jzero-rs

A compiler + bytecode VM for Jzero — a small subset of Java — written entirely in Rust.

This project follows the book Build Your Own Programming Language (Edition 2) by Clinton L. Jeffery, replacing the original Java/Unicon tooling with idiomatic Rust equivalents.

Using as a library

Add to your Cargo.toml:

[dependencies]
jzero = "0.1.0"

Then use the Compiler API:

use jzero::Compiler;

let out = Compiler::new()
    .source(r#"
        public class hello {
            public static void main(String argv[]) {
                System.out.println("hello, jzero!");
            }
        }
    "#)
    .run(&[])
    .unwrap();

println!("{}", out.stdout); // hello, jzero!

What is Jzero?

Jzero is a strict subset of Java designed for teaching compiler construction. Every valid Jzero program is also a valid Java program. It supports a minimal but complete set of features: classes, methods, control flow, basic types (int, double, bool, string), arrays, and simple I/O.

Roadmap

Chapters 10 (IDE/syntax coloring), 11 (transpiler), 14 (native code), 16 (domain control structures), and 17 (garbage collection) were intentionally skipped as detours unrelated to the core compiler pipeline or beyond the scope of Jzero.

Architecture

jzero-rs/
├── Cargo.toml
├── crates/
│   ├── jzero/              # Public facade crate (published to crates.io)
│   ├── jzero-lexer/        # Lexical analysis (Logos)
│   ├── jzero-parser/       # Parsing & syntax tree construction (LALRPOP)
│   ├── jzero-ast/          # Syntax tree data structures & DOT output
│   ├── jzero-symtab/       # Symbol table types (SymTab, SymTabEntry, TypeInfo)
│   ├── jzero-semantic/     # Symbol table construction & type checking
│   ├── jzero-codegen/      # TAC + bytecode generation
│   ├── jzero-vm/           # Bytecode interpreter + string pool
│   └── jzero-cli/          # CLI tool (j0, not published)
└── tests/
    └── examples/
        ├── hello.java           # Minimal hello-world
        ├── hello_loop.java      # Loop + array + I/O
        ├── concat.java          # String concatenation
        ├── countdown.java       # While loop + argv.length
        ├── fizzbuzz.java        # Nested if/else + modulo + String.valueOf
        └── greet.java           # String concatenation + String.valueOf

Clean one-way dependency chain:

jzero-lexer → jzero-symtab → jzero-ast → jzero-parser → jzero-semantic → jzero-codegen → jzero-vm → jzero

Tool Mapping

Building & Testing

# Build everything
cargo build

# Run all tests (single-threaded to avoid global ID counter races)
cargo test --workspace -- --test-threads=1

# Parse a file and visualize the syntax tree
cargo run --bin j0 -- tests/examples/hello.java --png

# Print TAC intermediate code (Chapter 9)
cargo run --bin j0 -- tests/examples/hello_loop.java --codegen

# Compile to bytecode and print assembler listing (Chapter 13)
cargo run --bin j0 -- tests/examples/hello_loop.java --bytecode

# Compile and execute in the VM (Chapters 12+13)
cargo run --bin j0 -- tests/examples/hello_loop.java --run a b c d e

# String concatenation example (Chapter 15)
cargo run --bin j0 -- tests/examples/concat.java --run

End-to-End Examples

Loop + array access

// tests/hello_loop.java
public class hello_loop {
   public static void main(String argv[]) {
      int x;
      x = argv.length;
      x = x + 2;
      while (x > 3) {
         System.out.println("hello, jzero!");
         x = x - 1;
      }
   }
}

Running j0 tests/examples/hello_loop.java --run a b c d e executes the program end-to-end:

hello, jzero!
hello, jzero!
hello, jzero!
hello, jzero!
no errors

String concatenation

// tests/concat.java
public class concat {
    public static void main(String argv[]) {
        String s;
        s = "hello, " + "jzero!";
        System.out.println(s);
    }
}

Running j0 tests/examples/concat.java --run produces:

hello, jzero!
no errors

Semantic Analysis (Chapters 6–8)

Symbol table construction (Ch. 6)

A two-pass tree traversal builds one SymTab per scope:

• First pass registers all field and method signatures, enabling forward references within a class.
• Second pass walks method bodies to insert parameters and local variables.
• System.out.println is pre-registered in the global scope.
• Each Tree node gets its stab field set to the nearest enclosing scope (inherited, top-down).

