OK to do bit-packing with GC pointers?

[Steven Schveighoffer]

On 7/22/22 12:50 PM, Ben Jones wrote:
> I'm looking to store a pointer to one of 2 unrelated (no inheritance 
> relationship) classes and use the LSb to track which type I have.  Is 
> this going to cause any problems with the GC? For one of the classes 
> I'll have a "pointer" to 1 byte past the start of the object.  It seems 
> like std.bitmanip.taggedClassRep does something similar, so I assume 
> it's OK, but wanted to double check.

It's specifically undefined behavior by the spec, but in practice, I 
think it will work, as long as the block you have isn't marked as not 
allowing interior pointers.

See: https://dlang.org/spec/garbage.html#pointers_and_gc

Specifically "Do not take advantage of alignment of pointers to store 
bit flags in the low order bits"

-Steve