Re: [PATCH] arm64: enable GENERIC_FIND_FIRST_BIT

Will Deacon
2020-12-07T11:26:32Z

On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
> ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't
> enable it in config. It leads to using find_next_bit() which is less
> efficient:

[...]

> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 1515f6f153a0..2b90ef1f548e 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -106,6 +106,7 @@ config ARM64
> select GENERIC_CPU_AUTOPROBE
> select GENERIC_CPU_VULNERABILITIES
> select GENERIC_EARLY_IOREMAP
> + select GENERIC_FIND_FIRST_BIT

Does this actually make any measurable difference? The disassembly with
or without this is _very_ similar for me (clang 11).

Will