[PATCH 4/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

Jisheng Zhang
2023-05-11T15:31:13Z

When trying to run linux with various opensource riscv core on
resource limited FPGA platforms, for example, those FPGAs with less
than 16MB SDRAM, I want to save mem as much as possible. One of the
major technologies is kernel size optimizations, I found that riscv
does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION, which
passes -fdata-sections, -ffunction-sections to CFLAGS and passes the
--gc-sections flag to the linker.

This not only benefits my case on FPGA but also benefits defconfigs.
Here are some notable improvements from enabling this with defconfigs:

nommu_k210_defconfig:
text data bss dec hex
1112009 410288 59837 1582134 182436 before
962838 376656 51285 1390779 1538bb after

rv32_defconfig:
text data bss dec hex
8804455 2816544 290577 11911576 b5c198 before
8692295 2779872 288977 11761144 b375f8 after

defconfig:
text data bss dec hex
9438267 3391332 485333 13314932 cb2b74 before
9285914 3350052 483349 13119315 c82f53 after

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
---
arch/riscv/Kconfig | 1 +
arch/riscv/kernel/vmlinux.lds.S | 6 +++---
2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index f0663b52d052..a5feab2c3037 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -110,6 +110,7 @@ config RISCV
select HAVE_KPROBES if !XIP_KERNEL
select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
select HAVE_KRETPROBES if !XIP_KERNEL
+ select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
select HAVE_MOVE_PMD
select HAVE_MOVE_PUD
select HAVE_PCI
diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
index e5f9f4677bbf..492dd4b8f3d6 100644
--- a/arch/riscv/kernel/vmlinux.lds.S
+++ b/arch/riscv/kernel/vmlinux.lds.S
@@ -85,11 +85,11 @@ SECTIONS
INIT_DATA_SECTION(16)

.init.pi : {
- *(.init.pi*)
+ KEEP(*(.init.pi*))
}

.init.bss : {
- *(.init.bss) /* from the EFI stub */
+ KEEP(*(.init.bss*)) /* from the EFI stub */
}
.exit.data :
{
@@ -112,7 +112,7 @@ SECTIONS
. = ALIGN(8);
.alternative : {
__alt_start = .;
- *(.alternative)
+ KEEP(*(.alternative))
__alt_end = .;
}
__init_end = .;
--
2.40.1