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Tree-structured soft-in/soft-out (SISO) processors provide an exponential speed-up relative to the standard forward-backward algorithm (FBA). These tree-SISOs were originally described analogously to fast tree-structured adders and later as standard message-passing on a binary tree graphical model for a finite state machine (FSM). In this paper, we summarize and unify these theoretical results and also summarize recent efforts to implement high-speed iterative decoders based on tree-SISOs. Specifically, we design a tree-SISO based on a traditional synchronous design flow and another based on our asynchronous design flow. The asynchronous design offers significant advantages in terms of throughput/area of the resulting high-speed iterative decoder at the cost of some additional energy consumption.