Re: [PATCH] arm64: enable GENERIC_FIND_FIRST_BIT
From: Will Deacon
Date: Wed Feb 24 2021 - 04:35:39 EST
On Tue, Feb 23, 2021 at 09:27:44PM -0800, Yury Norov wrote:
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On Tue, Dec 08, 2020 at 10:35:50AM +0000, Will Deacon wrote:
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> On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote:
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> > (CC: Alexey Klimov)
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> >
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> > On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <will@xxxxxxxxxx> wrote:
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> > >
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> > > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
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> > > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't
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> > > > enable it in config. It leads to using find_next_bit() which is less
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> > > > efficient:
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> > >
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> > > [...]
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> > >
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> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
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> > > > index 1515f6f153a0..2b90ef1f548e 100644
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> > > > --- a/arch/arm64/Kconfig
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> > > > +++ b/arch/arm64/Kconfig
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> > > > @@ -106,6 +106,7 @@ config ARM64
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> > > > select GENERIC_CPU_AUTOPROBE
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> > > > select GENERIC_CPU_VULNERABILITIES
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> > > > select GENERIC_EARLY_IOREMAP
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> > > > + select GENERIC_FIND_FIRST_BIT
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> > >
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> > > Does this actually make any measurable difference? The disassembly with
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> > > or without this is _very_ similar for me (clang 11).
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> > >
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> > > Will
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> >
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> > On A-53 find_first_bit() is almost twice faster than find_next_bit(),
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> > according to
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> > lib/find_bit_benchmark. (Thanks to Alexey for testing.)
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>
>
> I guess it's more compiler dependent than anything else, and it's a pity
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> that find_next_bit() isn't implemented in terms of the generic
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> find_first_bit() tbh, but if the numbers are as you suggest then I don't
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> have a problem selecting this on arm64.
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>
Ping?
Not sure what happened to this. Maybe resend at -rc1?
Will