Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Sedat Dilek <sedat.dilek@gmail.com>]

On Thu, Feb 25, 2021 at 12:21 PM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> When looking at kernel size optimizations, I found that arm64
> does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION,
> which enables the --gc-sections flag to the linker.
>
> I see that for a defconfig build with llvm, there are some
> notable improvements from enabling this, in particular when
> combined with the recently added CONFIG_LTO_CLANG_THIN
> and CONFIG_TRIM_UNUSED_KSYMS:
>
>    text    data     bss     dec     hex filename
> 16570322 10998617 506468 28075407 1ac658f defconfig/vmlinux
> 16318793 10569913 506468 27395174 1a20466 trim_defconfig/vmlinux
> 16281234 10984848 504291 27770373 1a7be05 gc_defconfig/vmlinux
> 16029705 10556880 504355 27090940 19d5ffc gc+trim_defconfig/vmlinux
> 17040142 11102945 504196 28647283 1b51f73 thinlto_defconfig/vmlinux
> 16788613 10663201 504196 27956010 1aa932a thinlto+trim_defconfig/vmlinux
> 16347062 11043384 502499 27892945 1a99cd1 gc+thinlto_defconfig/vmlinux
> 15759453 10532792 502395 26794640 198da90 gc+thinlto+trim_defconfig/vmlinux
>

Thanks for the numbers.
Does CONFIG_TRIM_UNUSED_KSYMS=y have an impact to the build-time (and
disc-usage - negative way means longer/bigger)?
Do you have any build-time for the above numbers?

BTW, is CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y setable for x86 (64bit)?
( Did not look or check for it. )

- Sedat -

> I needed a small change to the linker script to get clean randconfig
> builds, but I have not done any meaningful boot testing on it to
> see if it works. If there are no regressions, I wonder whether this
> should be autmatically done for LTO builds, given that it improves
> both kernel size and compile speed.
>
> Link: https://lore.kernel.org/lkml/CAK8P3a05VZ9hSKRzVTxTn+1nf9E+gqebJWTj6N23nfm+ELHt9A@mail.gmail.com/
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  arch/arm64/Kconfig              | 1 +
>  arch/arm64/kernel/vmlinux.lds.S | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b94a678afce4..75e13cc52928 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2,6 +2,7 @@
>  config ARM64
>         def_bool y
>         select ACPI_CCA_REQUIRED if ACPI
> +       select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>         select ACPI_GENERIC_GSI if ACPI
>         select ACPI_GTDT if ACPI
>         select ACPI_IORT if ACPI
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index bad2b9eaab22..926cdb597a45 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -217,7 +217,7 @@ SECTIONS
>                 INIT_CALLS
>                 CON_INITCALL
>                 INIT_RAM_FS
> -               *(.init.altinstructions .init.bss .init.bss.*)  /* from the EFI stub */
> +               *(.init.altinstructions .init.data.* .init.bss .init.bss.*)     /* from the EFI stub */
>         }
>         .exit.data : {
>                 EXIT_DATA
> --
> 2.29.2
>
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