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

Yury Norov
2025-02-24T22:30:11Z

On Mon, Feb 24, 2025 at 02:21:13PM -0800, H. Peter Anvin wrote:
> On February 24, 2025 2:17:29 PM PST, Yury Norov <yury.norov@gmail.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.

Yes, I could, but I will not. :) I also feel like bool looks more
natural here.