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

H. Peter Anvin
2025-02-24T22:21:41Z

On February 24, 2025 2:08:05 PM PST, Uros Bizjak <ubizjak@gmail.com> wrote:
>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.
>

Same thing, really, but yes, using test is cleaner.