Type checking (Ch. 7–8 + 15)

Three cooperative functions — check_type, check_kids, check_types — perform a selective post-order traversal of method bodies.

Key design decisions:

• TypeInfo enum replaces the book's typeinfo class hierarchy — Base, Array, Method, Class variants with exhaustive matching.
• "return" dummy symbol — each method's stab gets a "return" entry with the declared return type. ReturnStmt nodes look it up directly.
• mkcls pass — after symbol tables are fully built, a dedicated pass constructs a complete ClassType for each class.
• String concatenation — String + String is allowed by the type checker and emits SADD in the TAC layer. String - String is rejected.

Intermediate Code Generation (Chapter 9)

Five passes over the typed syntax tree produce TAC:

assign_addresses → genfirst → genfollow → gentargets → gencode → emit

Bytecode Generation + VM (Chapters 12 + 13)

Bytecode format

Each instruction is a fixed 8-byte word: [opcode:1][region:1][operand:6, little-endian].

The .j0 binary file layout:

Word 0:   magic   "Jzero!!\0"
Word 1:   version "1.0\0\0\0\0\0"
Word 2:   first-instruction word offset
Word 3…:  data section (string literals, NUL-terminated)
Word N…:  startup sequence + instructions

The startup sequence calls main with the real argc (number of CLI arguments passed after --run), so argv.length behaves correctly without hardcoding.

Instruction set (27 opcodes)

HALT NOOP ADD SUB MUL DIV MOD NEG PUSH POP CALL RETURN GOTO BIF
LT LE GT GE EQ NEQ LOCAL LOAD STORE
SPUSH SPOP SADD ITOS

SPUSH, SPOP, SADD were added in Chapter 15 for string operations. ITOS converts an integer to a string pool key (String.valueOf).

TAC → bytecode translation

Label resolution is two-pass: pass 1 records byte offsets, pass 2 patches branch targets. All GOTO/BIF targets are relocated by code_base_bytes to be absolute offsets from word 0.

VM calling convention

Saved registers (ip, bp, fn_slot) are kept in an off-stack call_stack: Vec<(usize, i64, i64)>, leaving the data stack clean for locals:

stack[bp+0] = fn_addr  (loc:0)
stack[bp+1] = arg0     (loc:8)
stack[bp+2] = local0   (loc:16)
…

LOCAL n pre-allocates all local slots at function entry so expression temporaries never overwrite them.

String Operations (Chapter 15)

String concatenation (+) is implemented via a runtime StringPool:

• SPUSH offset — reads a NUL-terminated string from the data section at offset (or resolves a pool key if the value is negative), interns it in the pool, and pushes a negative integer key onto the stack.
• SADD — pops two pool keys, concatenates the strings, interns the result, and pushes the new key.
• SPOP dst — pops a pool key and stores it in the destination stack slot.
• ITOS — pops an integer, converts it to a decimal string via String.valueOf, interns it in the pool, and pushes the pool key.
• do_println — resolves its argument via resolve_string(), which handles both data-section offsets (≥ 0) and pool keys (< 0) transparently.

Pool keys are negative integers (-1, -2, …) so they are visually distinct from data-section offsets on the stack.

Parser Design Notes

Why LALRPOP over grmtools/lrpar? The original grammar has inherent LALR(1) ambiguities. grmtools resolved conflicts silently in ways that broke dotted method calls like System.out.println(...). LALRPOP's LR(1) lane table algorithm handles more grammars without conflicts, and its explicit conflict reporting made it easier to restructure the grammar correctly.

References

• Build Your Own Programming Language (Edition 2) — Clinton L. Jeffery
• Logos — fast lexer generator for Rust
• LALRPOP — LR(1) parser generator for Rust