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

H. Peter Anvin
2025-02-24T21:56:54Z

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" : "=@ccnp" (parity) : "q" (val));
return parity;
}

static inline __attribute_const__ bool _arch_parity16(u16 val)
{
bool parity;
asm("xor %h0,%b0" : "=@ccnp" (parity), "+Q" (val));
return parity;
}

In the generic algorithm, you probably should implement parity16() in
terms of parity8(), parity32() in terms of parity16() and so on:

static inline __attribute_const__ bool parity16(u16 val)
{
#ifdef ARCH_HAS_PARITY16
if (!__builtin_const_p(val))
return _arch_parity16(val);
#endif
return parity8(val ^ (val >> 8));
}

This picks up the architectural versions when available.

Furthermore, if a popcnt instruction is known to exist, then the parity
is simply popcnt(x) & 1.

-hpa