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

Uros Bizjak
2025-02-24T22:08:19Z

On Mon, Feb 24, 2025 at 10:56 PM H. Peter Anvin <hpa@zytor.com> 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.

Uros.