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

Uros Bizjak
2025-02-24T15:24:19Z

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@gmail.com>

Thanks,
Uros.

>
> Co-developed-by: Yu-Chun Lin <eleanor15x@gmail.com>
> Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.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;