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

[H. Peter Anvin]

On 2/24/25 14:08, Uros Bizjak wrote:
> On Mon, Feb 24, 2025 at 10:56 PM H. Peter Anvin <hpa@xxxxxxxxx> 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" : "=@ccnp" (parity) : "q" (val));
>
> asm("test %0,%0" : "=@ccnp" (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