It is an LVB (a Loaded [reference] Value Barrier), as documented and fully descibed in https://www.azul.com/files/c4_paper_acm.pdf. The Mark/Compact GC algorithm used by ZGC (see section 2 in the paper) and the LVB barrier that powers it in the ZGC implementation are identical to the ones C4 uses in each of its generations, including the details of barrier invariants and guanatees, self healing, use of (non-addressing) metadata information in references, and even the collector phase behavior down to the mark/remap overlap are all the same. As expalined in the paper, the self-healing nature of the LVB (first described and implemented in https://www.usenix.org/legacy/events/vee05/full_papers/p46-c...) makes its actual dynamic cost several orders of magnitude smaller (e.g. during lengthy concurrent GC cycles) than that of prior read barrier approaches (which tend to have a "hot slow path" during long periods of GC cycle execution), making LVB quite practical for use in high throughoput collector implementations.

BTW, it's not a read barrier but a load barrier. It's not encountered every time when reading an object field, but only when reading object reference that's stored on the heap (i.e. loading it into a register).

Isn’t that the traditional definition of a read barrier? E.g. a Baker read barrier is triggered when a field of object reference type is loaded from a heap object. It checks if the loaded pointer is in from space, and if so forwards the object.

it would be interesting to have a database of small assumptions (Do(S)A) and watch how they evolve over time. Seems like a common theme in innovation is to spot the assumption inversion and make the obvious move.