Due to the statistical nature of gate delays in current day technologies, measures such as path criticality and node/edge criticality are required for timing optimization. Node criticalities are usually computed using the complementary path delay. In order to speed up computations, it has been recently proposed that the circuit delay be used instead. In this paper, we show that there is a monotonic relationship between the node criticalities computed using the circuit delay and the complementary delay. They are not equal, but they can be used interchangeably. We discuss the sources of error in this computation and propose methods for more accurate computations. We also introduce a measure that is very easy to compute and is an approximate indicator of criticality. Since it is easy to compute, it can also be used effectively for pruning the number of edges involved in criticality computations thus improving the speed of criticality computations. The speedup obtained can be as large as an order of magnitude for some of larger circuits in the ISCAS benchmarks.
The statistical nature of gate delays in current day technologies necessitates the use of measures, such as path criticality and node/edge criticality for timing optimization. Node criticalities are typically computed using the complementary path delay. An alternative approach to compute the criticality using the circuit delay has been recently proposed. In this paper, we discuss in detail, the use of circuit delay to compute node criticalities and show that the criticality thus found is not equal to the conventional measure found using complementary path delay. However, there is a monotonic relationship between them and the two measures can be used interchangeably. We derive new bounds for the global criticality and propose a pruning algorithm based on these bounds to improve the accuracy and speed of computation. The use of this pruning technique results in a significant speedup in criticality computations. We obtain an order of magnitude average speedup for ISCAS benchmarks.
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