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

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Message-ID: <68F1ED19-B0C2-4E78-B504-2F7C040ACC0A@zytor.com>
Date: Mon, 24 Feb 2025 14:21:13 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Yury Norov <yury.norov@...il.com>
CC: Uros Bizjak <ubizjak@...il.com>, Kuan-Wei Chiu <visitorckw@...il.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 February 24, 2025 2:17:29 PM PST, Yury Norov <yury.norov@...il.com> wrote:
>On Mon, Feb 24, 2025 at 01:55:28PM -0800, H. Peter Anvin 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__.)
>
>There was a discussion regarding return type when parity8() was added.
>The integer type was taken over bool with a sort of consideration that
>bool should be returned as an answer to some question, like parity_odd().
>
>To me it's not a big deal. We can switch to boolean and describe in
>comment what the 'true' means for the parity() function.

Bool is really the single-bit type, and gives the compiler more information. You could argue that the function really should be called parity_odd*() in general, but that's kind of excessive IMO.