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This particular engine, V8, doesn't use NaN-boxing, because of the memory costs on 32-bit systems of using 64-bit values for boxed pointers. Instead, we use pointer tagging, relying that with word alignment, we have 2 bits free at the bottom of pointers.

If the lsb is unset, then the remaining 31 bits represent a "small integer", a.k.a. a Smi (on 64-bit, the top 32 bits make the Smi, so that they can be read out of memory with a normal 32-bit access).

If the lsb is set, then the remaining bits are the pointer, where the bottom bits have to be masked off to actually dereference it (practically, most accesses are offset-based field accesses anyway, so we just subtract 1 from the field offset). The other bit, the 2nd lsb, has recently started being used as a weak pointer marker.

So, the type tag isn't stored in bits of the pointer, but rather in the object being pointed to, not dissimilar to a vtable. This has the side-advantage that we can swap the type of an object in-place, without having to update pointers to it. BigInts are immutable, so they are stored as a heap object which holds the object tag (actually another pointer to a "map", that's a story for another time), a bitfield that stores sign and length, and then data inline (very similar to how we store strings).

(pointer|0x1) -> [BigIntMap, bitfield (length+sign), digit0, digit1, ...]




Thanks for the explanation! If it functions similarly to strings, which are also immutable, when you do any type of operation it causes a reallocation?


I believe so, in this baseline implementation. In fact, that's also true of operations on doubles (non-Smi) in unoptimized code, which are stored as "heap numbers". Avoiding these allocations is one of the things an optimizing compiler can do (and does do, for doubles).

That said, for short-lived BigInts, the allocations may not be as expensive as you might think, since the garbage collector is generational and short-lived objects won't leave the nursery.




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