Field-programmable gate arrays (FPGAs) require a much longer compilation cycle than conventional computing platforms such as CPUs. In this article, we shorten the overall compilation time by co-optimizing the HLS compilation (C-to-RTL) and the back-end physical implementation (RTL-to-bitstream). We propose a split compilation approach based on the pipelining flexibility at the HLS level, which allows us to partition designs for parallel placement and routing. We outline a number of technical challenges and address them by breaking the conventional boundaries between different stages of the traditional FPGA tool flow and reorganizing them to achieve a fast end-to-end compilation. Our research produces RapidStream, a parallelized and physical-integrated compilation framework that takes in a latency-insensitive program in C/C++ and generates a fully placed and routed implementation. We present two approaches. The first approach (RapidStream 1.0) resolves inter-partition routing conflicts at the end when separate partitions are stitched together. When tested on the Xilinx U250 FPGA with a set of realistic HLS designs, RapidStream achieves a 5 to 7× reduction in compile time and up to 1.3× increase in frequency when compared with a commercial off-the-shelf toolchain. In addition, we provide preliminary results using a customized open-source router to reduce the compile time up to an order of magnitude in cases with lower performance requirements. The second approach (RapidStream 2.0) prevents routing conflicts using virtual pins. Testing on Xilinx U280 FPGA, we observed 5 to 7× compile time reduction and 1.3× frequency increase.

FPGAs require a much longer compilation cycle than conventional computing platforms like CPUs. In this paper, we shorten the overall compilation time by co-optimizing the HLS compilation (C-to-RTL) and the back-end physical implementation (RTL-to-bitstream). We propose a split compilation approach based on the pipelining flexibility at the HLS level, which allows us to partition designs for parallel placement and routing then stitch the separate partitions together. We outline a number of technical challenges and address them by breaking the conventional boundaries between different stages of the traditional FPGA tool flow and reorganizing them to achieve a fast end-to-end compilation. Our research produces RapidStream, a parallelized and physical-integrated compilation framework that takes in an HLS dataflow program in C/C++ and generates a fully placed and routed implementation. When tested on the Xilinx U250 FPGA with a set of realistic HLS designs, RapidStream achieves a 5-7X reduction in compile time and up to 1.3X increase in frequency when compared to a commercial-off-the-shelf toolchain. In addition, we provide preliminary results using a customized open-source router to reduce the compile time up to an order of magnitude in the cases with lower performance requirements. The tool is open-sourced at github.com/Licheng-Guo/RapidStream.

One of the key obstacles to pervasive deployment of FPGA accelerators in data centers is their cumbersome programming model. Open source tooling is suggested as a way to develop alternative EDA tools to remedy this issue. Open source FPGA CAD tools have traditionally targeted academic hypothetical architectures, making them impractical for commercial devices. Recently, there have been efforts to develop open source back-end tools targeting commercial devices. These tools claim to follow an alternate data-driven approach that allows them to be more adaptable to the domain requirements such as faster compile time. In this paper, we present RWRoute, the first open source timing-driven router for UltraScale+ devices. RWRoute is built on the RapidWright framework and includes the essential and pragmatic features found in commercial FPGA routers that are often missing from open source tools. Another valuable contribution of this work is an open-source lightweight timing model with high fidelity timing approximations. By leveraging a combination of architectural knowledge, repeating patterns, and extensive analysis of Vivado timing reports, we obtain a slightly pessimistic, lumped delay model within 2% average accuracy of Vivado for UltraScale+ devices. Compared to Vivado, RWRoute results in a 4.9× compile time improvement at the expense of 10% Quality of Results (QoR) loss for 665 synthetic and six real designs. A main benefit of our router is enabling fast partial routing at the back-end of a domain-specific flow. Our initial results indicate that more than 9× compile time improvement is achievable for partial routing. The results of this paper show how such a router can be beneficial for a low touch flow to reduce dependency on commercial tools.

Circuit rewiring can be used to explore a larger solution space by modifying circuit structure to suit a given optimization problem. Among several rewiring techniques that have been proposed, SPFD-based rewiring has been shown to be more effective in terms of solution space coverage. However, its adoption in practice has been limited due to its long runtime. We propose a novel SAT-based algorithm that is much faster than the traditional BDD-based methods. Unlike BDD-based methods that completely specify all pairs of SPFD using BDDs, our algorithm uses a few SAT instances to perform rewiring for a given wire without explicitly enumerating all SPFDs. Experimental results show that our algorithm's runtime is only 13% of that of a conventional one when each wire has at most 25 candidate wires and the runtime scales well with the number of candidate wires considered. Our approach evaluates each rewiring instance independently in the order of milliseconds, rendering deployment of an SPFD-based rewiring inside the optimization loop of synthesis tools a possibility.

