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

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Message-ID: <Z7zv-c4A76jeMAKf@thinkpad>
Date: Mon, 24 Feb 2025 17:17:29 -0500
From: Yury Norov <yury.norov@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Uros Bizjak <ubizjak@...il.com>, 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 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.