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> For example, currently on x86-64, only the lower 48 bits of a pointer are actually used. The other 16 bits could carefully be used for other information

Well, you can't dereference these fat pointers directly on amd64, you have to remove the tag.

aarch64 actually has a Top Byte Ignore flag though :)

I'd also expect that bit-fiddling on pointers, and then de referencing them, is undefined behavior.

In C, yes, but not in amd64 or i386 (or most instruction sets.)

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