SysY Compiler - RISC-V Backend Implementation (2025 Compiler System Design Contest)

This project is the preliminary round submission by OUC/Team水军出击 for the 2025 National College Student Computer System Ability Competition - Compiler System Design Contest - Compiler Implementation Contest (2025年全国大学生计算机系统能力大赛-编译系统设计赛-编译系统实现赛).

The compiler translates SysY language (a subset of C) into RISC-V RV64GC assembly code, covering the complete compilation workflow: from lexical and syntax analysis, AST construction, intermediate representation (IR) generation and optimization, to final target assembly code generation.

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🛠 Build and Usage

This project uses CMake as the build system and supports automatically generating the lexer and parser from .l (Flex) and .y (Bison) files.

Dependencies

• CMake ≥ 3.10
• Flex
• Bison
• C++17 compiler (g++ or clang++)

Example: Install Dependencies on Ubuntu

sudo apt update
sudo apt install cmake flex bison g++ -y

Build the Project

git clone -b compile https://github.com/yangpluseven/OUCC2025.git
cd oucc2025
mkdir build && cd build
cmake ..
make -j

The generated executable will be located in the build/ directory with the default name compiler2025-0.

Usage

./build/compiler2025-0 path/to/source.sy -S -o output.s

This command compiles a SysY source file into RISC-V assembly code.

Note: The project includes locally generated parser and lexer files. You may choose to regenerate them or use the provided ones.

To view all available command-line options, use the -h flag:

./build/compiler2025-0 -h

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📁 Project Structure

oucc2025/
├── CMakeLists.txt        # Build configuration
├── .clang-format         # Code formatting configuration
├── README.md             # This file
├── include/              # Public header files
├── src/                  # Source code implementation
│   ├── parser/           # AST traversal & IR generation
│   ├── ir/               # IR system implementation
│   ├── pass/             # Optimization passes
│   ├── riscv/            # IR to RISC-V assembly generation
│   ├── sysy/             # Flex/Bison lexer & parser
│   ├── main.cpp          # Program entry point
├── 3rd/                  # Third-party libraries (fmt, CLI11)
├── generated/            # Auto-generated Flex/Bison code

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🧠 Compiler Architecture Overview

Frontend

• Uses Flex/Bison to build the lexer and parser
• Constructs an Abstract Syntax Tree (AST)
• Implements the Visitor Pattern (accept + visit double dispatch) to traverse the AST and generate intermediate representation (IR)

Intermediate Representation (IR)

• Core design follows LLVM IR but is a simplified subset

• Supports function-level, basic-block-level, and instruction-level optimizations

• All IR objects are managed using smart pointers (e.g., std::unique_ptr)

• Optimizations implemented for the preliminary round include:

  • Dead Code Elimination (DCE)
  • Branch Optimization
  • Constant Folding (CF)
  • Memory to Register Promotion (Mem2Reg)

• Optimizations added for the final round include:

  • Strength Reduction
  • Common Subexpression Elimination (CSE)
  • Move Instruction Reduction
  • Jump Optimization
  • Loop Invariant Code Motion (LICM)

Backend

• Uses a separately designed backend IR (referred to as MIR), closer to RISC-V assembly
• Initially keeps backend IR in Static Single Assignment (SSA) form
• Provides machine related optimizations.
• Applies graph coloring register allocation for mapping virtual registers to physical registers
• After register allocation, SSA form is broken, and appropriate prologue/epilogue code is added to produce the final RISC-V RV64GC assembly

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📦 Third-Party Open Source Tools Used

Name	Purpose	Link
Flex	Lexer generator	https://github.com/westes/flex
Bison	Parser generator	https://www.gnu.org/software/bison/
fmt	Fast C++ formatting lib	https://github.com/fmtlib/fmt
CLI11	Command-line parsing	https://github.com/CLIUtils/CLI11

All these libraries are open-source and have been validated for stability and compatibility on mainstream platforms. The respective copyright notices are marked in the source.