To drastically improve energy efficiency, we believe future processors need to go beyond parallelization and provide architecture support for customization, enabling systems to adapt to different application domains. In particular, we believe future architectures will make extensive use of accelerators to significantly reduce energy consumption. Such architectures present many new challenges and opportunities, such as accelerator synthesis, scheduling, sharing, virtualization, memory hierarchy optimization, and efficient compilation and runtime support. With respect to these areas, we review the progress of our research in the Center for Domain-Specific Computing (supported by the NSF Expeditions-in-Computing Award), and discuss ongoing work and additional challenges.

Customized instructions (CIs) implemented using custom functional units (CFUs) have been proposed as a way of improving performance and energy efficiency of software while minimizing cost of designing and verifying accelerators from scratch. However, previous work allows CIs to only communicate with the processor through registers or with limited memory operations. In this work we propose an architecture that allows CIs to seamlessly execute memory operations without any special synchronization operations to guarantee program order of instructions. Our results show that our architecture can provide 24\% energy savings with 14% performance improvement for 2-issue and 4-issue superscalar processor cores.

The rise of multicore architectures across all computing domains has opened the door to heterogeneous multiprocessors, where processors of different compute characteristics can be combined to effectively boost the performance per watt of different application kernels. GPUs, in particular, are becoming very popular for speeding up compute-intensive kernels of scientific, imaging, and simulation applications. New programming models that facilitate parallel processing on heterogeneous systems containing GPUs are spreading rapidly in the computing community. By leveraging these investments, the developers of other accelerators have an opportunity to significantly reduce the programming effort by supporting those accelerator models already gaining popularity. In this work, we adapt one such language, the CUDA programming model, into a new FPGA design flow called FCUDA, which efficiently maps the coarse- and fine-grained parallelism exposed in CUDA onto the reconfigurable fabric. Our CUDA-to-FPGA flow employs AutoPilot, an advanced high-level synthesis tool (available from Xilinx) which enables high-abstraction FPGA programming. FCUDA is based on a source-to-source compilation that transforms the SIMT (Single Instruction, Multiple Thread) CUDA code into task-level parallel C code for AutoPilot. We describe the details of our CUDA-to-FPGA flow and demonstrate the highly competitive performance of the resulting customized FPGA multicore accelerators. To the best of our knowledge, this is the first CUDA-to-FPGA flow to demonstrate the applicability and potential advantage of using the CUDA programming model for high-performance computing in FPGAs.

The growth of 3D technology had led to opportunities for stacked multiprocessor-accelerator computing platforms with high-bandwidth and low-latency TSV connections between them, resulting in high computing performance and better energy efficiency. This work evaluates the performance and energy benefits of such an advanced architecture and addresses associated design problems. To better utilize the reconfigurable hardware resource and to explore the opportunity of kernel sharing across applications, we propose to use a dedicated domain-specific computing platform. In particular, we have chosen medical image processing as the domain in this work to accelerate due to its growing for real-time processing demand yet inadequete performance on conventional computing architectures. A design flow is proposed in this work for the 3D multiprocessor-accelerator platform and a number of methods are applied to optimize the average performance of all the applications in the targeted domain under area and bandwidth constraints. Experiments show that the applications in this domain can gain a 7.4× speed-up and 18.8× energy savings on average running on our platform using CMP cores and domain-specific accelerators as compared to their counterparts coded in CPU only.

Recent progress in High-Level Synthesis (HLS) techniques has helped raise the abstraction level of FPGA programming. However implementation and performance evaluation of the HLS-generated RTL, involves lengthy logic synthesis and physical design flows. Moreover, mapping of different levels of coarse grained parallelism onto hardware spatial parallelism affects the final FPGA-based performance both in terms of cycles and frequency. Evaluation of the rich design space through the full implementation flow - starting with high level source code and ending with routed net list - is prohibitive in various scientific and computing domains, thus hindering the adoption of reconfigurable computing. This work presents a framework for multilevel granularity parallelism exploration with HLS-order of efficiency. Our framework considers different granularities of parallelism for mapping CUDA kernels onto high performance FPGA-based accelerators. We leverage resource and clock period models to estimate the impact of multi-granularity parallelism extraction on execution cycles and frequency. The proposed Multilevel Granularity Parallelism Synthesis (ML-GPS) framework employs an efficient design space search heuristic in tandem with the estimation models as well as design layout information to derive a performance near-optimal configuration. Our experimental results demonstrate that ML-GPS can efficiently identify and generate CUDA kernel configurations that can significantly outperform previous related tools whereas it can offer competitive performance compared to software kernel execution on GPUs at a fraction of the energy cost.

