[PATCH v3] RISC-V: Enable dead code elimination

[Zhangjin Wu]

Hello, Maintainers

No changes from v2 to v3, only rebased on v6.4-rc2, it compiled and
booted.

This is part of my tinylinux work for RISC-V, the other patches are
coming soon.

Thanks,
Zhangjin Wu

> 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
> 
> Signed-off-by: Zhangjin Wu <falcon at tinylab.org>
> ---
>  arch/riscv/Kconfig              | 1 +
>  arch/riscv/kernel/vmlinux.lds.S | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 348c0fa1fc8c..d2eb1a862ea8 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -111,6 +111,7 @@ config RISCV
>  	select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
>  	select HAVE_KRETPROBES if !XIP_KERNEL
>  	select HAVE_RETHOOK 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..0f5dfbc113d4 100644
> --- a/arch/riscv/kernel/vmlinux.lds.S
> +++ b/arch/riscv/kernel/vmlinux.lds.S
> @@ -112,7 +112,7 @@ SECTIONS
>  	. = ALIGN(8);
>  	.alternative : {
>  		__alt_start = .;
> -		*(.alternative)
> +		KEEP(*(.alternative))
>  		__alt_end = .;
>  	}
>  	__init_end = .;