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

Yury Norov
2025-02-24T22:17:33Z

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.