High-level synthesis

Short description: Creation of hardware designs from software code

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.

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, 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.

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. 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 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.

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, the "SDC modulo scheduling" technique for optimal scheduling of pipelines was developed. This technique reduces the task of optimally scheduling operations into different pipeline stages while minimizing registers, which is naively an integer linear programming problem, to a linear programming problem, by making use of the fact that the constraint matrix is totally unimodular. For high level synthesis of ASIC designs this algorithm works well, but for FPGA use there are typically complex resource constraints that must be implemented using a brute force search on top of the SDC scheduling algorithm.

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. 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.

• 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. 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.

Vendors

Data reported on recent Survey

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	Yes	Yes	Yes
	AUGH	TIMA Lab.	Academic	C subset	VHDL	2012	All	Yes	No	No
	eXCite	Y Explorations	Commercial	C	VHDL/Verilog	2001	All	Yes	No	Yes
	Bambu	PoliMi	Academic	C	VHDL/Verilog	2012	All	Yes	Yes	No
	Bluespec	BlueSpec Inc.	BSD-3	BSV	SystemVerilog	2007	All	No	No	No
	QCC	CacheQ Systems, Inc.	Commercial	C/C++/Fortan	Host Executable + FPGA Bit file (SystemVerilog is intermediate)	2018	All - multi-core and heterogeneous compute	Yes (C++)	Yes	Yes
	CHC	Altium	Commercial	C subset	VHDL/Verilog	2008	All	No	Yes	Yes
	CoDeveloper	Impulse Accelerated	Commercial	Impulse-C	VHDL	2003	Image Streaming	Yes	Yes	No
	HDL Coder	MathWorks	Commercial	MATLAB, Simulink, Stateflow, Simscape	VHDL / Verilog	2003	Control Systems, Signal Processing, Wireless, Radar, Communications, Image and Computer Vision	Yes	Yes	Yes
	CyberWorkBench	NEC	Commercial	C, BDL, SystemC	VHDL/Verilog	2004	All	Cycle/ Formal	Yes	Yes
	Catapult	Mentor (Siemens business)	Commercial	C, C++, SystemC	VHDL/Verilog	2004	All	Yes	Yes	Yes
	DWARV	TU. Delft	Academic	C subset	VHDL	2012	All	Yes	Yes	Yes
	GAUT	U. Bretagne	Academic	C/C++	VHDL	2010	DSP	Yes	No	Yes
	Hastlayer	Lombiq Technologies	BSD-3	C#/C++/F#... (.NET)	VHDL	2015	.NET	Yes	Yes	Yes
	Instant SoC	FPGA Cores	Commercial	C/C++	VHDL/Verilog	2019	All	Yes	No	No
	Intel High Level Synthesis Compiler	Intel FPGA (Formerly Altera)	Commercial	C/C++	Verilog	2017	All	Yes	Yes	Yes
	LegUp HLS	LegUp Computing	Commercial	C/C++	Verilog	2015	All	Yes	Yes	Yes
	LegUp	U. Toronto	Academic	C	Verilog	2010	All	Yes	Yes	No
	MaxCompiler	Maxeler	Commercial	MaxJ	RTL	2010	DataFlow	No	Yes	No
	ROCCC	Jacquard Comp.	Commercial	C subset	VHDL	2010	Streaming	No	Yes	No
	Symphony C	Synopsys	Commercial	C/C++	VHDL/Verilog/ SystemC	2010	All	Yes	No	Yes
	VivadoHLS (formerly AutoPilot from AutoESL)	Xilinx	Commercial	C/C++/SystemC	VHDL/Verilog/ SystemC	2013	All	Yes	Yes	Yes
	Kiwi	U. Cambridge	Academic	C#	Verilog	2008	.NET	No	Yes	Yes
	CHiMPS	U. Washington	Academic	C	VHDL	2008	All	No	No	No
	gcc2verilog	U. Korea	Academic	C	Verilog	2011	All	No	No	No
	HercuLeS	Ajax Compilers	Commercial	C/NAC	VHDL	2012	All	Yes	Yes	Yes
	Shang	U. Illinois	Academic	C	Verilog	2013	All	Yes	?	?
	Trident	Los Alamos NL	Academic	C subset	VHDL	2007	Scientific	No	Yes	No
Aban- doned	AccelDSP	Xilinx	Commercial	MATLAB	VHDL/Verilog	2006	DSP	Yes	Yes	Yes
	C2H	Altera	Commercial	C	VHDL/Verilog	2006	All	No	No	No
	CtoVerilog	U. Haifa	Academic	C	Verilog	2008	All	No	No	No
	DEFACTO	U. South Cailf.	Academic	C	RTL	1999	DSE	No	No	No
	Garp	U. Berkeley	Academic	C subset	bitstream	2000	Loop	No	No	No
	MATCH	U. Northwest	Academic	MATLAB	VHDL	2000	Image	No	No	No
	Napa-C	Sarnoff Corp.	Academic	C subset	VHDL/Verilog	1998	Loop	No	No	No
	PipeRench	U.Carnegie M.	Academic	DIL	bistream	2000	Stream	No	No	No
	SA-C	U. Colorado	Academic	SA-C	VHDL	2003	Image	No	No	No
	SeaCucumber	U. Brigham Y.	Academic	Java	EDIF	2002	All	No	Yes	Yes
	SPARK	U. Cal. Irvine	Academic	C	VHDL	2003	Control	No	No	No

