It's a cool technique, but only useful in very limited scenarios. In particular, it is unsuitable when the number of reallocations is unbounded, because the gen

It's a cool technique, but only useful in very limited scenarios. In particular, it is unsuitable when the number of reallocations is unbounded, because the generation counter can easily overflow. Even if you can squeeze in 32 tag bits (you would need to leverage the virtual memory system in some platform-specific way), counting to 4-billion-and-change takes no time on modern CPUs, so you would realistically overflow the tag in milliseconds. But if your system is aware of virtual memory, you can still do interesting things, like remapping a whole arena with a different "tag" and use the information in GC write barriers.