High-level synthesis explained

High-level synthesis (HLS), sometimes referred to as C synthesis, electronic system-level (ESL) synthesis, algorithmic synthesis, or behavioral synthesis, is an automated design process that takes an abstract behavioral specification of a digital system and finds a register-transfer level structure that realizes the given behavior.^{[1] [2] [3]}

Synthesis begins with a high-level specification of the problem, where behavior is generally decoupled from low-level circuit mechanics such as clock-level timing. Early HLS explored a variety of input specification languages,^[4] although recent research and commercial applications generally accept synthesizable subsets of ANSI C/C++/SystemC/MATLAB. The code is analyzed, architecturally constrained, and scheduled to transcompile from a transaction-level model (TLM) into a register-transfer level (RTL) design in a hardware description language (HDL), which is in turn commonly synthesized to the gate level by the use of a logic synthesis tool.

The goal of HLS is to let hardware designers efficiently build and verify hardware, by giving them better control over optimization of their design architecture, and through the nature of allowing the designer to describe the design at a higher level of abstraction while the tool does the RTL implementation. Verification of the RTL is an important part of the process.^[5]

Hardware can be designed at varying levels of abstraction. The commonly used levels of abstraction are gate level, register-transfer level (RTL), and algorithmic level.

While logic synthesis uses an RTL description of the design, high-level synthesis works at a higher level of abstraction, starting with an algorithmic description in a high-level language such as SystemC and ANSI C/C++. The designer typically develops the module functionality and the interconnect protocol. The high-level synthesis tools handle the micro-architecture and transform untimed or partially timed functional code into fully timed RTL implementations, automatically creating cycle-by-cycle detail for hardware implementation.^[6] The (RTL) implementations are then used directly in a conventional logic synthesis flow to create a gate-level implementation.

History

Early academic work extracted scheduling, allocation, and binding as the basic steps for high-level-synthesis. Scheduling partitions the algorithm in control steps that are used to define the states in the finite-state machine. Each control step contains one small section of the algorithm that can be performed in a single clock cycle in the hardware. Allocation and binding maps the instructions and variables to the hardware components, multiplexers, registers and wires of the data path.

First generation behavioral synthesis was introduced by Synopsys in 1994 as Behavioral Compiler^[7] and used Verilog or VHDL as input languages. The abstraction level used was partially timed (clocked) processes. Tools based on behavioral Verilog or VHDL were not widely adopted in part because neither languages nor the partially timed abstraction were well suited to modeling behavior at a high level. 10 years later, in early 2004, Synopsys end-of-lifed Behavioral Compiler.^[8]

In 1998, Forte Design Systems introduced its Cynthesizer tool which used SystemC as an entry language instead of Verilog or VHDL. Cynthesizer was adopted by many Japanese companies in 2000 as Japan had a very mature SystemC user community. The first high-level synthesis tapeout was achieved in 2001 by Sony using Cynthesizer. Adoption in the United States started in earnest in 2008.

In 2006, an efficient and scalable "SDC modulo scheduling" technique was developed on control and data flow graphs ^[9] and was later extended to pipeline scheduling.^[10] This technique uses the integer linear programming formulation. But it shows that the underlying constraint matrix is totally unimodular (after approximating the resource constraints). Thus, the problem can be solved in polynomial time optimally using a linear programming solver in polynomial time. This work was inducted to the FPGA and Reconfigurable Computing Hall of Fame 2022.^[11]

The SDC scheduling algorithm was implemented in the xPilot HLS system^[12] developed at UCLA,^[13] and later licensed to the AutoESL Design Technologies, a spin-off from UCLA. AutoESL was acquired by Xilinx (now part of AMD) in 2011, and the HLS tool developed by AutoESL became the base of Xilinx HLS solutions, Vivado HLS and Vitis HLS, widely used for FPGA designs.

Source input

The most common source inputs for high-level synthesis are based on standard languages such as ANSI C/C++, SystemC and MATLAB.