• Dynamatic from EPFL/ETH Zurich
• MATLAB HDL Coder [1] from Mathworks
• HLS-QSP from CircuitSutra Technologies
• C-to-Silicon from Cadence Design Systems
• Concurrent Acceleration from Concurrent EDA
• Symphony C Compiler from Synopsys
• QuickPlay from PLDA
• PowerOpt from ChipVision
• 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
• PipelineC [2]
• CyberWorkBench from NEC
• Mega Hardware
• C2R from CebaTech
• CoDeveloper from Impulse Accelerated Technologies
• HercuLeS by Nikolaos Kavvadias
• PICO from Synfora, acquired by Synopsys in June 2010 (PICO = Program In/Code Out)
• xPilot from University of California, Los Angeles
• Vsyn from vsyn.ru
• ngDesign from SynFlow

See also

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

Further reading

• Michael Fingeroff (2010). High-Level Synthesis Blue Book. Xlibris Corporation. ISBN 978-1-4500-9724-6. 
• Coussy, P.; Gajski, D. D.; Meredith, M.; Takach, A. (2009). "An Introduction to High-Level Synthesis". IEEE Design & Test of Computers 26 (4): 8–17. doi:10.1109/MDT.2009.69. 
• Ewout S. J. Martens; Georges Gielen (2008). High-level modeling and synthesis of analog integrated systems. Springer. ISBN 978-1-4020-6801-0. 
• Saraju Mohanty; N. Ranganathan; E. Kougianos; P. Patra (2008). Low-Power High-Level Synthesis for Nanoscale CMOS Circuits. Springer. ISBN 978-0387764733. 
• Alice C. Parker; Yosef Tirat-Gefen; Suhrid A. Wadekar (2007). "System-Level Design". in Wai-Kai Chen. The VLSI handbook (2nd ed.). CRC Press. chapter 76. ISBN 978-0-8493-4199-1. 
• Shahrzad Mirkhani; Zainalabedin Navabi (2007). "System Level Design Languages". in Wai-Kai Chen. The VLSI handbook (2nd ed.). CRC Press. chapter 86. ISBN 978-0-8493-4199-1.  covers the use of C/C++, SystemC, TML and even UML
• Liming Xiu (2007). VLSI circuit design methodology demystified: a conceptual taxonomy. Wiley-IEEE. ISBN 978-0-470-12742-1. 
• John P. Elliott (1999). Understanding behavioral synthesis: a practical guide to high-level design. Springer. ISBN 978-0-7923-8542-4. 
• Nane, Razvan; Sima, Vlad-Mihai; Pilato, Christian; Choi, Jongsok; Fort, Blair; Canis, Andrew; Chen, Yu Ting; Hsiao, Hsuan et al. (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. doi:10.1109/TCAD.2015.2513673. 
• Gupta, Rajesh; Brewer, Forrest (2008). "High-Level Synthesis: A Retrospective". "High-level Synthesis: A Retrospective". Springer. pp. 13–28. doi:10.1007/978-1-4020-8588-8_2. ISBN 978-1-4020-8587-1. 

External links

• Vivado HLS course on Youtube
• Deepchip Discussion Forum