  Adders are key building blocks of many error-tolerant applications.
Leveraging the application-level error tolerance, a number of approximate
adders were proposed recently. Many of them belong to the category of
block-based approximate adders. For approximate circuits, besides normal
metrics such as area and delay, another important metric is the error
measurement. Given the popularity of block-based approximate adders, in this
work, we propose an accurate and efficient method to obtain the error
statistics of these adders. We first show how to calculate the error rates.
Then, we demonstrate an approach to get the exact error distribution, which can
be used to calculate other error characteristics, such as mean error distance
and mean square error.
