A partial archive of discourse.wicg.io as of Saturday February 24, 2024.

SIMD: should they be returned from / passed to FFI?


Hi everyone,

In exploratory work for SIMD support in Odin, we were wondering if FFI should accept SIMD arguments and return SIMD values.

Returning SIMD values from FFI is quite easy: it will only mean that we need to take an interpreter representation of a SIMD value and spill components into a register.

Passing SIMD value arguments to FFI is more tricky: we need to re-materialize interpreter’s SIMD values, and that means re-constructing objects, which involves garbage collection and has a cost for each SIMD argument.

For Float32, we finally decided to avoid having Float FFI arguments and return values, to keep things simpler. Any opinions about that?


I could go either way. On the one hand, having SIMD types be <: extern is sortof a performance footgun since it entails a GC allocation when the whole point of SIMD is to stay in registers. If you need to pass out a SIMD vector, you’d be better off passing the components by hand. On the other hand, this could be optimized in the future (with significant work on both the Odin/Ion side). However, asm.js is all about standardizing the “fast path” so maybe we should wait until there is a use case.