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It's more complicated than that -- x86-64 systems only use 48 bits of addressable space. JS engines use all the remaining bits for tagging, not just bits resulting from 8- or 16-byte alignment. This works great until you want to run on another CPU architecture that actually has 64-bits of addressable space and uses it regularly...

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I'm surprised they need so many bits.. you really just need to know if the item is an object (so the type can be embedded in the object), or a primitive type. But there are not so many primitive types.

Some dynamically typed systems put the tag at the beginning of a page: so all items of a particular type are allocated out of a page tagged with that type. You don't waste any bits from the pointer or data, but you do have to do some masking and worse, a remote memory access to get the type.

Probably safest and fastest to do something like: struct { int type; union { char *s; int n; double d; }};

Many JavaScript engines use IEEE-754 double-precision floats as their immediate value type, and fit pointers and integer within the NaN payload of the double representation. See https://wingolog.org/archives/2011/05/18/value-representatio... for a discussion of a few similar approaches and the tradeoffs of each.

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