Traditionally, research in fault tolerance has required architectural state to be numerically perfect for program execution to be correct. However, in many programs, even if execution is not 100% numerically correct, the program can still appear to execute correctly from the user's perspective. To quantify user satisfaction, application-level fidelity metrics (such as PSNR) can be used. The output for such applications is defined to be correct if the fidelity metrics satisfy a certain threshold. However, such applications still contain instructions whose outputs are critical - i.e. their correctness decides if the overall quality of the program output is acceptable. In this paper, we present an analysis technique for identifying such critical program segments. More importantly, our technique is capable of guaranteeing application-level correctness through a combination of static analysis and runtime monitoring. Our static analysis consists of data flow analysis followed by control flow analysis to find static critical instructions which affect several instructions. Critical instructions are further refined into likely non-critical and likely critical sets in a profiling phase. At runtime, we use a monitoring scheme to monitor likely non-critical instructions and take remedial actions if some likely non-critical instructions become critical. Based on this analysis, we minimize the number of instructions that are duplicated and checked at runtime using a software-based fault detection and recovery technique [20]. Put together, our approach can lead to 22% average energy savings for multimedia applications while guaranteeing application-level correctness, when compared to a recent work [9], which cannot guarantee application-level correctness. Comparing to the approach proposed in [20] which guarantees both application-level and numerical correctness, our method achieves 79% energy reduction.

Monte Carlo based SSTA serves as the golden standard against alternative SSTA algorithms, but it is seldom used in practice due to its high computation time. In this paper, we accelerate Monte Carlo based SSTA using the FPGA platform. A simple dataflow pipeline technique will not work well due to the excessive usage of FPGA logic slices. We leverage the recently proposed pattern matching method to identify common circuit structures, and further use a mathematical programming based formulation to explore the trade-off between performance and logic slices consumption. The proposed design provides two orders of magnitude speedup compared to the CPU-based implementation.

In this paper, we propose a novel, energy aware scheduling algorithm for applications running on DVS-enabled multiprocessor systems, which exploits variation in execution times of individual tasks. In particular, our algorithm takes into account latency and resource constraints, precedence constraints among tasks and input-dependent variation in execution times of tasks to produce a scheduling solution and voltage assignment such that the average energy consumption is minimized. Our algorithm is based on a mathematical programming formulation of the scheduling and voltage assignment problem and runs in polynomial time. Experiments with randomly generated task graphs show that up to 30% savings in energy can be obtained by using our algorithm over existing techniques. We perform experiments on two real-world applications - MPEG-4 decoder and MJPEG encoder. Simulations show that the scheduling solution generated by our algorithm can provide up to 25% reduction in energy consumption over greedy dynamic slack reclamation algorithms.

Precision analysis and optimization is very important when transforming a floating-point algorithm into fixed-point hardware implementations. The core analysis techniques are either based on dynamic analysis or static analysis. We believe in static error analysis, as it is the only technique that can guarantee the desired worst-case accuracy. In this paper we study various underlying arithmetic candidates that can be used in static error analysis and compare their computed sensitivities. The approaches studied include Affine Arithmetic(AA), General Interval Arithmetic (GIA) and Automatic Differentiation (Symbolic Arithmetic). Our study shows that symbolic method is preferred for expressions with higher order cancellation. For programs without strong cancellation, any method works fairly well and GIA slightly outperforms others. We also study the impact of program transformations on these arithmetics.

Reconfigurable high-performance computing systems (RHPC) have been attracting more and more attention over the past few years. RHPC systems are a promising solution for accelerating system performance, lowering power consumption and minimizing operation cost. In order to achieve high performance on this hybrid system, it is important to effectively explore the design space, which includes accelerator synthesis, resource allocation and job scheduling. In this paper we propose novel algorithms for reconfigurable resource allocation and job scheduling to optimize performance of multicore RHPC systems. Specifically, we first propose an interesting approximation algorithm to assign jobs to processors with consideration of coprocessors at the global optimization step. Then we present an optimal solution for coprocessor selection in the local optimization step. In this paper we also demonstrate that designers can quickly explore a large number of accelerator design choices with the help of high-level synthesis tools. Experiments show that our proposed techniques provide efficient solutions for real-life benchmarks and generate higher quality of results. When compared to other heuristic algorithms, our results can achieve up to 47% performance improvement.