High-level synthesis typically also includes a bit-accurate executable specification as input, since to derive an efficient hardware implementation, additional information is needed on what is an acceptable Mean-Square Error or Bit-Error Rate etc. For example, if the designer starts with an FIR filter written using the "double" floating type, before he can derive an efficient hardware implementation, they need to perform numerical refinement to arrive at a fixed-point implementation. The refinement requires additional information on the level of quantization noise that can be tolerated, the valid input ranges etc. This bit-accurate specification makes the high level synthesis source specification functionally complete.^[14] Normally the tools infer from the high level code a Finite State Machine and a Datapath that implement arithmetic operations.

Process stages

The high-level synthesis process consists of a number of activities. Various high-level synthesis tools perform these activities in different orders using different algorithms. Some high-level synthesis tools combine some of these activities or perform them iteratively to converge on the desired solution.^[15]

• Lexical processing
• Algorithm optimization
• Control/Dataflow analysis
• Library processing
• Resource allocation
• Scheduling
• Functional unit binding
• Register binding
• Output processing
• Input Rebundling

Functionality

In general, an algorithm can be performed over many clock cycles with few hardware resources, or over fewer clock cycles using a larger number of ALUs, registers and memories. Correspondingly, from one algorithmic description, a variety of hardware microarchitectures can be generated by an HLS compiler according to the directives given to the tool. This is the same trade off of execution speed for hardware complexity as seen when a given program is run on conventional processors of differing performance, yet all running at roughly the same clock frequency.

Architectural constraints

Synthesis constraints for the architecture can automatically be applied based on the design analysis.^[5] These constraints can be broken into

• Hierarchy
• Interface
• Memory
• Loop
• Low-level timing constraints
• Iteration

Interface synthesis

Interface Synthesis refers to the ability to accept pure C/C++ description as its input, then use automated interface synthesis technology to control the timing and communications protocol on the design interface. This enables interface analysis and exploration of a full range of hardware interface options such as streaming, single- or dual-port RAM plus various handshaking mechanisms. With interface synthesis the designer does not embed interface protocols in the source description. Examples might be: direct connection, one line, 2 line handshake, FIFO.^[16]

Vendors

Data reported on recent Survey^[17]

Status	Compiler	Owner	License	Input	Output	Year	Domain	Test bench	FP	FixP
In use	Stratus HLS	Cadence Design Systems	Commercial	C–C++ SystemC	RTL	2015	All
	AUGH	TIMA Lab.	Academic	C subset	VHDL	2012	All
	eXCite	Y Explorations	Commercial	C	VHDL–Verilog	2001	All
	Bambu	PoliMi	Academic	C	VHDL–Verilog	2012	All
	Bluespec	BlueSpec, Inc.	BSD-3	Bluespec SystemVerilog (Haskell)	SystemVerilog	2007	All
	QCC	CacheQ Systems, Inc.	Commercial	C, C++, Fortran	Host executable + FPGA bit file (SystemVerilog is intermediate)	2018	All - multi-core and heterogeneous compute	(C++)
	CHC	Altium	Commercial	C subset	VHDL–Verilog	2008	All
	CoDeveloper	Impulse Accelerated	Commercial	Impulse-C	VHDL	2003	Image streaming
	HDL Coder	MathWorks	Commercial	MATLAB, Simulink, Stateflow, Simscape	VHDL, Verilog	2003	Control systems, signal processing, wireless, radar, communications, image and computer vision
	CyberWorkBench	NEC	Commercial	C, BDL, SystemC	VHDL–Verilog	2004	All	Cycle, formal
	Catapult	Siemens EDA	Commercial	C–C++ SystemC	VHDL–Verilog	2004	All
	DWARV	TU. Delft	Academic	C subset	VHDL	2012	All
	GAUT	University of Western Brittany	Academic	C, C++	VHDL	2010	DSP
	Hastlayer	Lombiq Technologies	BSD-3	C#, C++, F#, ... (.NET)	VHDL	2015	.NET
	Instant SoC	FPGA Cores	Commercial	C, C++	VHDL–Verilog	2019	All
	Intel High Level Synthesis Compiler	Intel FPGA (Formerly Altera)	Commercial	C, C++	Verilog	2017	All
	LegUp HLS	LegUp Computing	Commercial	C, C++	Verilog	2015	All
	LegUp	University of Toronto	Academic	C	Verilog	2010	All
	MaxCompiler	Maxeler	Commercial	MaxJ	RTL	2010	Data-flow analysis
	ROCCC	Jacquard Comp.	Commercial	C subset	VHDL	2010	Streaming
	Symphony C	Synopsys	Commercial	C, C++	VHDL–Verilog, SystemC	2010	All
	VivadoHLS (formerly AutoPilot from AutoESL^[18])	Xilinx	Commercial	C–C++ SystemC	VHDL–Verilog, SystemC	2013	All
	Kiwi	University of Cambridge	Academic	C#	Verilog	2008	.NET
	CHiMPS	University of Washington	Academic	C	VHDL	2008	All
	gcc2verilog	Korea University	Academic	C	Verilog	2011	All
	HercuLeS	Ajax Compilers	Commercial	C/NAC	VHDL	2012	All
	Shang	University of Illinois Urbana-Champaign	Academic	C	Verilog	2013	All		?	?
	Trident	Los Alamos NL	Academic	C subset	VHDL	2007	Scientific
Aban- doned	AccelDSP	Xilinx	Commercial	MATLAB	VHDL–Verilog	2006	DSP
	C2H	Altera	Commercial	C	VHDL–Verilog	2006	All
	CtoVerilog	University of Haifa	Academic	C	Verilog	2008	All
	DEFACTO	University South Cailf.	Academic	C	RTL	1999	DSE
	Garp	University of California, Berkeley	Academic	C subset	bitstream	2000	Loop
	MATCH	Northwest University	Academic	MATLAB	VHDL	2000	Image
	Napa-C	Sarnoff Corp.	Academic	C subset	VHDL–Verilog	1998	Loop
	PipeRench	Carnegie Mellon University	Academic	DIL	bistream	2000	Stream
	SA-C	University of Colorado	Academic	SA-C	VHDL	2003	Image
	SeaCucumber	Brigham Young University	Academic	Java	EDIF	2002	All
	SPARK	University of California, Irvine	Academic	C	VHDL	2003	Control

