Basic RISC-V Test Programs

This repository contains programs used to demonstrate the compilation of bare-metal programs for the CV32E40P RISC-V core in preparation for compiling the the TinyMLPerf benchmarking suite.

Prerequisites

Toolchain

The Vanilla GNU Toolchain was used for this project. For details about the installation process and shell scripts to complete the process automatically go to toolchain-setup and read the README.

Verilator Model

The CV32E40P is currently undergoing verification in preparation for tape-out. Part of this verification process is exercising a Verilator model of the processor. This Verilator model can be initialized with an arbitrary hex file loaded into memory. This model is used to run the example programs.

Run the commands below to generate the Verilator model of the Standard CV32E40P. For instructions on installing the model for the verilator model for the Accelerated CV32E40P please the repository core-v-verif-ava

sudo apt install verilator
git clone https://github.com/openhwgroup/core-v-verif.git
cd ./core-v-verif/cv32/sim/core
make    # Clone the CV32E40P, build the Verilator model, run a bare-metal hello-world example

Add the verilator model to your PATH for easy access elsewhere in your system making sure you replace /path/to in the first statement with the full path to the core-v-verif repo.

export CV32SIM=/path/to/core-v-verif/cv32/sim/core
export PATH=$PATH:$CV32SIM

However this change will be undone when you close your terminal. To make the change permanent you must add the commands above to your ~/.profile file. Open the file with the command:

nano ~/.profile

Copy and paste the export commands above to the bottom of the file, remembering to replace /path/to/core-v-verif/ with the full path to the core-v-verif repo.
With both the verilator model and toolchain on your path, the bottom of your ~/.profile file should now look something like this:

...
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi

# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/.local/bin" ] ; then
    PATH="$HOME/.local/bin:$PATH"
fi

export RISCV=/opt/riscv
export PATH=$PATH:$RISCV/bin

export CV32SIM=/path/to/core-v-verif/cv32/sim/core
export PATH=$PATH:$CV32SIM

Running the Programs

The test programs are stored in sub-directories. To run a program navigate to its sub-directory and call make all to build the hex file of the program and run it on the Verilog model.

Build System

The build system used in these example programs is the same as is used to compile the test programs in the core-v-verif repo. Details on this build system can be found here: https://github.com/openhwgroup/core-v-verif/tree/master/cv32/bsp#building-and-using-the-bsp-library

In each C program sub-directory there is a lib directory. This directory contains the files libcv-verif.a and link.ld.

In each C++ program lib directory there is also a file called crtbegin.o.

libcv-verif.a

This archive file contains bare-metal implementations of essential functions usually provided by the OS. It includes a port of the C Runtime (crt0.S), a port of syscalls that does not rely on an OS, an interrupt system, and a vector table.

The syscall port is pretty bare-bones, the lack of an OS means the best many of the syscall functions can do is fail gracefully (e.g: file handling). Initial experimentation showed that the toolchain can sometimes try to add two different versions of syscalls if standard C functions are called from libraries other than stdio.h or stdlib.h, e.g: time.h.

link.ld

This is the linker script used to arrange the sections correctly in the ELF and HEX files produced at the end of compilation. It has been slightly edited from the default RISC-V linker script to accommodate changes in the address space in the CV32E40P. It is used to place the C Runtime start up code at the correct address. When this linker script is not used GCC makes incorrect assumptions about where to put the boot code. Often it will assume the program is being built to run on top of Linux and place the boot code accordingly.

crtbegin.o

This file contains for handling constructors for C++ objects, hence why it is not needed for a C environment. This file is usually linked with the other start files by GCC. However, in order to use the custom C Runtime and syscalls discussed above, we have removed all default start files, hence, we link this file manually.
If the linker ever starts complaining that it can't find a symbol called __dso_handle make sure this file is being linked properly.

Usage

Below is an example taken from the core-v-verif repo of how the above files should be used during the build process:

C

gcc test-program.o extra1.o extra2.o \
     -nostartfiles -T/path/to/link/link.ld -L/path/to/cv-verif/ -lcv-verif

C++

g++ test-program.o extra1.o extra2.o \
     -nostartfiles -T/path/to/link/link.ld -L/path/to/cv-verif/ -lcv-verif ./crtbegin.o

The -nostartfiles flag tells GCC not to include the default C Runtime boot code.
The -T option allows a user to specify the linker script used.
The -L option is used to tell GCC where to look for library files and the -l option is used to indicate an archive file to link into the project.
The inclusion of ./crtbegin.o here tells GCC to link this file alongside the contents of libcv-verif.a at the final linking stage. It is assumed that this file is in the directory that G++ is being called from.

These extra options are only required when linking. Object files can be produced without these options.

Compiling and Linking Options

On top of the options discussed in Usage, other compilation and linking options are required to get code running bare-metal.
Below is an excerpt from the makefile in the helloworld sub-directory. It shows the options used to compile each object files (RV_COMPILER_OPTIONS) and the compiler options used in the final linking stage (RV_LINKING_OPTIONS). The linking options are largely the same as the compilation options with the -nostartfiles flag added to the end.

RV_COMPILER_OPTIONS = -Os -g -static -mabi=ilp32 -march=rv32imc -Wall -pedantic 
RV_LINKING_OPTIONS = ${RV_COMPILER_OPTIONS} -nostartfiles

AI-Vector-Accelerator/riscv-toolchain-setup-tests