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

H. Peter Anvin
2025-02-25T03:37:48Z

On 2/24/25 14:08, Uros Bizjak 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.
>

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