linux-kernel - Re: [PATCH 03/17] x86: Replace open-coded parity calculation with parity8()

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Message-ID: <d080a2d6-9ec7-1c86-4cf4-536400221f68@gmail.com>
Date: Mon, 24 Feb 2025 16:24:14 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>, tglx@...utronix.de,
 Ingo Molnar <mingo@...hat.com>, bp@...en8.de, dave.hansen@...ux.intel.com,
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 Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH 03/17] x86: Replace open-coded parity calculation with
 parity8()



On 23. 02. 25 17:42, Kuan-Wei Chiu wrote:
> Refactor parity calculations to use the standard parity8() helper. This
> change eliminates redundant implementations and improves code
> efficiency.

The patch improves parity assembly code in bootflag.o from:

   58:	89 de                	mov    %ebx,%esi
   5a:	b9 08 00 00 00       	mov    $0x8,%ecx
   5f:	31 d2                	xor    %edx,%edx
   61:	89 f0                	mov    %esi,%eax
   63:	89 d7                	mov    %edx,%edi
   65:	40 d0 ee             	shr    %sil
   68:	83 e0 01             	and    $0x1,%eax
   6b:	31 c2                	xor    %eax,%edx
   6d:	83 e9 01             	sub    $0x1,%ecx
   70:	75 ef                	jne    61 <sbf_init+0x51>
   72:	39 c7                	cmp    %eax,%edi
   74:	74 7f                	je     f5 <sbf_init+0xe5>
   76:

to:

   54:	89 d8                	mov    %ebx,%eax
   56:	ba 96 69 00 00       	mov    $0x6996,%edx
   5b:	c0 e8 04             	shr    $0x4,%al
   5e:	31 d8                	xor    %ebx,%eax
   60:	83 e0 0f             	and    $0xf,%eax
   63:	0f a3 c2             	bt     %eax,%edx
   66:	73 64                	jae    cc <sbf_init+0xbc>
   68:

which is faster and smaller (-10 bytes) code.

Reviewed-by: Uros Bizjak <ubizjak@...il.com>

Thanks,
Uros.

> 
> 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>
> ---
>   arch/x86/kernel/bootflag.c | 18 +++---------------
>   1 file changed, 3 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kernel/bootflag.c b/arch/x86/kernel/bootflag.c
> index 3fed7ae58b60..314ff0e84900 100644
> --- a/arch/x86/kernel/bootflag.c
> +++ b/arch/x86/kernel/bootflag.c
> @@ -8,6 +8,7 @@
>   #include <linux/string.h>
>   #include <linux/spinlock.h>
>   #include <linux/acpi.h>
> +#include <linux/bitops.h>
>   #include <asm/io.h>
>   
>   #include <linux/mc146818rtc.h>
> @@ -20,26 +21,13 @@
>   
>   int sbf_port __initdata = -1;	/* set via acpi_boot_init() */
>   
> -static int __init parity(u8 v)
> -{
> -	int x = 0;
> -	int i;
> -
> -	for (i = 0; i < 8; i++) {
> -		x ^= (v & 1);
> -		v >>= 1;
> -	}
> -
> -	return x;
> -}
> -
>   static void __init sbf_write(u8 v)
>   {
>   	unsigned long flags;
>   
>   	if (sbf_port != -1) {
>   		v &= ~SBF_PARITY;
> -		if (!parity(v))
> +		if (!parity8(v))
>   			v |= SBF_PARITY;
>   
>   		printk(KERN_INFO "Simple Boot Flag at 0x%x set to 0x%x\n",
> @@ -70,7 +58,7 @@ static int __init sbf_value_valid(u8 v)
>   {
>   	if (v & SBF_RESERVED)		/* Reserved bits */
>   		return 0;
> -	if (!parity(v))
> +	if (!parity8(v))
>   		return 0;
>   
>   	return 1;