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Date:   Mon, 7 Dec 2020 11:25:30 +0000
From:   Will Deacon <will@kernel.org>
To:     Yury Norov <yury.norov@gmail.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org
Subject: Re: [PATCH] arm64: enable  GENERIC_FIND_FIRST_BIT
Message-ID: <20201207112530.GB4379@willie-the-truck>
References: <20201205165406.108990-1-yury.norov@gmail.com>
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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