When it's logged...hahaha... sorry, couldn't resist.
I was asked this question "When is a task graph not a tree in TBB?" I couldn't think of a good answer. Does anyone have an example of a task graph that isn't a tree?
I suppose technically it's a forest, because there can be multiple root tasks. If you want to simulate a directed acyclic graph of dependencies (examples aplenty), you have to find a supporting tree (would that always be a subgraph?) that is represented directly in TBB (which doesn't do dags), and fill in the missing dependencies by adding empty_task children of the dependent tasks to be explicitly spawned by their dependencies on completion (or use release_ref() as implemented in the "Pipeline" thread).