Re: [PATCH v3 00/16] Introduce and use generic parity16/32/64 helper

Kuan-Wei Chiu
2025-04-03T14:46:14Z

On Tue, Mar 25, 2025 at 12:43:25PM -0700, H. Peter Anvin wrote:
> On 3/23/25 08:16, Kuan-Wei Chiu wrote:
> >
> > Interface 3: Multiple Functions
> > Description: bool parity_odd8/16/32/64()
> > Pros: No need for explicit casting; easy to integrate
> > architecture-specific optimizations; except for parity8(), all
> > functions are one-liners with no significant code duplication
> > Cons: More functions may increase maintenance burden
> > Opinions: Only I support this approach
> >
>
> OK, so I responded to this but I can't find my reply or any of the
> followups, so let me go again:
>
> I prefer this option, because:
>
> a. Virtually all uses of parity is done in contexts where the sizes of the
> items for which parity is to be taken are well-defined, but it is *really*
> easy for integer promotion to cause a value to be extended to 32 bits
> unnecessarily (sign or zero extend, although for parity it doesn't make any
> difference -- if the compiler realizes it.)
>
> b. It makes it easier to add arch-specific implementations, notably using
> __builtin_parity on architectures where that is known to generate good code.
>
> c. For architectures where only *some* parity implementations are
> fast/practical, the generic fallbacks will either naturally synthesize them
> from components via shift-xor, or they can be defined to use a larger
> version; the function prototype acts like a cast.
>
> d. If there is a reason in the future to add a generic version, it is really
> easy to do using the size-specific functions as components; this is
> something we do literally all over the place, using a pattern so common that
> it, itself, probably should be macroized:
>
> #define parity(x) \
> ({ \
> typeof(x) __x = (x); \
> bool __y; \
> switch (sizeof(__x)) { \
> case 1: \
> __y = parity8(__x); \
> break; \
> case 2: \
> __y = parity16(__x); \
> break; \
> case 4: \
> __y = parity32(__x); \
> break; \
> case 8: \
> __y = parity64(__x); \
> break; \
> default: \
> BUILD_BUG(); \
> break; \
> } \
> __y; \
> })
>
Thank you for your detailed response and for explaining the rationale
behind your preference. The points you outlined in (a)–(d) all seem
quite reasonable to me.

Yury,
do you have any feedback on this?
Thank you.

Regards,
Kuan-Wei