEEP()

Changes in v3:
- Move --print-gc-sections to ifneq ($(DEBUG),)
- Move "ifneq ($(DEBUG),)" to the latter part of the Makefile, ensuring
that "ELFFLAGS  +=      $(USE_LD_FLAG)" remains at the beginning.


Kele Zhang (1):
  Makefile: enable --gc-sections

 Makefile             | 15 +++++++++------
 firmware/fw_base.ldS |  1 +
 2 files changed, 10 insertions(+), 6 deletions(-)


base-commit: c4940a9517486413cd676fc8032bb55f9d4e2778
-- 
2.43.0

[PATCH v3 1/1] Makefile: enable --gc-sections

[Kele Zhang]

The --gc-sections option enables the linker to perform garbage
collection of unreferenced code and data, thereby reducing the binary
size.

The -ffunction-sections option will place each function into a separate
section, so it is necessary to add .text.* to the linker script.

Signed-off-by: Kele Zhang <zhangcola2003@gmail.com>
Signed-off-by: Yuan Tan <tanyuan@tinylab.org>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
---
 Makefile             | 15 +++++++++------
 firmware/fw_base.ldS |  1 +
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index e5a0f19..30ba3b6 100644
--- a/Makefile
+++ b/Makefile
@@ -355,12 +355,7 @@ GENFLAGS	+=	$(libsbiutils-genflags-y)
 GENFLAGS	+=	$(platform-genflags-y)
 GENFLAGS	+=	$(firmware-genflags-y)
 
-CFLAGS		=	-g -Wall -Werror -ffreestanding -nostdlib -fno-stack-protector -fno-strict-aliasing
-ifneq ($(DEBUG),)
-CFLAGS		+=	-O0
-else
-CFLAGS		+=	-O2
-endif
+CFLAGS		=	-g -Wall -Werror -ffreestanding -nostdlib -fno-stack-protector -fno-strict-aliasing -ffunction-sections -fdata-sections
 CFLAGS		+=	-fno-omit-frame-pointer -fno-optimize-sibling-calls
 # Optionally supported flags
 ifeq ($(CC_SUPPORT_SAVE_RESTORE),y)
@@ -406,6 +401,7 @@ ASFLAGS		+=	$(firmware-asflags-y)
 ARFLAGS		=	rcs
 
 ELFFLAGS	+=	$(USE_LD_FLAG)
+ELFFLAGS	+=	-Wl,--gc-sections
 ifeq ($(OPENSBI_LD_EXCLUDE_LIBS),y)
 ELFFLAGS	+=	-Wl,--exclude-libs,ALL
 endif
@@ -424,6 +420,13 @@ MERGEFLAGS	+=	-m elf$(PLATFORM_RISCV_XLEN)lriscv
 
 DTSCPPFLAGS	=	$(CPPFLAGS) -nostdinc -nostdlib -fno-builtin -D__DTS__ -x assembler-with-cpp
 
+ifneq ($(DEBUG),)
+CFLAGS		+=	-O0
+ELFFLAGS	+=  -Wl,--print-gc-sections
+else
+CFLAGS		+=	-O2
+endif
+
 # Setup functions for compilation
 define dynamic_flags
 -I$(shell dirname $(2)) -D__OBJNAME__=$(subst -,_,$(shell basename $(1) .o))
diff --git a/firmware/fw_base.ldS b/firmware/fw_base.ldS
index fb47984..a33746a 100644
--- a/firmware/fw_base.ldS
+++ b/firmware/fw_base.ldS
@@ -20,6 +20,7 @@
 		PROVIDE(_text_start = .);
 		*(.entry)
 		*(.text)
+		*(.text.*)
 		. = ALIGN(8);
 		PROVIDE(_text_end = .);
 	}
-- 
2.43.0

[PATCH v3 1/1] Makefile: enable --gc-sections

[Bin Meng]

On Thu, Sep 5, 2024 at 5:31?PM Kele Zhang <zhangcola2003@gmail.com> wrote:
>
> The --gc-sections option enables the linker to perform garbage
> collection of unreferenced code and data, thereby reducing the binary
> size.
>
> The -ffunction-sections option will place each function into a separate
> section, so it is necessary to add .text.* to the linker script.
>
> Signed-off-by: Kele Zhang <zhangcola2003@gmail.com>
> Signed-off-by: Yuan Tan <tanyuan@tinylab.org>
> Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
> ---
>  Makefile             | 15 +++++++++------
>  firmware/fw_base.ldS |  1 +
>  2 files changed, 10 insertions(+), 6 deletions(-)
>

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

[PATCH v3 1/1] Makefile: enable --gc-sections

[Jessica Clarke]

On 5 Sep 2024, at 10:31, Kele Zhang <zhangcola2003@gmail.com> wrote:
> 
> The --gc-sections option enables the linker to perform garbage
> collection of unreferenced code and data, thereby reducing the binary
> size.
> 
> The -ffunction-sections option will place each function into a separate
> section, so it is necessary to add .text.* to the linker script.
> 
> Signed-off-by: Kele Zhang <zhangcola2003@gmail.com>
> Signed-off-by: Yuan Tan <tanyuan@tinylab.org>
> Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
> ?
> ...
> @@ -424,6 +420,13 @@ MERGEFLAGS += -m elf$(PLATFORM_RISCV_XLEN)lriscv
> 
> DTSCPPFLAGS = $(CPPFLAGS) -nostdinc -nostdlib -fno-builtin -D__DTS__ -x assembler-with-cpp
> 
> +ifneq ($(DEBUG),)
> +CFLAGS += -O0
> +ELFFLAGS +=  -Wl,--print-gc-sections

This line is using spaces not tabs.

Jess