This work details the capabilities of a major new release of the Verilog-to-Routing (VTR) open source FPGA CAD tool flow. Enhancements include generalizations of VTR’s architecture modeling language and optimizers to enable a more diverse set of programmable routing fabrics, FPGAs with embedded hard Networks-on-Chip (NoCs) and three-dimensional 3D FPGA systems that leverage stacked silicon integration. The new Parmys logic synthesis flow improves language coverage and result quality, and the physical implementation flow includes a more efficient placement engine, floorplanning constraints to guide placement, the ability to perform single-stage (flat) routing to improve quality, and parallel routing algorithms to reduce CPU time. This release also includes new architecture captures of recent commercial devices (Xilinx’s 7-series and Altera’s Stratix 10) and new benchmark suites (Titanium25 and Hermes) to aid FPGA architecture investigation. Verilog language coverage is greatly improved with the new Parmys logic synthesis flow, enabling more designs to be used with VTR. Finally, the placement and routing engines have beeenbeen sped up by 4 \(\times\) and 2.2 \(\times\) vs. VTR 8, respectively, leading to an overall physical implementation flow CPU time reduction of 48% with better result quality on average compared to VTR 8.

This is a corrigendum for the article “VTR 9: Open-Source CAD for Fabric and Beyond FPGA Architecture Exploration” published in ACM Trans. Reconfig. Technol. Syst. 18, 3, Article 39 (August 2025), 53 pages.

Simulated annealing (SA) is one of the most common FPGA placement techniques, and is used both as a standalone algorithm and to improve an initial analytical placement. While SA-based placers can achieve high-quality results, they suffer from long runtimes. In this article, we introduceRLPlace, a novel SA-based FPGA placer that utilizes both reinforcement learning (RL) and targeted perturbations (directed moves). The proposed moves target both wirelength and timing optimization and explore the solution space more efficiently than traditional random moves while preventing oscillation in the Quality of Results (QoR). RL techniques are used to dynamically select the most effective move types as optimization progresses. The experimental results show thatRLPlaceoutperforms the widely used VTR 8 placer across all runtime/quality tradeoff points, achieving better QoR placement solutions in less runtime. On average, across the Titan23 suite of large FPGA benchmarks, RLPlace can reduce CPU time by$2.5\times $with result quality comparable to VTR 8, or improve wirelength by 8% (at a high CPU time budget) −26% (at a low CPU time budget) versus VTR 8.0 given the same CPU time.

Routing is a key step in the FPGA design process, which significantly impacts design implementation quality. Routing is also very time-consuming, and can scale poorly to very large designs. This paper describes the Adaptive Incremental Router (AIR), a high-performance timing-driven FPGA router. AIR dynamically adapts to the routing problem, which it solves `lazily' to minimize work. Compared to the widely used VPR 7 router, AIR significantly reduces route-time ($7.1 \times$ faster), while also improving quality (15% wirelength, and 18% critical path delay reductions). We also show how these techniques enable efficient incremental improvement of existing routing.

Developing Field-programmable Gate Array (FPGA) architectures is challenging due to the competing requirements of various application domains and changing manufacturing process technology. This is compounded by the difficulty of fairly evaluating FPGA architectural choices, which requires sophisticated high-quality Computer Aided Design (CAD) tools to target each potential architecture. This article describes version 8.0 of the open source Verilog to Routing (VTR) project, which provides such a design flow. VTR 8 expands the scope of FPGA architectures that can be modelled, allowing VTR to target and model many details of both commercial and proposed FPGA architectures. The VTR design flow also serves as a baseline for evaluating new CAD algorithms. It is therefore important, for both CAD algorithm comparisons and the validity of architectural conclusions, that VTR produce high-quality circuit implementations. VTR 8 significantly improves optimization quality (reductions of 15% minimum routable channel width, 41% wirelength, and 12% critical path delay), run-time (5.3× faster) and memory footprint (3.3× lower). Finally, we demonstrate VTR is run-time and memory footprint efficient, while producing circuit implementations of reasonable quality compared to highly-tuned architecture-specific industrial tools—showing that architecture generality, good implementation quality, and run-time efficiency are not mutually exclusive goals.

Hardened adder and carry logic is widely used in commercial field-programmable gate arrays (FPGAs) to improve the efficiency of arithmetic functions. There are many design choices and complexities associated with such hardening, including circuit design, FPGA architectural choices, and the computer-aided design (CAD) flow. However, these choices have not been studied much and hence we explore a number of possibilities. We also highlight front-end elaboration optimization that helps ameliorate the restrictions placed on logic synthesis by hardened arithmetic. We show that hard adders and carry chains increase the performance of simple adders by a factor of 4 or more, but on larger benchmark designs that contain arithmetic improve the overall performance by 15%. Our results also show that for complete application circuits simple hardened ripple-carry adders perform as well as more complex carry-lookahead adders. Our best non-fracturable lookup table (non-fLUT) architecture with hardened arithmetic yields 12% better area-delay product than architectures without hardened arithmetic. We also investigate the impact of fLUTs and their interaction with hardened arithmetic. We find that fLUTs offer significant (12%-15%) area reduction, which is complementary to the delay reduction of hardened arithmetic. Therefore, our best fLUT architectures which use two bits of hardened arithmetic achieve 25% better area-delay product than non-fLUT architectures without hardened arithmetic.

