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

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Message-ID: <B5E017BC-8BE4-482A-B1A2-3F3074968B56@zytor.com>
Date: Mon, 24 Feb 2025 14:18:17 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>
CC: Kuan-Wei Chiu <visitorckw@...il.com>, tglx@...utronix.de,
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        Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH 03/17] x86: Replace open-coded parity calculation with parity8()

On February 24, 2025 2:08:05 PM PST, Uros Bizjak <ubizjak@...il.com> wrote:
>On Mon, Feb 24, 2025 at 10:56 PM H. Peter Anvin <hpa@...or.com> wrote:
>>
>> On 2/24/25 07:24, Uros Bizjak wrote:
>> >
>> >
>> > 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.
>> >
>>
>> Of course, on x86, parity8() and parity16() can be implemented very simply:
>>
>> (Also, the parity functions really ought to return bool, and be flagged
>> __attribute_const__.)
>>
>> static inline __attribute_const__ bool _arch_parity8(u8 val)
>> {
>>         bool parity;
>>         asm("and %0,%0" : "=@...p" (parity) : "q" (val));
>
>asm("test %0,%0" : "=@...p" (parity) : "q" (val));
>
>because we are interested only in flags.
>
>Uros.
>

Same thing, really, but yes, using test is cleaner.