# Simodense: a RISC-V softcore for custom SIMD instructions

(mirrors: [philippos.info](https://philippos.info/simdsoftcore/), [GitHub](https://github.com/pphilippos/simodense))


### Welcome to the source code repository for a RISC-V softcore optimised for exploring advanced reconfigurable SIMD instructions

This repository contains a softcore built from scratch for supporting custom SIMD instruction development by Philippos Papaphilippou @ Imperial College London. See below for additional material from the publication, including a [video tutorial](https://youtu.be/gdD6353bzzo).


The source code is divided into 4 directories. A separate README file accompanies each directory, to explain how the source code can be used to reproduce the experiments in the paper.

- ``RTL_and_simulation`` This folder contains the main code of the softcore for simulation, as well as the template and examples for custom SIMD instructions. 
- ``Benchmarks`` This directory contains the benchmarks and micro-benchmarks (sort & prefix sum) used in the evaluation section.
- ``FPGA_Implementation_AXI_peripheral`` The FPGA directory provides additional Verilog code for interfacing with the an AXI interconnect (such as for Avnet's Ultra96 with the Xilinx device ZU3EG) and the relevant information for creating a Vivado project
- ``ARM_Linux_Client_for_Ultra96`` The optional Linux (for ARM) client mainly provides C code for interfacing with the softcore in our heterogeneous platform. Having Linux and the specific board is not necessary, as AXI is a versatile protocol, at least for Xilinx boards. Though [bridges](https://github.com/ZipCPU/wb2axip) to other protocols exist as well. 


### Setting up the environment 

- In order to run the simulations, [Icarus Verilog](https://steveicarus.github.io/iverilog/) is the main requirement for running the provided examples (tested with version 11.0 (stable) (v11_0)). For the debugging functionality through waveforms, a waveform viewer is recommended, such as [GTKWave Analyzer](http://gtkwave.sourceforge.net/) (tested on version 3.3.108).

- For experimentation on a Xilinx FPGA, the freely-available Webpack version of [Vivado](https://www.xilinx.com/support/download.html) is enough, given that your FPGA board is mentioned in the release notes (see [here](https://www.xilinx.com/support/documents/sw_manuals/xilinx2022_2/ug973-vivado-release-notes-install-license.pdf)).

- For software development for the softcore, we need to install the [RISC-V GNU Compiler Toolchain](https://github.com/riscv/riscv-gnu-toolchain), and modify it for supporting the custom SIMD instructions for inline assembly. Note that for simply testing that simulator works there already is a sample binary at ``RTL_and_simulation`` by default.

First we fetch the code using the bash command (6.65GB of data, about 10GB after compilation):

	$ git clone https://github.com/riscv/riscv-gnu-toolchain

and change directory:

	$ cd riscv-gnu-toolchain

*Update* - the file structure and behaviour of the RISC-V GNU toolchain has changed since the softcore has been developped. Revert to a tested version of that repository using the following commands:

	$ git switch b715e4f --detach
    
Then we provide the target directory where GCC and related tools are installed. This is for bare-metal applications, based on the 32-bit "RV32I" base specification and "M" multiplication/division extension. The size of the prefix directory will become about 1.1 GB after compilation.

	$ ./configure --prefix=/opt/riscv32im_custom --with-arch=rv32im --with-abi=ilp32    

Before compiling, we need to add the support of custom instructions to binutils (see next section).

Finally, we compile the tools by running make, preferably with multiple-threads (15 in the example), as it can take a while.

	$ make newlib -j15


### GNU toolchain support for custom instructions

In order to do that, we add the following lines to the C file `riscv-gnu-toolchain/riscv-binutils/opcodes/riscv-opc.c` inside the lengthy `const struct riscv_opcode riscv_opcodes[]` array:

	{"c0_lv",      0, INSN_CLASS_I, "d,s,t,j",   MATCH_CUSTOM0, MASK_CUSTOM0, match_opcode, 0 },
	{"c0_sv",      0, INSN_CLASS_I, "d,s,t,j",   MATCH_CUSTOM0_RS1, MASK_CUSTOM0_RS1, match_opcode, 0 },
	{"c1",      0, INSN_CLASS_I, "d,s,j",   MATCH_CUSTOM1, MASK_CUSTOM1, match_opcode, 0 },
	{"c2",      0, INSN_CLASS_I, "d,s,j",   MATCH_CUSTOM2, MASK_CUSTOM2, match_opcode, 0 },
	{"c3",      0, INSN_CLASS_I, "d,s,j",   MATCH_CUSTOM3, MASK_CUSTOM3, match_opcode, 0 },

where ``c0_lv``, ``c0_sv`` are two custom instructions for vector load and store respectively, using the same opcode and the S' instruction type. `c1`, `c2` and `c3` are for the custom instructions, and are using the I' instruction type. Currently, `c1`, `c2` and `c3` are used for the merge, sort and prefix SIMD instruction examples, but can be replaced as well as increased and aliased. For more information on the pre-defined opcodes for custom instructions, and for adding more, see file `riscv-gnu-toolchain/riscv-binutils/include/opcode/riscv-opc.h`. The equivalent files are also included in the `riscv-gnu-toolchain/riscv-gdb`, should this be of use.

