c-ll-verilog

An LLVM based mini-C to Verilog High-level Synthesis tool

Overview

In this project, we are proposing an LLVM based mini-C to Verilog high-level synthesis (HLS) tool, called c-ll-verilog (C to Verilog). HLS is a trending topic in both the academia and industry which aims to reduce the complexity of hardware design by automatically generating hardware description language (HDL) code from a higher level language such as C. An important different between programming languages like C and HDLs is that, C and similar languages are meant to be used on a predefined architectures, which usually are conventional von neumann architectures, while HDL itself (in the domain of FPGA programming) is meant to model and design the underlying architecture or hardware. There are different HLS tools which their generated HDL code can be implemented on the latest field programmable gate array (FPGA) or system-on-chips. However, due to the inherent syntax and symantec differences in general purpose high level languages such as C and HDLs and the complexity of this conversion, current HLS tools cannot yet satisfy all of the performance, area, and power requirements of the complex systems. Therefore, in the view of the author these tools are either used in a highly annotated manner (for e.g. using PRAGMA or compiler directives) for a specific FPGA device and application or used for experimental as well as educational purposes. In this project we investigated the HLS process using an LLVM based framework, and by compiling a subset of branchless C statements to dataflow modeling Verilog code.

What's inside and how it works?

The overall goal of this project was to convert a C function to a Verilog module, with the following features:

• C function arguments are used as Verilog module input ports (no pointer, array, and struct support)
• C function return value are used as Verilog module output port
• Only a subset of branchless C statements is supported
• Only a subset of arithmetic and logical operations are supported (addition, subtraction, signed greater than, equality, logical shift right, bitwise XOR, bitwise AND, and bitwise OR)
• Only combinational logic with data modeling Verilog style is supported (no registers and clock)

The following UML diagram represents the summarized class hierarchy use in the LLVM-to-Verilog code generator:

Building the toolchain

A simple LLVM parser and code generator are built upon a MIPS code generator LLVM pass. Build the c-ll-verilog tool (as verilog LLVM opt pass) in a linux machine with proper installation of LLVM framework, after cloning the repository and run make command in the main directory. You can clean the generated files using the make clean command.

Let's go live!

To run the HLS tool pass your c file to the simpleHLS.bash script as below (make sure simpleHLS.bash is set to executable using the chmod +x simpleHLS.bash command): ./simpleHLS.bash <filename>.c You should see the generated Verilog file in the same directory as .v. So the only thing that is needed is to pass the input C file as an argument to the script and it produces the corresponding Verilog file with the same name.

Note that after the parsing process mem2reg LLVM optimization is used to simplify the LLVM code remove memory allocation and accesses as it is not supported by our simple HLS tool. An example C code (filter.c),

bool adder(int in1, int in2, int threshold,
int mask)
{
	return ( ( ( (in1+in2) & mask ) >> 2 ) > threshold );
}

and its synthesis to Verilog code (filter.v):

//entry:
        module adder (
        input wire [31:0] in1,
        input wire [31:0] in2,
        input wire [31:0] threshold,
        input wire [31:0] mask,
        output wire result
        );
//L0:
        wire [31:0] wire000;
        assign wire000 = in1 + in2;
        wire [31:0] wire001;
        assign wire001 = wire000 & mask;
        wire [31:0] wire003;
        assign wire003 = 2;
        wire [31:0] wire002;
        assign wire002 = wire001 >> wire003;
        wire wire004;
        assign wire004 = wire002 > threshold;
        assign result = wire004;
//exit:
        endmodule

License

This project is licensed under the MIT License - see the license file for details

Acknowledgments

This project was based on the materials presented and taught by professor Refael Ubal for the Compilers course (EECE-7398) at Northeastern University in Fall 2016.