linux-kernel - Re: [PATCH 02/17] bitops: Add generic parity calculation for u64

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Message-ID: <bde62fee-4617-4db7-b92c-59fb958c4ca6@kernel.org>
Date: Mon, 24 Feb 2025 08:09:43 +0100
From: Jiri Slaby <jirislaby@...nel.org>
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 Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH 02/17] bitops: Add generic parity calculation for u64

On 23. 02. 25, 17:42, Kuan-Wei Chiu wrote:
> Several parts of the kernel open-code parity calculations using
> different methods. Add a generic parity64() helper implemented with the
> same efficient approach as parity8().
> 
> Co-developed-by: Yu-Chun Lin <eleanor15x@...il.com>
> Signed-off-by: Yu-Chun Lin <eleanor15x@...il.com>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
>   include/linux/bitops.h | 22 ++++++++++++++++++++++
>   1 file changed, 22 insertions(+)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index fb13dedad7aa..67677057f5e2 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -281,6 +281,28 @@ static inline int parity32(u32 val)
>   	return (0x6996 >> (val & 0xf)) & 1;
>   }
>   
> +/**
> + * parity64 - get the parity of an u64 value
> + * @value: the value to be examined
> + *
> + * Determine the parity of the u64 argument.
> + *
> + * Returns:
> + * 0 for even parity, 1 for odd parity
> + */
> +static inline int parity64(u64 val)
> +{
> +	/*
> +	 * One explanation of this algorithm:
> +	 * https://funloop.org/codex/problem/parity/README.html
> +	 */
> +	val ^= val >> 32;

Do we need all these implementations? Can't we simply use parity64() for 
any 8, 16 and 32-bit values too? I.e. have one parity().

> +	val ^= val >> 16;
> +	val ^= val >> 8;
> +	val ^= val >> 4;
> +	return (0x6996 >> (val & 0xf)) & 1;
> +}
> +
>   /**
>    * __ffs64 - find first set bit in a 64 bit word
>    * @word: The 64 bit word


-- 
js
suse labs