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Using Feedback to Improve VLSI Designs
January-February 1997 (vol. 12 no. 1)
pp. 67-73

A new design-optimization technique, called Constrained-Redo, uses feedback from solutions analysis to improve both power and coverage of an existing design system, the DAA system.

The VLSI High-Level Synthesis problem involves creating a register-transfer-level description of a chip that satisfies a given behavioral specification, and simultaneously maximizing the chip's speed and minimizing the area usage. The input to an HLS system is an algorithmic (functional) description of a chip, in a high-level language (for example, VHDL): for example, a microprocessor or a digital filter. The output is an RTL description of the chip consisting of components specification-for example, adders, multipliers and registers, connections between the components, and a controller that determines when the components execute and when data is transferred between components. The behavior of the RTL description must satisfy the functional requirements embedded in the algorithmic description. Examples of chips specified using this scheme range from simple circuits (for example, digital filters) to complex microprocessors (for example, the Motorola MC68000).

Optimization is a very important part of the VLSI design process. Considerable time and design effort are spent determining how to generate a solution that is optimized for a particular combination of criteria-for example, area or speed, or both. VLSI HLS is a well-understood problem, so generating correct solutions is easy, but finding a good solution remains difficult.

The Design Automation System is a VLSI system for designing microprocessors. The system incorporates a large amount of domain knowledge in the form of complex evaluation functions, optimizations, and rule bases for selecting appropriate library modules. One weakness of the system that is also present in many other systems lies in its inability to use information (feedback) gleaned from analysis of its solutions to improve its own performance. The knowledge of how to analyze the solutions is present either in textbooks or in the minds of VLSI designers who can critique the designs.

This article describes how feedback based on the analysis of solutions has improved both the power and coverage of the DAA system. The technique, called Constrained-Redo, uses information from critiques of an existing solution to guide the design system to generate a new, different, and hopefully better solution. This technique is part of the Feedback Directed Optimization framework. The extended system has been tested on several standard microprocessor designs and has improved many of the solutions that DAA has generated.

Citation:
C.w. Liew, "Using Feedback to Improve VLSI Designs," IEEE Intelligent Systems, vol. 12, no. 1, pp. 67-73, Jan.-Feb. 1997, doi:10.1109/64.577418
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