StressTest: an automatic approach to test generation via activity monitors

The challenge of verifying a modern microprocessor design is an overwhelming one: increasingly complex micro-architectures combined with heavy time-to-market pressure have forced microprocessor vendors to employ immense verification teams in the hope of finding the most critical bugs in a timely manner. Unfortunately, too often size doesn't seem to matter for verification teams, as design schedules continue to slip and microprocessors find their way to the marketplace with design errors. In this paper, we describe a simulation-based random test generation tool, called StressTest, that provides assistance in locating hard-to-find corner-case design bugs and performance problems. StressTest is based on a Markov-model-driven random instruction generator with activity monitors. The model is generated from the user-specified template programs and is used to generate the instructions sent to the design under test (DUT). In addition, the user specifies key activity points within the design that should be stressed and monitored throughout the simulation. The StressTest engine then uses closed-loop feedback techniques to transform the Markov model into one that effectively stresses the points of interest. In parallel, StressTest monitors the correctness of the DUT response to the supplied stimuli, and if the design behaves unexpectedly, a bug and a trace that leads to it are reported. Using two micro-architectures as example testbeds, we demonstrate that StressTest finds more bugs with less effort than open-loop random instruction test generation techniques.