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An algorithm for incrementally updating LoopInfo within a LoopPassManager. The incremental update should be extremely cheap in most cases and can be used in places where it's not feasible to regenerate the entire loop forest. - "Unloop" is a node in the loop tree whose last backedge has been removed. - Perform reverse dataflow on the block inside Unloop to propagate the nearest loop from the block's successors. - For reducible CFG, each block in unloop is visited exactly once. This is because unloop no longer has a backedge and blocks within subloops don't change parents. - Immediate subloops are summarized by the nearest loop reachable from their exits or exits within nested subloops. - At completion the unloop blocks each have a new parent loop, and each immediate subloop has a new parent. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137276 91177308-0d34-0410-b5e6-96231b3b80d8
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