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

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Message-ID: <20210224052744.GA1168363@yury-ThinkPad>
Date:   Tue, 23 Feb 2021 21:27:44 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Alexey Klimov <aklimov@...hat.com>
Subject: Re: [PATCH] arm64: enable GENERIC_FIND_FIRST_BIT

On Tue, Dec 08, 2020 at 10:35:50AM +0000, Will Deacon wrote:
> On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote:
> > (CC: Alexey Klimov)
> > 
> > On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <will@...nel.org> wrote:
> > >
> > > 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
> > 
> > On A-53 find_first_bit() is almost twice faster than find_next_bit(),
> > according to
> > lib/find_bit_benchmark. (Thanks to Alexey for testing.)
> 
> I guess it's more compiler dependent than anything else, and it's a pity
> that find_next_bit() isn't implemented in terms of the generic
> find_first_bit() tbh, but if the numbers are as you suggest then I don't
> have a problem selecting this on arm64.

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