Timing analysis is a key step in the digital design process. By modeling device delay variations statistical static timing analysis (SSTA) reduces pessimism compared to traditional static timing analysis (STA). However, it ignores the circuit's logic which causes some timing paths to never, or only rarely, be sensitized. We introduce a general timing analysis approach and tool to calculate the probability that individual timing paths are sensitized, enabling the calculation of bounding delay distributions over all input combinations. We show how this analysis is related to the well-known #SAT problem and present approaches to improve scalability, achieving, on average, results 75% to 37% less pessimistic than STA while running 569 to 16 times faster than Monte-Carlo timing simulation.

Static Timing Analysis (STA) is used to evaluate the correctness and performance of a digital circuit implementation. In addition to final sign-off checks, STA is called numerous times during placement and routing to guide optimization. As a result, STA consumes a significant fraction of the time required for design implementation; to make progress reducing FPGA compile times we need faster STA. We evaluate the suitability of both GPU and multi-core CPU platforms for accelerating STA. On core STA algorithms our GPU kernel achieves a 6.2 times kernel speed-up but data transfer overhead reduces this to 0.9 times. Our best CPU implementation achieves a 9.2 times parallel speed-up on 32 cores, yielding a 15.2 times overall speed-up compared to the VPR analyzer, and a 6.9 times larger parallel speed-up than a recent parallel ASIC timing analyzer. We then show how reducing the run-time cost of STA can be leveraged to improve optimization quality, reducing critical path delay by 4%.

Timing analysis is a cornerstone of the digital design process. Statistical Static Timing Analysis was introduced to reduce pessimism by modelling device delay variations. However it ignores circuit logic, which may cause some timing paths to never or only rarely be sensitized. We introduce a general timing analysis approach and tool to calculate the probability that individual timing paths are sensitized, enabling the calculation of bounding delay distributions over all input combinations. We show the connection to the well-known #SAT problem and present approaches to improve scalability, achieving average results 46 to 32% less pessimistic than Static Timing Analysis while running 14.6 to 44.0 times faster than Monte-Carlo timing simulation.

Floorplanning is an approach to improve the scalability of existing CAD algorithms, facilitate team-based design, and also plays an important role in partial reconfiguration. This work introduces HETRIS, a new automated floorplanning tool for heterogeneous FPGAs. HETRIS uses an adaptive legality approach to target arbitrary FPGA architectures. It includes enhancements enabling it to run on average 15.6× faster than previous work, while producing denser floorplans than a commercial tool. Using HETRIS we perform the first evaluation of an FPGA floorplanner using real-world benchmarks, allowing us to investigate the relationship between partitioning, floorplanning and FPGA architecture.

Benchmarks play a key role in Field-Programmable Gate Array (FPGA) architecture and CAD research, enabling the quantitative comparison of tools and architectures. It is important that these benchmarks reflect modern large-scale systems that make use of heterogeneous resources; however, most current FPGA benchmarks are both small and simple. In this artile, we present Titan, a hybrid CAD flow that addresses these issues. The flow uses Altera’s Quartus II FPGA CAD software to perform HDL synthesis and a conversion tool to translate the result into the academic Berkeley Logic Interchange Format (BLIF). Using this flow, we created the Titan23 benchmark set, which consists of 23 large (90K--1.8M block) benchmark circuits covering a wide range of application domains. Using the Titan23 benchmarks and an enhanced model of Altera’s Stratix IV architecture, including a detailed timing model, we compare the performance and quality of VPR and Quartus II targeting the same architecture. We found that VPR is at least 2.8 × slower, uses 6.2 × more memory, 2.2 × more wire, and produces critical paths 1.5 × slower compared to Quartus II. Finally, we identified that VPR’s focus on achieving a dense packing and an inability to take apart clusters is responsible for a large portion of the wire length and critical path delay gap.

Latency insensitive communication offers many potential benefits for FPGA designs, including easier timing closure by enabling automatic pipelining, and easier interfacing with embedded NoCs. However, it is important to understand the costs and trade-offs associated with any new design style. This paper presents optimized implementations of latency insensitive communication building blocks, quantifies their overheads in terms of area and frequency, and provides guidance to designers on how to generate high-speed and area-efficient latency insensitive systems.

Benchmarks play a key role in FPGA architecture and CAD research, enabling the quantitative comparison of tools and architectures. It is important that these benchmarks reflect modern designs which are large scale systems that make use of heterogeneous resources; however, most current FPGA benchmarks are both small and simple. In this paper we present Titan, a hybrid CAD flow that addresses these issues. The flow uses Altera's Quartus II FPGA CAD software to perform HDL synthesis and a conversion tool to translate the result into the academic BLIF format. Using this flow we created the Titan23 benchmark set, which consists of 23 large (90K-1.8M block) benchmark circuits covering a wide range of application domains. Using the Titan23 benchmarks and a detailed model of Altera's Stratix IV architecture we compared the performance and quality of VPR and Quartus II targeting the same architecture. We found that VPR is at least 2.7× slower, uses 5.1× more memory and 2.6× more wire compared to Quartus II. Finally, we identified that VPR's focus on achieving a dense packing is responsible for a large portion of the wire length gap.

Realistic benchmarks are important for FPGA Architecture and CAD evaluation. This paper provides a demo illustrating how designs described in HDL can be converted to BLIF using the Titan flow, and used in academic CAD tools.