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

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Message-ID: <d0067e6a-a6af-4b38-ac72-f655af4d3b3d@zytor.com>
Date: Mon, 24 Feb 2025 19:36:25 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Kuan-Wei Chiu <visitorckw@...il.com>, tglx@...utronix.de,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.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 2/24/25 14:08, Uros Bizjak 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.
> 

Also, needs to be %1,%1 (my mistake, thought flags outputs didn't count.)

Finally, this is kind of an obvious improvement:

  static void __init sbf_write(u8 v)
  {
         unsigned long flags;

         if (sbf_port != -1) {
-               v &= ~SBF_PARITY;
                 if (!parity(v))
-                       v |= SBF_PARITY;
+                       v ^= SBF_PARITY;

	-hpa