In the later versions of the riscv-gnu-toolchain, the custom opcodes need to be added manually inside the file `riscv-gnu-toolchain/riscv-binutils/include/opcode/riscv-opc.h`. For example, the following snippet can be copied from an earlier version of `riscv-opc.h` from [here](https://github.com/riscv-collab/riscv-binutils-gdb/blob/rvv-1.0.x/include/opcode/riscv-opc.h), as shown below.

	#define MATCH_CUSTOM0 0xb
	#define MASK_CUSTOM0  0x707f
	#define MATCH_CUSTOM0_RS1 0x200b
	#define MASK_CUSTOM0_RS1  0x707f	 // ...
	#define MATCH_CUSTOM1 0x2b
	#define MASK_CUSTOM1  0x707f		// ...
	#define MATCH_CUSTOM2 0x5b
	#define MASK_CUSTOM2  0x707f		// ...
	#define MATCH_CUSTOM3 0x7b
	#define MASK_CUSTOM3  0x707f

As before, the toolchain now needs to be recompiled to include the changes:

	$ make newlib -j15

### License

The following license applies to this source code, except on code of other projects like some benchmarks in the Benchmarks folder.


```
Copyright 2021-2024 Philippos Papaphilippou
   
Simodense Software License v1.0

You are free to use, learn from, modify and distribute this creative work under 
  the following conditions:
1. No code, text, or other elements of this work can be used as an input to models
  for the purposes of training. Generative A.I. and language models are prohibited
  from using this work. These do not include search engines and related tools whose
  main purpose is to index and appropriately point to the original source and author.
2. It comes under no warranties.
3. Any modification to the code shall be made available in the style of GNU General
  Public License v2.0 (GPL-2.0), but published under the same license (not GPL-2.0,
  and clause 7 still refers to the original copyright holder).
4. A proper attribution of the author (copyright holder) is required when the work 
  is used. (If applicable, it would be appreciated to cite the related academic
  publication that introduced the work.)
5. It cannot be used for commercial purposes unless specific permission is given by
  the author. 
6. A redistribution shall include this license in its entirety.
7. The author has the right to change this license for future versions of this work,
  as well as to update and clarify the author's original intentions of the current
  version, such as with regard to what is considered "fair use" by the author for
  future entities and technologies.  
```

   
### Related publications

**(cite this)** Philippos Papaphilippou, Paul H. J. Kelly and Wayne Luk "Simodense: a RISC-V softcore optimised for exploring custom SIMD instructions" The International Conference on Field-Programmable Logic and Applications (FPL) 2021 [pdf](http://philippos.info/papers/fpl21simodense.pdf) [link](https://ieeexplore.ieee.org/document/9556386/) [source](http://philippos.info/simodense/) [video](https://youtu.be/hYdqvmoQx3E) [program](https://cfaed.tu-dresden.de/fpl2021/program) [bib](http://philippos.info/papers/simodense.bib)

Philippos Papaphilippou, Paul H. J. Kelly and Wayne Luk "Extending the RISC-V ISA for exploring advanced reconfigurable SIMD instructions" The Fifth Workshop on Computer Architecture Research with RISC-V (CARRV 2021) (co-located with ISCA 2021) [pdf](https://carrv.github.io/2021/papers/CARRV2021_paper_86_Papaphilippou.pdf) [slides](https://carrv.github.io/2021/slides/CARRV2021_slides_86_Papaphilippou.pdf) [source](http://philippos.info/simdsoftcore/) [video](https://carrv.github.io/2021/videos/CARRV2021_full_86_Papaphilippou.html) [program](https://carrv.github.io/2021) [bib](http://philippos.info/papers/simdcarrv.bib)   

Philippos Papaphilippou, Paul H. J. Kelly and Wayne Luk "Demonstrating custom SIMD instruction development for a RISC-V softcore (demo abstract)" The International Conference on Field-Programmable Logic and Applications (FPL) 2021 [pdf](http://philippos.info/papers/fpl21simdemo.pdf) [link](https://ieeexplore.ieee.org/document/9556425) [source](https://github.com/pphilippos/simodense) [video](https://youtu.be/gdD6353bzzo) [bib](http://philippos.info/papers/simdemo.bib)

Philippos Papaphilippou, Myrtle Shah "FPGA-extended General Purpose Computer Architecture" The 18th International Symposium on Applied Reconfigurable Computing (ARC) 2022 [pdf](https://arxiv.org/pdf/2203.10359.pdf) [link](https://doi.org/10.1007/978-3-031-19983-7_7) [source](https://philippos.info/fpga-ext-arch/) *(framework is partly-based on Simodense)* [program](https://nicsefc.ee.tsinghua.edu.cn/detail.html?id=1030) [slides](https://www.researchgate.net/publication/363652284_FPGA-extended_General_Purpose_Computer_Architecture_slides) [video](https://youtu.be/B-UI6G1Cws8) [bib](http://philippos.info/papers/fpgaext.bib)

Philippos Papaphilippou "LUTstructions: Fast-Reconfigurable FPGA-Based Instructions" SBAC-PAD 2026 (in-press) [preprint](https://arxiv.org/abs/2602.20802) [source](https://philippos.info/lutstructions/) *(framework is partly-based on Simodense)*


### Thanks

General comments or feedback on this repository has been provided by: [Tim Todman](http://www.doc.ic.ac.uk/~tjt97/), Hakam Atassi, [Yutaka Nagashima](https://yutakang.github.io/), Ninghao Miao.


