Building an LLVM-based Toy Compiler in Rust

Writing a compiler from scratch gives you a deep, visceral understanding of what CPU instructions actually execute when high-level code runs. In this article, I want to walk through the architectural phases of constructing a modern compiler backend using LLVM and Rust.

Pipeline Architecture

Every production-grade compiler pipeline decomposes the translation process into three fundamental stages:

1. Frontend: Lexical analysis (tokenization) and parsing into an Abstract Syntax Tree (AST).
2. Middle-end: Type validation, symbol resolution, and high-level intermediate representation (IR) optimizations.
3. Backend: Lowering the AST into LLVM IR, running SSA optimization passes, and generating machine code for specific architectures (x86_64, AArch64, RISC-V).

Step 1: Lexing and Recursive Descent

Rust's pattern matching makes writing parser combinators or recursive descent parsers enjoyable and safe. We define our tokens using an enum:

With clean token definitions, we parse statements into strongly typed AST nodes:

Step 2: Emitting LLVM IR

Using the inkwell crate (safe Rust wrappers for LLVM), we can initialize an LLVM Context, Module, and IRBuilder:

When visiting a binary addition expression, lowering is straightforward:

Step 3: Architecture-Specific Code Generation

Because LLVM abstracts away machine architecture details while providing target machines, target triples like riscv64gc-unknown-linux-gnu or x86_64-unknown-linux-gnu allow our frontend to produce optimized native assembly with zero target-specific rewrite:

Key Takeaways

• Clear separation between AST representation and code emission keeps the codebase maintainable.
• LLVM's SSA form handles dead-code elimination, loop unrolling, and register allocation automatically.
• Writing your own compiler is the best way to master low-level computing principles.