[kg]

Yeah, this is a historical design difference between Firefox's Spidermonkey JS engine and Chrome's V8.

Spidermonkey uses (I'm simplifying here, there are cases where this isn't true) a trick where all values are 64-bits, and for anything that isn't a double-precision float they smuggle it inside of the bits of a NaN. This means that you can store a double, a float32, an int, or an object pointer all in a field of the same size. Great, but creates some problems and complications for asm.js/wasm because you can't rely on all the bits of a NaN surviving a trip through the JS engine.

V8 instead allocates doubles on the heap. I forget the exact historical reason why they do this. IIRC they also do some fancy stuff with integers - if your integer is 31 bits or less it counts as a "smi" in that engine, or small int, and gets special performance treatment. So letting your integers get too big is also a performance trap, not just having double-precision numbers.

EDIT: I found something just now that suggests Smis are now 32-bits instead of 31-bits in 64-bit builds of v8, so that's cool!

[kannanvijayan]

I suspect it's just circumstantial - two different design approaches. Both of the approaches have their advantages and disadvantages.

IMHO the bigger issue with NaN-boxing is that on 64-bit systems it relies on the address space only needing <50 bits or so, as the discriminator is stored on the high bits. It's ok for now when virtual address spaces typically only need 48 bits of representation, but that's already starting to slip with newer systems.

On the other hand, I love the fact that NaN-boxing basically lets you eliminate all heap allocations for doubles.

I actually wrote a small article a while back on a hybrid approach called Ex-boxing (exponent boxing), which tries to get at the best of both worlds: decouple the boxing representation from virtual address significant bits, and also represent most (almost all) doubles that show up at runtime as immediates.

https://medium.com/@kannanvijayan/exboxing-bridging-the-divi...

[addaon]

> IMHO the bigger issue with NaN-boxing is that on 64-bit systems it relies on the address space only needing <50 bits or so, as the discriminator is stored on the high bits.

Is this right? You get 51 tag bits, of which you must use one to distinguish pointer-to-object from other uses of the tag bits (assuming Huffman-ish coding of tags). But objects are presumedly a minimum of 8-byte sized and aligned, and on most platforms I assume they'd be 16-byte sized and aligned, which means the low three (four) bits of the address are implicit, giving 53 (54) bit object addresses. This is quite a few years of runway...

[kannanvijayan]

There's a bit of time yes, but for an engine that relies on this format (e.g. spidermonkey), the assumptions associated with the value boxing format would have leaked into the codebase all over the place. It's the kind of thing that's far less painful to take care of when you don't need to do it than when you need to do it.

But fair point on the aligned pointers - that would give you some free bits to keep using, but it gets ugly.

You're right about the 51 bits - I always get mixed up about whether it's 12 bits of exponent, or the 12 includes the sign. Point is it puts some hard constraints on a pretty large number of high bits of a pointer being free, as opposed to an alignment requirement for low-bit tagging which will never run out of bits.

[afiori]

48/56 bits numbers would be useful in many circumstances