Alas, compilers have historically fudged the behavior on some 32-bit targets, e.g., x86 targets without SSE2 [0], so you'd have to go all the way to soft-float implementations if you really want guarantees. But these targets are rare, and very unlikely to be the cause of the original issue.

(And NaN payloads in general are a mess across platforms and ABIs, but that can be solved by treating all NaNs as identical.)

[0] https://github.com/rust-lang/rust/issues/114479