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retiming
Retiming is the technique of moving the structural location of latches or registers in a digital circuit to improve its performance, area, and/or power characteristics in such a way that preserves its functional behavior at its outputs. Retiming was first described by Charles E. Leiserson and James B. Saxe in 1983.〔http://citeseer.ist.psu.edu/context/96547/0〕
The technique uses a directed graph where the vertices represent asynchronous combinational blocks and the directed edges represent a series of registers or latches (the number of registers or latches can be zero). Each vertex has a value corresponding to the delay through the combinational circuit it represents. After doing this, one can attempt to optimize the circuit by pushing registers from output to input and vice versa - much like bubble pushing. Two operations can be used - deleting a register from each input of a vertex while adding a register to all outputs, and conversely adding a register to each input of vertex and deleting a register from all outputs. In all cases, if the rules are followed, the circuit will have the same functional behavior as it did before retiming.
==Formal description==
The initial formulation of the retiming problem as described by Leiserson and Saxe is as follows. Given a directed graph whose vertices represent logic gates or combinational delay elements in a circuit, assume there is a directed edge between two elements that are connected directly or through one or more registers. Let the ''weight'' of each edge be the number of registers present along edge in the initial circuit. Let be the propagation delay through vertex . The goal in retiming is to compute an integer ''lag'' value for each vertex such that the retimed weight of every edge is non-negative. There is a proof that this preserves the output functionality.〔C. E. Leiserson, J. B. Saxe, "Retiming Synchronous Circuitry," Algorithmica, Vol. 6, No. 1, pp. 5-35, 1991.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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