Is there a portable way to tag a 1 byte aligned pointer?

Asked Mar 27 '22 at 04:46

Active Mar 28 '22 at 03:42

Viewed 228 times

I want to create a union of 2 pointers which have 1 byte alignment, but I don't want to store an additional tag, to distinguish them. I know that on some platforms, at least the most significant bit of a pointer is unused, but that might change in the future.

Please does someone know a portable way to do this?

I'm hoping for a technique that does not invalidate the original pointer. That is, is there a function for which when I apply it to some number n, frees up at least one bit in the number and when I "unapply" it, gives me back the original number.

edited Mar 28 '22 at 03:42

user207421

305,947
44
307
483

asked Mar 27 '22 at 04:46

Samuel Okechukwu

In what language? – user207421 Mar 27 '22 at 05:24
2

see [Using the extra 16 bits in 64-bit pointers](https://stackoverflow.com/q/16198700/995714) – phuclv Mar 27 '22 at 06:12
For 32 bit pointers is there a way to do this? Also the feature that only 48 bits of a pointer are used is that a stable feature or might it change in the future. – Samuel Okechukwu Mar 27 '22 at 09:08
The language is Rust. I want to store a union of an Rc and &'static T. – Samuel Okechukwu Mar 27 '22 at 09:11
no, unfortunately in 32-bit it only works if the kernel/user split is 2GB/2GB. But why do you care about 32-bit platforms anyway? They're completely outdated nowadays – phuclv Mar 27 '22 at 14:49
What is your use case? – PitaJ Mar 28 '22 at 15:38
@phuclv They're still around in embedded. Most ARM microcontrollers are 32 bit. – Colonel Thirty Two Mar 30 '22 at 14:27
1

Pointer tagging is heavily dependent on the exact environment you are in, because the entire point is (ab)using the part of the pointer that just happen to be unused. You need to give more information about your environment, and accept that the solution is not going to be portable. – Colonel Thirty Two Mar 30 '22 at 14:31

Is there a portable way to tag a 1 byte aligned pointer?

0 Answers0