On Monday, 18 August 2025 at 15:26:21 UTC, monkyyy wrote:
On Monday, 18 August 2025 at 07:32:13 UTC, Steven Schveighoffer
wrote:
2. You can store data in the lower bits of an aligned pointer
(after all, this is just an interior pointer).
Will the gc count this as a reference while scaning in any case?
As long as it's stored as a pointer, it is treated as a pointer.
I'll try my hand at ascii art.
Let's say you have an integer 0x4321 allocated in memory at
address 0x100:
```
-------------------------
Address: | 100 | 101 | 102 | 103 |
-------------------------
Data: | 1 | 2 | 3 | 4 |
-------------------------
```
A pointer to the integer points at address 0x100
```
-------------------------
Address: | 100 | 101 | 102 | 103 |
-------------------------
Data: | 1 | 2 | 3 | 4 |
-------------------------
^ ptr
```
Now, you add a 1 to the pointer value to have it pointing at
0x101, it looks like this:
```
-------------------------
Address: | 100 | 101 | 102 | 103 |
-------------------------
Data: | 1 | 2 | 3 | 4 |
-------------------------
^ ptr
```
This is still pointing at the integer. So the GC believes the
integer memory block is still valid because there's a pointer
interior to the block
Add 2, and you get a pointer to the `3` byte, add 3 and you get a
pointer to the `4` byte. Add 4, and now it's pointing at the next
integer, and so now it's no longer pointing at the integer, and
this becomes invalid.
So you have 2 bits of space you can use to store a value from 0
to 3, and still have it be a valid pointer.
-Steve
