def _iterated_domfronts(cfg):
    """Compute the iterated dominance frontiers (DF+ in literatures).

    Returns a dictionary which maps block label to the set of labels of its
    iterated dominance frontiers.
    """
    domfronts = {k: set(vs) for k, vs in cfg.dominance_frontier().items()}
    keep_going = True
    while keep_going:
        keep_going = False
        for k, vs in domfronts.items():
            inner = reduce(operator.or_, [domfronts[v] for v in vs], set())
            if inner.difference(vs):
                vs |= inner
                keep_going = True
    return domfronts
Compute the iterated dominance frontiers (DF+ in literatures).

    Returns a dictionary which maps block label to the set of labels of its
    iterated dominance frontiers.
    
numba/core/ssa.py