Branch divergence
Beginner
When workers on the same team need to do different things, they have to take turns, and the team slows down.
Novice
When threads in one warp take different sides of an if/else, the warp runs each side in turn with the other threads switched off, wasting part of the hardware.
Expert
Handled with an active mask and, classically, a reconvergence stack that rejoins paths at the immediate post-dominator. Lane efficiency falls to the fraction of active lanes; Volta-style per-thread PCs allow interleaving but still execute one path per issue.
Explained in SIMT and GPUs (Architectures).
See also: SIMT (single instruction, multiple threads), Warp (wavefront).