I really appreciate the elegance of pointer tagging.

[ammarfaizi2@social.gnuweeb.org]

RE: https://mstdn.social/@hkrn/116216511686844081

I really appreciate the elegance of pointer tagging. A standard 64-bit pointer isn't actually fully utilized for addressing. On most systems, virtual addresses are only 48 bits wide, leaving the upper 16 bits theoretically free to embed extra user data

[ammarfaizi2@social.gnuweeb.org]

BTW, using those bits to store extra user data can drastically simplify object identification.

I often use it when dealing with async I/O user data in epoll or io_uring.

How I typically macro this out to pack and unpack event bits:
```
#define EV_BIT_ALL (0xFFFFull << 48)
#define GET_EV_BIT(X) ((X) & EV_BIT_ALL)
#define CLEAR_EV_BIT(X) ((X) & ~EV_BIT_ALL)
#define SET_EV_BIT(X, EV) ((X) | ((uint64_t)(EV) << 48))
```

[ammarfaizi2@social.gnuweeb.org]

I doubt that it will be forward compatible, though.

[viro_ssfs@social.gnuweeb.org]

@ammarfaizi2

Yeah, since Ice Lake (2019), Intel supports 5-level paging, it extends the virtual memory from 48 bits to 57 bits.

Intel 5-level paging - Wikipedia

(en.wikipedia.org)

At least you can still use the remaining unused 7 bits wkwkwk