This paper revisits the classical bitwidth optimization problem for fixed-point designs. Our approach also starts from static analysis (range analysis and precision analysis) techniques. We first point out that AA-based precision analysis, which is widely used in many previous works, are not correctly applied for some corner cases. We propose to use interval coefficients to model the sensitivity (the mathematic form is similar to classical generalized interval arithmetic) to correct those issues. Based on the analysis, we use a convex optimization problem to optimize the area costs. In the formulation, we also allow different bitwidths for the multiple uses of the same signal. We get an additional 2% to 3% area reduction compared with previous AA-based analysis and optimizations. We also conduct some optimality study to see how much the gap between the current methods and the possible optimal point is.

In this work, we propose a new FPGA design flow that combines the CUDA programming model from Nvidia with the state of the art high-level synthesis tool AutoPilot from AutoESL, to efficiently map the exposed parallelism in CUDA kernels onto reconfigurable devices. The use of the CUDA programming model offers the advantage of a common programming interface for exploiting parallelism on two very different types of accelerators -- FPGAs and GPUs. Moreover, by leveraging the advanced synthesis capabilities of AutoPilot we enable efficient exploitation of the FPGA configurability for application specific acceleration. Our flow is based on a compilation process that transforms the SPMD CUDA thread blocks into high-concurrency AutoPilot-C code. We provide an overview of our CUDA-to-FPGA flow and demonstrate the highly competitive performance of the generated multi-core accelerators.

The application specific multiprocessor system-on-a-chip is a promising design alternative because of its high degree of flexibility, short development time, and potentially high performance attributed to application specific optimizations. However, designing an optimal application specific multiprocessor system is still challenging because there are a number of important metrics, such as throughput, latency, and resource usage, which need to be explored and optimized. This paper addresses the problem of synthesizing an application-specific multiprocessor system for stream-oriented embedded applications to minimize system latency under the throughput constraint. We employ a novel framework for this problem, similar to that of technology mapping in the logic synthesis domain, and develop a set of efficient algorithms, including labeling and clustering for efficient generation of the multiprocessor architecture with application specific optimized latency. Specifically, the result of our algorithm is latency optimal for directed acyclic task graphs. Application of our approach to the Motion JPEG example on Xilinx's Virtex II Pro platform FPGA shows interesting design tradeoffs.

The problem of hardware-software codesign for embedded systems using configurable architectures has been studied extensively in the past decade. In this work we studied the feasibility of utilizing Commercial Off-The-Shelf (COTS) FPGA systems for codesign. We partitioned the implementation of a set of benchmark applications on hardware and software and studied the performance and resource consumption in the system. The result of experiments demonstrated that the communication between the processor and the reconfigurable architecture is the major hurdle in the codesign, especially when using COTS System on Chips. It is demonstrated that although implementing algorithms in hardware can lead to enormous speedup, the communication overhead for transferring data variables between the configurable architecture and the processor can destroy all the achieved speedup. We especially showed that in COTS FPGAs this bottleneck is more restricting because of the weak communication structure between different IPs. Furthermore, analyzing the experimental results, we propose a partitioning mechanism; the evaluation results show that the achieved speedup using the proposed partitioning mechanism is between 2 to 300 based on application's data dependency

The ability to integrate diverse components such as processor cores, memories, custom hardware blocks and complex network-on-chip (NoC) communication frameworks onto a single chip has greatly increased the design space available for system-on-chip (SoC) designers. Efficient and accurate performance estimation tools are needed to assist the designer in making design decisions. In this paper, we present MC-Sim, a heterogeneous multi-core simulator framework which is capable of accurately simulating a variety of processor, memory, NoC configurations and application specific coprocessors. We also describe a methodology to automatically generate fast, cycle-true behavioral, C-based simulators for coprocessors using a high-level synthesis tool and integrate them with MC-Sim, thus augmenting it with the capacity to simulate coprocessors. Our C-based simulators provide on an average 45times improvement in simulation speed over that of RTL descriptions. We have used this framework to simulate a number of real-life applications such as the MPEG4 decoder and litho-simulation, and experimented with a number of design choices. Our simulator framework is able to accurately model the performance of these applications (only 7% off the actual implementation) and allows us to explore the design space rapidly and achieve interesting design implementations.