Re: [PATCH] arm64: enable GENERIC_FIND_FIRST_BIT
From: Yury Norov
Date: Wed Feb 24 2021 - 00:28:30 EST
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.
Ping?