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authors Stok, Leon
year 1991
title Architectural synthesis and optimization of digital systems
source Eindhoven University of Technology
summary High level synthesis means going from an functional specification of a digits-system at the algorithmic level to a register transfer level structure. Different appli-cations will ask for different design styles. Despite this diversity in design styles many tasks in the synthesis will be similar. There is no need to write a new synthesis system for each design style. The best way to go seems a decomposition of the high level synthesis problems in several well defined subproblems. How the problem is decomposed depends heavily on a) the type of network architecture chosen, b) the constraints applied to the design and c) on the functional description itself. From this architecture style, the constraints and the functional description a synthesis scheme can be derived. Once this scheme is fixed, algorithms can be chosen which fit into this scheme and solve the subproblems in a fast and, when possible, optimal way. To support such a synthesis philosophy, a framework is needed in which all design information can be stored in a unique way during the various phases of the design process. This asks for a design data base capable of handling all design information with a formally defined interface to all design tools. This thesis gives a formal way to describe both the functional representation, the register transfer level structure and the controller and the relations between all three of them. Special attention has been paid to the efficient representation of mutual exclusive operations and array accesses. The scheduling and allocation problems are defined as mappings between these formal representations. Both the existing synthesis algorithms and the new algorithms described in this thesis fit into this framework. Three new allocation algorithms are presented in this thesis: an algorithm for optimal register allocation in cyclic data flow graphs, an exact polynomial algorithm to do the module allocation and a new scheme to minimize the number of interconnections during all stages of the data path allocation. Cyclic data flow graphs result from high level behavioral descriptions that contain loops. Algorithms for register allocation in high level synthesis published up till now, only considered loop free data flow graphs, When these algorithms are applied to data flow graphs with loops, unnecessary register transfer operations are introduced. A new algorithm is presented that performs a minimal register allocation and eliminates all superfluous register transfer operations. The problem is reformulated as a multicommodity network flow problem for which very efficient solutions exist. Experiments on a benchmark set have shown that in all test cases all register transfers could be eliminated at no increase in register cost. Only heuristic algorithms appeared in literature to solve the module allocation problem. The module allocation problem is usually defined as a clique cover problem on a so-called module allocation graph. It is shown that, under certain conditions, the module allocation graph belongs to the special class of comparability graphs. A polynomial time algorithm can optimally find a clique cover of such a graph. Even when interconnect weights are taken into account, this can be solved exactly. This problem can be transformed into a maximal cost network flow problem, which can be solved exactly in polynomial time. An algorithm is described which solves the module allocation problem with interconnect weights exactly, with a complexity O(kn2), where n is the number of operations In previous research, interconnection was optimized when the module allocation for the operations and the register allocation for the variables already had been done. However, the amount of multiplexing and interconnect are crucial factors to both the delay and the area of a circuit. A new scheme is presented to minimize the number of interconnections during the data path allocation. This scheme first groups all values based on their read and write times. Values belonging to the same group can share a register file. This minimizes the number of data transfers with different sources and destinations. Secondly, registers are allocated for each group separately. Finally the interconnect allocation is done. During the interconnect allocation, the module allocation is determined. The value grouping is based on edge coloring algorithms providing a sharp upper bound on the number of colors needed two techniques: splitting read and write phases of values and introducing serial (re-)write operations for the same value, make that even more efficient exact edge coloring algorithms can be used. It is shown that when variables are grouped into register files and operations are assigned to modules during the interconnection minimization, significant savings (20%) can be obtained in the number of local interconnections and the amount of global interconnect, at the expense of only slightly more register area.
keywords Digital Systems; Digital Systems
series thesis:PhD
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