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

On February 24, 2025 2:17:29 PM PST, Yury Norov [off-list ref] wrote:

On Mon, Feb 24, 2025 at 01:55:28PM -0800, H. Peter Anvin wrote:

quoted

On 2/24/25 07:24, Uros Bizjak wrote:

quoted

On 23. 02. 25 17:42, Kuan-Wei Chiu wrote:

quoted

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__.)

There was a discussion regarding return type when parity8() was added.
The integer type was taken over bool with a sort of consideration that
bool should be returned as an answer to some question, like parity_odd().

To me it's not a big deal. We can switch to boolean and describe in
comment what the 'true' means for the parity() function.

Bool is really the single-bit type, and gives the compiler more information. You could argue that the function really should be called parity_odd*() in general, but that's kind of excessive IMO.