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Comment by barrkel | Hacker News Reader

The issue isn't adding extra data to pointers; it's distinguishing a pointer from an integer of the same size. The extra bit needs to be "meta".

Some x64 targeted languages already use tagged pointers without specific hardware support; e.g. I recall reading about JVM using several bits of object pointers in 64-bit to encode a class index, so that polymorphic inline caches can be checked without an indirection.

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Right, any language could do this -- `malloc` and friends return pointers aligned to 16 bytes, giving you 4 bits at the bottom that are totally free.

It can go even further than that; most systems only use 48 bits for addresses -- this leaves more than enough space to "NaN-box" objects by putting the address inside the extraneous bits of a 64 bit floating NaN.

Right, any language could do this -- `malloc` and friends return pointers aligned to 16 bytes

How does the having the bottom 4 bits free in a pointer help if you can't use them to tell the difference between a pointer and an integer 1 or 2? He is asking for a reserved set of bits on every data type so that he can store minimal type information like pointer, not pointer.

To use this, you would have a tag for ints and a tag for pointers; when using the ints you shift right 4 bits, pointers would be masked before use.

Sure. This was just meant as an interesting tidbit. On ARM v8, you'll still need the compiler/VM to help you out as far as type precision goes, of course.

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