[PATCH v2 4/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

Jisheng Zhang
2023-05-23T17:06:31Z

From: Zhangjin Wu <falcon@tinylab.org>

Select CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION for RISC-V, allowing
the user to enable dead code elimination. In order for this to work,
ensure that we keep the alternative table by annotating them with KEEP.

This boots well on qemu with both rv32_defconfig & rv64 defconfig, but
it only shrinks their builds by ~1%, a smaller config is thereforce
customized to test this feature:

| rv32 | rv64
--------|------------------------|---------------------
No DCE | 4460684 | 4893488
DCE | 3986716 | 4376400
Shrink | 473968 (~10.6%) | 517088 (~10.5%)

The config used above only reserves necessary options to boot on qemu
with serial console, more like the size-critical embedded scenes:

- rv64 config: https://pastebin.com/crz82T0s
- rv32 config: rv64 config + 32-bit.config

Here is Jisheng's original commit-msg:
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: Zhangjin Wu <falcon@tinylab.org>
Co-developed-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Tested-by: Bin Meng <bmeng@tinylab.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 8f55aa4aae34..62e84fee2cfd 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -115,6 +115,7 @@ 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