Re: [PATCH v4 5/6] AutoFDO: Enable machine function split optimization for AutoFDO

[Masahiro Yamada]

On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@xxxxxxxxxx> wrote:
>
> Enable the machine function split optimization for AutoFDO in Clang.
>
> Machine function split (MFS) is a pass in the Clang compiler that
> splits a function into hot and cold parts. The linker groups all
> cold blocks across functions together. This decreases hot code
> fragmentation and improves iCache and iTLB utilization.
>
> MFS requires a profile so this is enabled only for the AutoFDO builds.
>
> Co-developed-by: Han Shen <shenhan@xxxxxxxxxx>
> Signed-off-by: Han Shen <shenhan@xxxxxxxxxx>
> Signed-off-by: Rong Xu <xur@xxxxxxxxxx>
> Suggested-by: Sriraman Tallam <tmsriram@xxxxxxxxxx>
> Suggested-by: Krzysztof Pszeniczny <kpszeniczny@xxxxxxxxxx>
> ---
>  include/asm-generic/vmlinux.lds.h | 6 ++++++
>  scripts/Makefile.autofdo          | 2 ++
>  2 files changed, 8 insertions(+)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index ace617d1af9b..20e46c0917db 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -565,9 +565,14 @@ defined(CONFIG_AUTOFDO_CLANG)
>                 __unlikely_text_start = .;                              \
>                 *(.text.unlikely .text.unlikely.*)                      \
>                 __unlikely_text_end = .;
> +#define TEXT_SPLIT                                                     \
> +               __split_text_start = .;                                 \
> +               *(.text.split .text.split.[0-9a-zA-Z_]*)                \
> +               __split_text_end = .;
>  #else
>  #define TEXT_HOT *(.text.hot .text.hot.*)
>  #define TEXT_UNLIKELY *(.text.unlikely .text.unlikely.*)
> +#define TEXT_SPLIT
>  #endif


Why conditional?


Where are __unlikely_text_start and __unlikely_text_end used?







-- 
Best Regards
Masahiro Yamada