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The Shang High-level Synthesis Framework

News

We will modernize Shang and push new release.

Overview

The Shang high-level synthesis framework, which is implemented as an LLVM backend, take as input C specification and generates Verilog RTL hardware desciption from LLVM IR. Unlike most other LLVM-based high-level synthesis frameworks, e.g. C-to-Verilog or Legup, which work on the LLVM-IR layer, Shang works on the LLVM machine code layer, which allow Shang to easily represent and optimize some high-level synthesis specific operation(instruction), e.g. reduction OR, concatenation, etc.

At the moment, Shang has several high-level synthesis specific (optimization) passes including:

• (pre-schedule) Arithmetic/bitwise operation strength reduction
• Pre-schedule logic synthesis with ABC (optional, maps all bitwise logic operations to look-up tables)
• SDC-based Scheduling pass which support multi-cycles chaining and global code motion (only apply to a specific kind of operations at the moment).
• Weighted compatibility graph-based unified register/functional-unit allocation and binding pass.
• Register-transfer level optimizations, e.g. common subexpression elimination by and-invert graph (AIG) based structural hashing.
• Verilog RTL code generation pass

Users can also schedule some "scripting passes" which apply Lua scripts to the design to accomplish tasks inluding:

• Generate vendor specific timing constraints for the design, this is necessary because Shang will always generates multi-cycles pathes (by the so-called Multi-cycles Chaining).
• Generate platform specific bus interface to allow the design generated by Shang cooperates with others components in the same system.
• ...

Combining these together, Shang outperforms the commercial tool eXCite by 10% and the old version of open source HLS tool LegUp by 30%, respectively, on the CHStone benchmark suite.

Getting Start

This guide should quickly get you started on using Shang to synthesize C into Verilog. We divided this guide into 2 parts. Installation will guide you to install required packages and complie the source code on Ubuntu. Main or Hybrid Flow will show you how to use Shang in different synthesis flow. These flows has been tested on the ubuntu-10.04.3, we assume that you are using a Linux 32/64-bit environment, we have not tested Shang on Windows or Mac OS.

1.Installation

Required Packages or Softwares on Ubuntu

#####To install the required packages on Ubuntu run:#####

#####To install systemc##### Download the source code here, when you do this you need an account in systemc's website to access source code, you can register as member employee to get an account. Among all the file provided by Accellera Systems Initiative there is a file called INSTALL which will guide you to install systemc on your computer whether for windows or unix. #####To install verilator##### You can download the verilator-3.833.tgz from the website. On the website it alse told you the way to install verilator on your computer, you can just do as it said. #####To install clang##### Manually download the latest version of clang here and add it to your path. For 32-bit machines:

#####To install the Quartus and Modelsim on Ubuntu##### You will need Modelsim to simulate the Verilog and Quartus to synthesize the Verilog for an FPGA. You can download Quartus Web Edition and Modelsim for free from Altera.

After installing Quartus update your environment to add quartus and modelsim to your path:

######Note###### You must edit the path above to point to your particular Quartus installation location. Shang has been tested with Quartus 10.1sp1.

Compile Shang Source and run the testsuite

#####Download Shang and compile it from source:##### First you should download the source code of llvm to the local computer, you’ll get a folder named llvm, cd into the ~\llvm\lib\Target folder and rename the VerilogBackend of Shang which was called Shang to VerilogBackend. Now you have got all the source code of Shang. To get both llvm and VBE you can run those:

Before compiling Shang, you need to checkout LLVM revsion 16436dffb50fac4677c7162639f8da0b73eb4e99, and a patch located in <path-to-Shang's-source>/util/0001-Minimal-patch-to-llvm-3.1svn.patch. You can do this by the below commands:

Then we use CMake ,a cross-platform, open-source build system to control the compilation process of Shang. On command line you can type the cmake command shows as follew to configure path for the environment variable used by Shang.

######Note###### In the command you should specified the relative path after “cmake” to point to the folder named llvm we have got from the remote repository. The environment variable alse should be specified absolute path on where the package installed or lib placed. #####Run the testsuite##### By running the testsuite you can verify your installation and checking the correction of verilog file converted by Shang. You can synthesis or simulate not only all the c file in CHStone but alse the specific c file at once. In order to simulate all programs in the benchmark, you can run:

In order to simulate and synthesis all programs in the benchmark, you can run:

Using float64_add.c as example, you can alse simulate the specific c program like this:

2.Main or Hybrid Flow

As main flow, Shang can compile an entire C program to hardware. It can also compile user designated functions to hardware while remaining program segments are executed in software on the Altera Nios II Soft Processor. This is referred to as the hybrid flow. #####Main flow##### For example, let’s synthesis the float64_add.c of the dfadd CHStone benchmark, you can run as this:

you will get the Verilog file float64_add_IMS_ASAP_main_DUT_RTL.v in shang-build/lib/Target/VerilogBackend/testsuite/benchmark/ChStone/dfadd/float64_add_IMS_ASAP_main/ #####Hybrid flow##### For example, let’s synthesis the float64_add.c of the dfadd CHStone benchmark, you can run as this:

you will get the Verilog file float64_add_IMS_ASAP_DUT_RTL.v in shang-build/lib/Target/VerilogBackend/testsuite/benchmark/ChStone/dfadd/float64_add_IMS_ASAP/

Writing Lua script

Shang uses the popular scripting language Lua (version 5.1) as the configure input.

