linux-kernel - Re: [PATCH] arm64: enable dead code elimination

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Date:   Mon, 17 Jul 2023 10:24:40 +0100
From:   Will Deacon <will@...nel.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: enable dead code elimination

On Mon, Jul 17, 2023 at 04:07:39PM +0800, Kefeng Wang wrote:
> Select CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION for arm64, allowing the
> user to enable dead code elimination. In order for this to work, ensure
> that we keep the necessary tables by annotating them with KEEP, also it
> requires further changes to linker script to KEEP some tables and wildcard
> compiler generated sections into the right place.
> 
> The following comparison is based 6.5-rc2 with defconfig,
> 
> $ ./scripts/bloat-o-meter vmlinux-base vmlinux-new
> add/remove: 3/1106 grow/shrink: 4102/6964 up/down: 35704/-99980 (-64276)
> Function                                     old     new   delta
> ...
> Total: Before=17888959, After=17824683, chg -0.36%
> 
> add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-44 (-44)
> Data                                         old     new   delta
> ...
> Total: Before=4820808, After=4820764, chg -0.00%
> 
> add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-1096 (-1096)
> RO Data                                      old     new   delta
> ...
> Total: Before=5179123, After=5178027, chg -0.02%
> 
> $ size vmlinux-base vmlinux
>    text	   data	     bss      dec       hex	filename
> 25433734  15385766  630656  41450156  2787aac	vmlinux-base
> 24756738  15360870  629888  40747496  26dc1e8	vmlinux-new
> 
> Memory available after booting, saving 704k on qemu,
> base: 8084532K/8388608K
> new:  8085236K/8388608K

Is that a 0.009% improvement? Is it really worth the hassle?

x86 doesn't select this and risc-v had to turn it off for LLD, so it feels
like we're just creating a rod for our own back by selecting it.

Will

> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> ---
>  arch/arm64/Kconfig              | 1 +
>  arch/arm64/kernel/vmlinux.lds.S | 5 +++--
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index a2511b30d0f6..73bb908ec62f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -148,6 +148,7 @@ config ARM64
>  	select GENERIC_VDSO_TIME_NS
>  	select HARDIRQS_SW_RESEND
>  	select HAS_IOPORT
> +	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>  	select HAVE_MOVE_PMD
>  	select HAVE_MOVE_PUD
>  	select HAVE_PCI
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 3cd7e76cc562..bb4ce6cd6896 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -238,7 +238,7 @@ SECTIONS
>  	. = ALIGN(4);
>  	.altinstructions : {
>  		__alt_instructions = .;
> -		*(.altinstructions)
> +		KEEP(*(.altinstructions))
>  		__alt_instructions_end = .;
>  	}
>  
> @@ -258,8 +258,9 @@ SECTIONS
>  		INIT_CALLS
>  		CON_INITCALL
>  		INIT_RAM_FS
> -		*(.init.altinstructions .init.bss)	/* from the EFI stub */
> +		KEEP(*(.init.altinstructions .init.bss*))	/* from the EFI stub */
>  	}
> +
>  	.exit.data : {
>  		EXIT_DATA
>  	}
> -- 
> 2.27.0
>