Hey, you may be interested to know that in the upcoming ARM v8's AArch64 mode, the upper 8 bits of all pointers can be used freely for tagged pointers.
See page 11 of: http://board.flatassembler.net/download.php?id=5698
See page 11 of: http://board.flatassembler.net/download.php?id=5698
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.
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.
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.