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My favorite trick: NaN boxing. NaN's aren't just for errors, but also for smuggling other data inside. For a double, you have whopping 53 bits of payload, enough to cram in a pointer and maybe a type tag, and many javascript engines do (since JS numbers are double's after all)

https://wingolog.org/archives/2011/05/18/value-representatio...

https://piotrduperas.com/posts/nan-boxing

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It is also how RISC-V floating point registers are required to store floats of smaller widths. Eg if your CPU supports 64-bit floats (D extension), its FPU registers will be 64-bit wide. If you use an instruction to load a 16-bit float (Zfh extension) into such a register, it will be boxed into a negative quiet NaN with all bits above the lower 16 bits set to 1.

52 bits of payload, and at least one bit must be set.

You can put stuff into the sign bit too, that makes 53. Yeah, the lower 52 bits can't all be zero - that'd be ±INF, but the other 2^53-2 values are all yours to use.

It's possible for the sign bit of a NaN to be changed by a "non-arithmetic" operation that doesn't trap on the NaN, so don't put anything precious in there.

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