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

From: Jisheng Zhang <jszhang@kernel.org>
To: Zhangjin Wu <falcon@tinylab.org>
Cc: palmer@rivosinc.com, conor@kernel.org, guoren@kernel.org,
	linux-riscv@lists.infradead.org, npiggin@gmail.com,
	paul.walmsley@sifive.com, paulburton@kernel.org,
	vincent.chen@sifive.com, tim.bird@sony.com
Subject: Re: [PATCH v3] RISC-V: Enable dead code elimination
Date: Sun, 21 May 2023 16:48:42 +0800	[thread overview]
Message-ID: <ZGna6sWmRZbMf3OH@xhacker> (raw)
In-Reply-To: <20230517082936.37563-1-falcon@tinylab.org>

On Wed, May 17, 2023 at 04:29:36PM +0800, Zhangjin Wu wrote:
> 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:

OOPS, I didn't noticed that you have sent out the patch and you are now
in v3. I may read your patch sevral months ago, but I forget it.
But your version missed an important preparation patch which is
to fix the typo in commom vmlinux.lds.h in asm-generic, thus LD_ORPHAN_WARN
will warn loudly. See[1] for more details.

Another missing point is we need to keep the .init.pi* and .init.bss*
sections. See[1] for more details.

Since you are the first person to send out the patch of
HAVE_LD_DEAD_CODE_DATA_ELIMINATION for riscv, do you mind if I update
my series and take your patch in my series v2, make you as the commmit
author, use your commit msg, and add my co-developped tag etc?

Thanks

> 
>           | 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@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 = .;
> -- 
> 2.25.1
> 
> 
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