This paper proposes two set-of-pairs-of-functions-to-be-distinguished (SPFD)-based rewiring algorithms to be used in a multi-tier rewiring framework, which employs multiple rewiring techniques. The first algorithm has two unique features: 1) a satisfiability problem (SAT) instance was devised so that an unsuccessful rewiring can be identified very quickly, and 2) unlike binary decision diagram-based methods that require all pairs of SPFD, our algorithm uses a few SAT instances to perform rewiring for a given wire without explicitly enumerating all SPFDs. Experimental results show that the runtime of our algorithm is about three times faster than that of a conventional one under a simulated setting of such a framework and it scales well with the number of candidate wires considered. The efficacy of the framework can be further improved by the second proposed algorithm. The algorithm relies on a theory presented herein to allow adding a new wire outside of the restricted set of dominator nodes, a feature common in automatic-test-pattern-generation-based rewiring, but absent in existing SPFD-based ones. Although this algorithm may suffer from long runtimes in the same way conventional SPFD-based techniques do, experiments show that the number of wires which can be rewired increases 13% on average and the number of alternative wires also increases.

Field-programmable gate arrays (FPGAs) have gained wide acceptance among low- to medium-volume applications. However, there are gaps between FPGA and custom implementations in terms of area, performance and power consumption. In recent years, specialized blocks - memories and multipliers in particular - have been shown to help reduce this gap. However, their usefulness has not been studied formally on a broad spectrum of designs. As FPGAs are prefabricated, an FPGA family must contain members of various sizes and combinations of specialized blocks to satisfy diverse design resource requirements. We formulate the family selection process as an ldquoFPGA family compositionrdquo problem and propose an efficient algorithm to solve it. The technique was applied to an architecture similar to Xilinx Virtex FPGAs. The results show that smart composition technique can reduce the expected silicon area up to 55%. The benefit of providing multiplier blocks in FPGAs is also shown to reduce total area by 20% using the proposed algorithm.

Traditionally, logic synthesis constrains the solution space of later design steps, such as physical design, because they are applied in sequence. Rewiring is a technique to restructure a circuit while maintaining its functionality. Since design properties and objectives can be considered during post-synthesis rewiring, it can help relieve constraints put forth by decisions made at earlier design steps. The extent of rewiring of a rewiring algorithm has a great impact on the success of the design flow. This paper presents a powerful rewiring technique that in addition to unifying all previously proposed Set-of-Pairs-of-Functions-to-be-Distinguished based rewiring techniques, it can perform rewiring with more than one wire which increases our ability to circumvent poorly-decided early design constraints. With this ability, the rewiring ability of using different numbers of wires is reported for the first time in this paper. Our technique can be used for runtime/quality trade-off in any given rewiring application.

According to the ITRS predictions, controlling manufacturing yield is going to be a challenging task in future technologies. The effective yield of future FPGA architectures considering configurable logic blocks, switch boxes, connection boxes and routing segments is estimated in this paper. The results show that some degree of redundancy for logic blocks, routing and switch boxes is necessary. However, no more than one spare logic block per cluster, and at most one spare wire is required to obtain a satisfactory effective yield. The results also indicate that it is beneficial to increase logic cluster size of future FPGA architectures for better yield.

In traditional field programmable gate array (FPGA) placement methods, there is virtually no coupling between placement and routing. Performing simultaneous placement and detailed routing has been shown to generate much better placement qualities, but at the expense of significant runtime penalties (Nag and Rutenbar, 1998). We propose a routing-aware partitioning-based placement algorithm for FPGAs in which a looser but effective coupling between the placement and routing stages is used. The placement engine incorporates a more accurate FPGA delay model and employs effective heuristics that minimize circuit delay. Delay estimations are obtained from routing profiles of selected circuits that are placed and routed using the timing-driven versatile place and route (TVPR) (Betz and Rose, 1997), (Marquardt et al., 2000). As a result, the delay predictions during placement more accurately resemble those observed after detailed routing, which in turn leads to better delay optimization. An efficient terminal alignment heuristic for delay minimization is applied during placement to further optimize the delay of the circuit. These two techniques help maintain harmony between placement and routing-delay optimization stages. Simulation results show that the proposed partitioning-based placement combined with more accurate delay models and the alignment heuristic can achieve postrouting circuit delays comparable to those obtained from TVPR, while achieving a fourfold speedup in total placement runtime. In another experiment, we augmented the original TVPR algorithm with the terminal alignment heuristic, and achieved, on average, a 5% improvement in circuit delay with negligible runtime penalty.

In this paper we propose a partitioning-based placement algorithm for FPGAs. The method incorporates simple, but effective heuristics that target delay minimization. The placement engine incorporates delay estimations obtained from previously placed and routed circuits using VPR [6]. As a result, the delay predictions during placement more accurately resemble those observed after detailed routing, which in turn leads to better delay optimization. An efficient terminal alignment heuristic for delay minimization is employed to further optimize the delay of the circuit in the routing phase. Simulation results show that the proposed technique can achieve comparable circuit delays (after routing) to those obtained with VPR while achieving a 7-fold speedup in placement runtime.