• Dynamatic from EPFL/ETH Zurich
• MATLAB HDL Coder https://www.mathworks.com/products/hdl-coder.html from Mathworks^[19]
• HLS-QSP from CircuitSutra Technologies^[20]
• C-to-Silicon from Cadence Design Systems
• Concurrent Acceleration from Concurrent EDA
• Symphony C Compiler from Synopsys
• QuickPlay from PLDA^[21]
• PowerOpt from ChipVision^[22]
• Cynthesizer from Forte Design Systems (now Stratus HLS from Cadence Design Systems)
• Catapult C from Calypto Design Systems, part of Mentor Graphics as of 2015, September 16. In November 2016 Siemens announced plans to acquire Mentor Graphics, Mentor Graphics became styled as "Mentor, a Siemens Business". In January 2021, the legal merger of Mentor Graphics with Siemens was completed - merging into the Siemens Industry Software Inc legal entity. Mentor Graphics' name was changed to Siemens EDA, a division of Siemens Digital Industries Software.^[23]
• PipelineC https://github.com/JulianKemmerer/PipelineC
• CyberWorkBench from NEC^[24]
• Mega Hardware ^[25]
• C2R from CebaTech^[26]
• CoDeveloper from Impulse Accelerated Technologies
• HercuLeS by Nikolaos Kavvadias^[27]
• Program In/Code Out (PICO) from Synfora, acquired by Synopsys in June 2010^[28]
• xPilot from University of California, Los Angeles^[29]
• Vsyn from vsyn.ru^[30]
• ngDesign from SynFlow^[31]

See also

• C to HDL
• Electronic design automation (EDA)
• Electronic system-level (ESL)
• Logic synthesis
• High-level verification (HLV)
• SystemVerilog
• Hardware acceleration

