linux-kernel - Re: [PATCH] arm64: enable GENERIC_FIND_FIRST

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Message-ID: <20201207112530.GB4379@willie-the-truck>
Date:   Mon, 7 Dec 2020 11:25:30 +0000
From:   Will Deacon <will@...nel.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH] arm64: enable  GENERIC_FIND_FIRST_BIT

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