Lua is a powerful, fast, lightweight, embeddable scripting language. If you are not familiar with the syntax of Lua, you should spend a little time and go over the Lua 5.1 reference book.

We will demonstrate how to write a Lua script to configure Shang as follows. Now we assume that you want to convert a C code named float64_add.c which is available at testsuite\benchmark\ChStone\dfadd into the corresponding RTL code.
######1. Setup the input and output path###### Now we should write a Lua script named "configure.lua" for Shang. To begin with, You should assign the input path of .bc or .ll file (float64_add.bc). We also presume that the output path is the same as the input path. We output the RLT code (float64_add.v) and timing constraints script(float64_add.sdc). a simple example:

InDir = [[your-work-dir]]
OutDir = Indir
InputFile = InDir .. 'float64_add.bc'
RTLOutput = OutDir .. 'float64_add.v'
SDCOutput = OutDir .. 'float64_add.sdc'

######2. Setup the convert function###### If we want to convert certain function (float64_add in this case) into hardware, we should have the following statement in the Lua script.

In this table, we create a table in which the "ModName" is the name of the converted verilog module, the "Scheduling" is the schedule mode of Shang (ASAP or ILP etc.), the "Pipeline" is the option whether we use software pipelining in Shang. ######3. Setup the platform information script.###### Supposed that we use the EP2C35F672C6 FPGA of altera as the hardware platform, we could create another lua script named "EP2C35F672C6.lua" to hold the platform information of EP2C35F672C6. The "EP2C35F672C6.lua" could be like this:

local FMAX = 100
PERIOD = 1000.0 / FMAX
FUs.ClkEnSelLatency = 1.535 / PERIOD --1.535
FUs.MaxAllowedMuxSize = 8
FUs.RegCost = 64
FUs.LUTCost = 64
FUs.MaxLutSize = 4
FUs.MaxMuxPerLUT = 2
FUs.LutLatency = 0.635 / PERIOD
-- Latency table for EP2C35F672C6
FUs.AddSub = { Latencies = { 1.994 / PERIOD, 2.752 / PERIOD, 4.055 / PERIOD, 6.648 / PERIOD },
               Costs = {128, 576, 1088, 2112, 4160}, StartInterval=1,
               ChainingThreshold = -1}
FUs.Shift  = { Latencies = { 3.073 / PERIOD, 3.711 / PERIOD, 5.209 / PERIOD, 6.403 / PERIOD },
               Costs = {64, 1792, 4352, 10176, 26240}, StartInterval=1,
               ChainingThreshold = -1}
FUs.Mult   = { Latencies = { 2.181 / PERIOD, 2.504 / PERIOD, 6.503 / PERIOD, 9.229 / PERIOD },
               Costs = {64, 4160, 8256, 39040, 160256}, StartInterval=1,
               ChainingThreshold = -1}
FUs.ICmp   = { Latencies = { 1.909 / PERIOD, 2.752 / PERIOD, 4.669 / PERIOD, 7.342 / PERIOD },
               Costs = {64, 512, 1024, 2048, 4096}, StartInterval=1,
               ChainingThreshold = -1}
    
FUs.MemoryBus = { Latency= 0.5, StartInterval=1, AddressWidth=POINTER_SIZE_IN_BITS, DataWidth=64 }
    
FUs.BRam = { Latency=1, StartInterval=1, DataWidth = 64, InitFileDir = [[the-dir-to-place the init files]],
             Template=[=[
    
// Block Ram $(num)
reg                      bram$(num)we;
reg   [$(addrwidth - 1):0]   bram$(num)addr;
reg   [$(datawidth - 1):0]   bram$(num)in;
reg   [$(datawidth - 1):0]   bram$(num)out;
    
reg   [$(datawidth - 1):0]  mem$(num)[0:$(2^addrwidth-1)];

#if filename ~= [[empty]] then 
initial begin
  $readmemh("$(filepath)$(filename)", mem$(num));
end
#end

always @ (posedge $(clk)) begin
  if (bram$(num)en) begin
    if (bram$(num)we)
      mem$(num)[bram$(num)addr] <= bram$(num)out;

    bram$(num)in <= mem$(num)[bram$(num)addr];
  end
end
]=]}

In this script, we configure the target period of the hardware implemenation and setup the parameters about the function units in the target FPGA platform. We make the latency table for the combinational logic.(to be continued...)

Then we can include the EP2C35F672C6.lua in configure.lua with the following statement:

-- load platform information script
dofile(InDir .. 'EP2C35F672C6.lua')

######4. Setup the other configuration.###### User will find the other configuration in the example configure.lua in the testsuit build directory. As a whole the example configure.lua should look like this(without the configuration information mentioned in setp 4):

Internal Representations

To be written.

Todo

• Transcational-level optimization