Further reading

• Jason Cong, Jason Lau, Gai Liu, Stephen Neuendorffer, Peichen Pan, Kees Vissers, Zhiru Zhang.  FPGA HLS Today: Successes, Challenges, and Opportunities. ACM Transactions on Reconfigurable Technology and Systems, Volume 15, Issue 4, Article No. 5, pp 1–42, December 2022, https://doi.org/10.1145/3530775.
• Book: Michael Fingeroff. High-Level Synthesis Blue Book. 2010. Xlibris Corporation. 978-1-4500-9724-6.
• 10.1109/MDT.2009.69. Coussy . P. . Gajski . D. D. . Meredith . M. . Takach . A. . An Introduction to High-Level Synthesis . IEEE Design & Test of Computers . 26 . 4 . 8–17 . 2009 . 52870966 .
• Book: Ewout S. J. Martens. Georges Gielen. High-level modeling and synthesis of analog integrated systems. 2008. Springer. 978-1-4020-6801-0.
• Book: Saraju Mohanty. Saraju Mohanty. N. Ranganathan. E. Kougianos. P. Patra. amp. Low-Power High-Level Synthesis for Nanoscale CMOS Circuits. 2008. Springer. 978-0387764733.
• Book: Wai-Kai Chen. The VLSI handbook. 2007. CRC Press. 978-0-8493-4199-1. 2nd. System-Level Design. chapter 76. Alice C. Parker. Alice C. Parker . Yosef Tirat-Gefen . Suhrid A. Wadekar .
• Book: Wai-Kai Chen. The VLSI handbook. 2007. CRC Press. 978-0-8493-4199-1. 2nd. System Level Design Languages. chapter 86. Shahrzad Mirkhani . Zainalabedin Navabi . covers the use of C/C++, SystemC, TML and even UML
• Book: Liming Xiu. VLSI circuit design methodology demystified: a conceptual taxonomy. 2007. Wiley-IEEE. 978-0-470-12742-1.
• Book: John P. Elliott. Understanding behavioral synthesis: a practical guide to high-level design. 1999. Springer. 978-0-7923-8542-4.
• 10.1109/TCAD.2015.2513673 . Nane . Razvan . Sima . Vlad-Mihai . Pilato . Christian . Choi . Jongsok . Fort . Blair . Canis . Andrew . Chen . Yu Ting . Hsiao . Hsuan . Brown . Stephen . Ferrandi . Fabrizio . Anderson . Jason . Bertels . Koen . A Survey and Evaluation of FPGA High-Level Synthesis Tools . IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems . 35 . 10 . 1591–1604 . 2016 . 11311/998432 . 8749577 . free .
• Book: 10.1007/978-1-4020-8588-8_2 . Gupta . Rajesh . Brewer . Forrest . "High-level Synthesis: A Retrospective" . High-Level Synthesis: A Retrospective . Springer . 2008. 13–28 . 978-1-4020-8587-1 .

