C++ SIMD operator== debate

Here is the brief summarized debate of whether SIMD vectors should have operator== return a SIMD vector or a boolean

operator== Should Returns a SIMD Vector Camp

• That’s what state-of-arts C++ libraries do
• It’s what people want for the most of times
• If returning a bool, it is not clear what the semantics is (all? at least one?)
• Boolean returning simd vector operator== can be misuse
  • e.g. std::simd x, y, z; auto result = (x == y) * z; /* spreading here */
• Generic code that works both on scalars and vectors need operator== returning vector

operator== Should Returns a Bool Camp

• regular types’ operator== return bool
  • Returning a vector can break generic code which expect operator== to return bool
• Rust’s std::simd does that
  • Lesley: this is related to how Rust’s equality is implemented (Eq trait), so in C++ the situation is different

Third Option: operator== Should not Exist

That’s my take. And it seems to solve a large numbers of concerns above

Cons

• Can’t easily take advantage of generic code (both the above two options do it somewhat)
• using explicit functions looks quite ugly for basic math stuff

Fourth Option: Implicitly Convert a simd<bool> to bool

IMO just horrible

References

• open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2892r0.pdf
• open-std.org/JTC1/SC22/WG21/docs/papers/2023/p2926r0.pdf
• The story of regularity and std::simd | Make it fast and readable!