Re: [PATCH v2 4/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

Kefeng Wang
2023-05-24T11:05:11Z

On 2023/5/24 0:55, Jisheng Zhang wrote:
> 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>

Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>

> ---
> 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 = .;