Formal verification has increased efficiency by detecting corner case design bugs but it has also introduced new challenges when failures are detected. Once a counter-example is returned by a formal tool, the user typically does not know if the failure is caused by a design bug, an incorrectly written assertion, or a missing assumption. Previous work in debug automation has focused on the former two cases. This paper introduces a novel methodology to automatically debug missing assumptions. It begins by generating multiple formal counter-examples for the error. Next, a function is extracted from these counter-examples that encodes the input combinations that cause the assertion to fail. This function is later used to generate a list of fixed cycle assumptions that prevent failures similar to the generated counter-examples. These filtered assumptions can then be used as hints for the actual missing assumption. Further, if a missing assumption is not the cause of the failure, the method offers the additional benefit that the counter-examples it generates can be utilized to debug the RTL and/or the assertion. An extensive set of experimental results on OpenCores designs and assertions show that the number of generated assumptions can be reduced by an average of 38% using ten counter-examples, while an average of 28 assumptions is returned to the user.

Modern designs are growing in size and complexity, becoming increasingly harder to verify. Today, they are architected to include multiple clock domains as a measure to reduce power consumption. Verifying them proves to be a computationally intensive and challenging task as it requires their clocks to be synchronized. To achieve synchronization, existing Boolean satisfiability-based methodologies add hardware to combine the clock domains before transforming them into their iterative logic array representation (ILA). As a consequence, this results in the addition of redundant time-frames adding overhead during verification. This paper introduces a novel framework to verify designs with multiple clocks using Quantified Boolean Formula satisfiability (QBF). We first present a formulation that models an ILA representation with symbolic universal quantification to achieve synchronization. This is later extended with the use of a clock divider to overcome inefficiencies. The net effect is the reduction in the number of redundant time-frames. Furthermore, the usage of QBF results in significant memory savings when compared to traditional methods. Experiments on bounded model checking demonstrate memory reductions of 76% on average with competitive run-time performance.

Register-transfer level (RTL) debug has become a resource-intensive bottleneck in modern very large scale integration computer-aided design flows, consuming as much as 32% of the total verification effort. This paper aims to advance the state-of-the-art in automated RTL debuggers, which return all potential bugs in the RTL, called solutions, along with corresponding corrections. First, an iterative algorithm is presented to compute the dominance relationships between RTL blocks. These relationships are leveraged to discover implied solutions with every new solution, thus significantly reducing the number of formal engine calls. Furthermore, a modern Boolean satisfiability (SAT) solver is tailored to detect debugging nonsolutions, sets of RTL blocks guaranteed to be bug-free, and to imply other nonsolutions using the precomputed RTL dominance relationships. Extensive experiments on industrial designs show a three-fold reduction in the number of SAT calls due to solution implications, coupled with faster SAT run-times due to nonsolution implications, resulting in a 2.63x overall speedup in total SAT solving time, demonstrating the robustness and practicality of the proposed approach.

The conflicting yet increasing demand for high performance and low power in multi-functional chips has pushed techniques for power reduction to the forefront of VLSI design. Although recent developments have automated most of the low power implementations, designers often manually modify the circuit in order to achieve further power savings. This human intervention is often paved with many errors that are bound to typical logic functional failures. Debugging these errors can be a resource intensive process that requires considerable manual effort. This discourages engineers and achieving power savings at the micro level of the design sometimes remains unrealized. This paper proposes a novel debugging methodology to rectify erroneous clock-gating implementations. With the use of Partial Max-SAT, the method localizes and rectifies the design error introduced in the circuit during a clock-gating implementation. The net effect of the proposed methodology leads to shorter debug time ensuring additional power savings. Extensive experiments on benchmark circuits confirm the effectiveness of the approach.

With the growing complexity of current designs and shrinking time-to-market, traditional ATPG methods fail to detect all electrical faults in the design. Debug teams have to spend considerable amount of time and effort to identify these faults during post silicon debug. This work proposes off-chip analysis to speed-up the effort of identifying hard-to-find electrical faults that are not detected using conventional test methods, but cause the chip to crash during functional testing or silicon-bring-up. With the goal of reducing the search space for reconstructing the failure trace path, formal methodology is used to analyze the reachable states along the path. Isolating the root cause of failure is also accelerated. Moreover, we propose a forward traversal technique on selected few possible faults to generate a complete failure trace starting from the initial state to the crash state. Experimental results show that the proposed approach can lead to a 44% reduction in actual silicon run with a commensurate reduction in off-chip debug time.

With the growing complexity of VLSI designs, functional debugging has become a bottleneck in modern CAD flows. To alleviate this cost, various SAT-based techniques have been developed to automate bug localization in the RTL. In this context, dominance relationships between circuit blocks have been recently shown to reduce the number of SAT solver calls, using the concept of solution implications. This paper first introduces the dual concepts of reverse domination and non-solution implications. A SAT solver is tailored to leverage reverse dominators for the early on-the-fly detection of bug-free components. These are non-solution areas and their early pruning significantly reduces the the debugging search-space. This process is expedited by branching on error-select variables first. Extensive experiments on tough real-life industrial debugging cases show an average speedup of 1.7x in SAT solving time over the state-of-the-art, a testimony of the practicality and effectiveness of the proposed approach.

We propose new hardware and software techniques for FPGA functional debug that leverage the inherent reconfigurability of the FPGA fabric to reduce functional debugging time. The functionality of an FPGA circuit is represented by a programming bitstream that specifies the configuration of the FPGA's internal logic and routing. The proposed methodology allows different sets of design internal signals to be traced solely by changes to the programming bitstream followed by device reconfiguration and hardware execution. Evidently, the advantage of this new methodology vs. existing debug techniques is that it operates without the need of iterative executions of the computationally-intensive design re-synthesis, placement and routing tools. In essence, with a single execution of the synthesis flow, the new approach permits a large number of internal signals to be traced for an arbitrary number of clock cycles using a limited number of external pins. Experimental results using commercial FPGA vendor tools demonstrate productivity (i.e. run-time) improvements of up to 30× vs. a conventional approach to FPGA functional debugging. These results demonstrate the practicality and effectiveness of the proposed approach.

Many CAD for VLSI problems can be naturally encoded as Quantified Boolean Formulas (QBFs) and solved with QBF solvers. Furthermore, such problems often contain circuit-based information that is lost during the translation to Conjunctive Normal Form (CNF), the format accepted by most modern solvers. In this work, a novel preprocessing framework for circuit-based QBF problems is presented. It leverages structural circuit dominators to reduce the problem size and expedite the solving process. Our circuit-based QBF preprocessor PReDom recursively reduces dominated subcircuits to return a simpler but equisatisfiable QBF instance. A rigorous proof is given for eliminating subcircuits dominated by single outputs, irrespective of input quantifiers. Experimental results are presented for circuit diameter computation problems. With preprocessing times of at most five seconds using PReDom, three state-of-the-art QBF solvers can solve 27% to 45% of our problem instances, compared to none without preprocessing.