External links

• Vivado HLS course on Youtube
• Deepchip Discussion Forum

Notes and References

1. Book: High-Level Synthesis - Springer . 10.1007/978-1-4020-8588-8 . 978-1-4020-8587-1 . 2008 . Coussy . Philippe . Morawiec . Adam .
2. McFarland. M.C.. Parker. A.C.. Camposano. R.. February 1990. The high-level synthesis of digital systems. Proceedings of the IEEE. 78. 2. 301–318. 10.1109/5.52214. 1558-2256.
3. Web site: HLS Book : Home . 2023-06-21 . www.hlsbook.com.
4. IEEE Xplore High-Level Synthesis: Past, Present, and Future DOI 10.1109/MDT.2009.83
5. Web site: Bowyer . Bryan . 2005-05-02 . The 'why' and 'what' of algorithmic synthesis . 2016-10-03 . EE Times.
6. Web site: UBS University, France. C-Based Rapid Prototyping for Digital Signal Processing. 2016-10-03.
7. Web site: Publications and Presentations . Bdti.com . 2016-10-03 . dead . https://web.archive.org/web/20080426075303/http://www.bdti.com/articles/info_dspx95asics.htm . 2008-04-26 .
8. Web site: Behavioral synthesis crossroad . EE Times . 2016-10-03.
9. Book: Cong . Jason . Fan . Yiping . Han . Guoling . Jiang . Wei . Zhang . Zhiru . Platform-Based Behavior-Level and System-Level Synthesis . September 2006 . 199–202 . 2006 IEEE International SOC Conference . http://dx.doi.org/10.1109/socc.2006.283880 . IEEE . 10.1109/socc.2006.283880. 0-7803-9782-7 .
10. Book: Zhang . Zhiru . 2013 IEEE/ACM International Conference on Computer-Aided Design (ICCAD) . Liu . Bin . 2013 . IEEE . 978-1-4799-1071-7 . 211–218.
11. Cong . Jason . Bin Liu . Neuendorffer . Stephen . Noguera . Juanjo . Vissers . Kees . Zhiru Zhang . April 2011 . High-Level Synthesis for FPGAs: From Prototyping to Deployment . IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems . 30 . 4 . 473–491 . 10.1109/tcad.2011.2110592 . 0278-0070.
12. Book: Cong . J. . Zhiru Zhang . An efficient and versatile scheduling algorithm based on SDC formulation . 2006 . 433–438 . 2006 43rd ACM/IEEE Design Automation Conference . http://dx.doi.org/10.1109/dac.2006.229228 . IEEE . 10.1109/dac.2006.229228. 1-59593-381-6 .
13. Web site: xPilot: Platform-based Behavior Synthesis System VAST lab . 2024-04-18 . vast.cs.ucla.edu.
14. http://www.hindawi.com/GetArticle.aspx?doi=10.1155/2008/916867&e=html Multiple Word-Length High-Level Synthesis
15. Web site: A look inside behavioral synthesis . EE Times . 2016-10-03.
16. Web site: DesignCon: InfoVault: Paper Library . www.designcon.com . 13 January 2022 . https://web.archive.org/web/20100925034609/http://www.designcon.com/infovault/paper.asp?PAPER_ID=407 . 25 September 2010 . dead.
17. Nane. R.. Sima. V. M.. Pilato. C.. Choi. J.. Fort. B.. Canis. A.. Chen. Y. T.. Hsiao. H.. Brown. S.. 2016. A Survey and Evaluation of FPGA High-Level Synthesis Tools. IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems. 35. 10. 1591–1604. 10.1109/TCAD.2015.2513673. 0278-0070. 11311/998432. 8749577. free.
18. Web site: Xilinx buys high-level synthesis EDA vendor . EE Times . 2011-02-05 . 2016-10-03 . https://web.archive.org/web/20111017170928/http://www.eetimes.com/electronics-news/4212668/Xilinx-buys-high-level-synthesis-EDA-vendor . 2011-10-17 . dead.
19. Web site: MathWorks: Makers of MATLAB and Simulink . Mathworks.com . 2016-10-03.
20. Web site: SystemC based ESL methodologies - SystemC based ESL methodologies . Circuitsutra.com . 2016-10-03.
21. Web site: John M. at a major ERP & DBMS Corporation . QuickPlay: Bringing FPGA Computing to the Masses . Quickplay.io . 2016-08-29 . 2016-10-03.
22. Web site: Chipvision - Fast Track to Low Power . www.chipvision.com . 13 January 2022 . https://web.archive.org/web/20020530041720/http://www.chipvision.com/ . 30 May 2002 . dead.
23. Web site: Mentor Finally Becomes Siemens EDA from January 2021 . 15 December 2020.
24. Web site: CyberWorkBench: Products . NEC . 2016-10-03.
25. Web site: Accueil mega-hardware . www.mega-hardware.com . 13 January 2022 . https://web.archive.org/web/20040115020254/http://www.mega-hardware.com/ . 15 January 2004 . dead.
26. Web site: Cebatech - Home . www.cebatech.com . 13 January 2022 . https://web.archive.org/web/20050507101111/http://www.cebatech.com/ . 7 May 2005 . dead.
27. Web site: Nikolaos Kavvadias - HercuLeS high-level synthesis tool . Nkavvadias.com . 2016-10-03.
28. Web site: Synopsys buys Synfora assets . EE Times . 2016-10-03 . https://web.archive.org/web/20110407145420/http://www.eetimes.com/electronics-news/4200083/Synopsys-buys-Synfora-assets . 2011-04-07 . dead.
29. Web site: The xPilot System . Cadlab.cs.ucla.edu . 2016-10-03.
30. Web site: vSyn.ru . vSyn.ru . 2016-06-16 . 2016-10-03 . https://web.archive.org/web/20160630171129/http://www.vsyn.ru/ . 2016-06-30 . dead.
31. Web site: Hardware design for all . Synflow . 2016-10-03.