Comment by dougall

The tag bit can be inverted.

V8 uses 1 for pointers, 0 for integers - you're usually loading from constant offsets from pointers anyway, so that mostly folds away nicely. Then:

x + y is translated to CPU instructions x + y

x * y is translated to CPU instructions (x >> 1) * y

x / y is translated to CPU instructions (((x >> 1) / (y >> 1)) << 1)

x lsl y is translated to CPU instructions (x << (y >> 1))