Chester Compiler Backend Architecture

Overview

This document outlines the backend architecture of the Chester compiler system. The backend is responsible for transforming Chester’s internal representation into executable code for various target platforms.

Backend Pipeline

The Chester compiler backend follows a multi-phase code generation pipeline:

Core Representation → Target AST Generation → Optimization → Code Generation → Executable Artifacts

Backend Components

The backend is composed of several key components:

Target-Specific AST Generation

Each target platform has a specialized AST generation phase:

• Target AST Construction: Builds an AST specific to the target language
• Type Mapping: Maps Chester types to target language types
• Effect Translation: Converts Chester effects to target language constructs
• Standard Library Binding: Connects to platform-specific libraries

Optimization Engine

The backend applies target-specific optimizations:

• Dead Code Elimination: Removes unused code
• Constant Folding: Evaluates constant expressions at compile time
• Inlining: Replaces function calls with function bodies where beneficial
• Specialization: Creates specialized versions of generic functions
• Tail Call Optimization: Optimizes tail-recursive calls

Code Generation

The final phase transforms the optimized target AST to executable code:

• Pretty Printing: Generates formatted source code
• Native Code Generation: For targets that compile directly to machine code
• Bytecode Generation: For VM-based targets like the JVM
• Source Maps: Generates debugging information

Supported Compiler Targets

Chester supports multiple compiler targets, each with its own backend implementation:

JavaScript/TypeScript

The JavaScript/TypeScript target (compiler/shared/src/main/scala/chester/targets/js/AST.scala) enables running Chester programs in web browsers and Node.js.

Architecture

The JavaScript backend consists of:

1. AST Nodes: Comprehensive representation of JavaScript/TypeScript constructs
2. Type Translator: Maps Chester types to TypeScript type annotations
3. Effect Handler: Implements Chester’s effect system using JavaScript promises
4. Module System: Generates ES module exports and imports

Key Features

• Complete JavaScript language support
• TypeScript type annotations
• ECMAScript module system
• Web API integration
• Sourcemap generation for debugging

JS AST Structure

The JavaScript AST is implemented as a set of case classes:

sealed trait ASTNode extends ToDoc {
  val meta: Option[Meta]
}

sealed trait Expression extends ASTNode
sealed trait Statement extends ASTNode
sealed trait Declaration extends Statement
sealed trait TypeAnnotation extends ASTNode

Each node includes a toDoc method for pretty printing and source generation.

JVM (Java/Scala)

The JVM target enables integration with the Java ecosystem and leverages the JVM runtime.

Architecture

The JVM backend consists of:

1. Bytecode Generation: Direct generation of JVM bytecode
2. Class Builder: Creates JVM class files
3. Runtime Library: Core runtime support for Chester on JVM
4. Java Interop: Enables calling Java code from Chester

Key Features

• Java interoperability
• Scala library integration
• JVM optimizations
• Access to the Java standard library
• Advanced JVM optimizations (inlining, specialization)

Native (Planned)

A native code target is planned for high-performance applications.

Potential Architecture

The native backend will likely include:

1. LLVM IR Generation: Translation to LLVM intermediate representation
2. Native Runtime: Minimal runtime support library
3. ABI Compatibility: Interoperability with C/C++ code
4. Platform Support: Cross-compilation for different CPU architectures

Planned Features

• LLVM-based code generation
• Native performance
• Low-level memory control
• System programming capabilities
• Cross-platform support

Type System Mapping

Chester’s rich type system needs careful mapping to target language types:

Effects Handling

Chester’s effect system is implemented differently for each target language:

• JavaScript/TypeScript: Using promises or custom effect handlers
• JVM: Using exceptions and monadic structures
• Native: Using error codes or custom effect handling

Implementation Example: JavaScript Backend

JavaScript/TypeScript AST Example

The JavaScript target provides a good example of target-specific AST:

// Example: Function declaration in JavaScript AST
FunctionDeclaration(
  id = Some(Identifier("greet")),
  params = List(Parameter(TypedIdentifier("name", StringTypeAnnotation()))),
  returnType = Some(StringTypeAnnotation()),
  body = BlockStatement(List(
    ReturnStatement(Some(
      BinaryExpression(
        BinaryOperator.Plus,
        StringLiteral("Hello, "),
        Identifier("name")
      )
    ))
  ))
)

This represents the TypeScript function:

function greet(name: string): string {
  return "Hello, " + name;
}

JavaScript AST Node Categories

The JavaScript AST supports a wide range of node types:

Expressions

• Literals: Numbers, strings, booleans, null, BigInt, RegExp
• Identifiers: Named references (typed and untyped)
• Operators: Binary, logical, assignment, unary, update
• Function Expressions: Regular functions and arrow functions
• Object and Array Expressions: Object literals and array literals
• Class Expressions: Class definitions with inheritance and method definitions

Statements

• Block Statements: Groups of statements
• Expression Statements: Expressions used as statements
• Control Flow Statements: if/else, while, do-while, for, switch
• Declaration Statements: let, const, var declarations

TypeScript Features

• Type Annotations: For variables, parameters, return types
• Interface and Type Declarations: For defining complex types
• Generics: Type parameters for functions and classes
• Union and Intersection Types: Type combinations

Build System Integration

The Chester compiler backend integrates with build systems through:

• SBT Plugin: For JVM builds
• NPM Package: For JavaScript/TypeScript integration
• CLI Interface: For command-line usage

Future Directions

Planned improvements to the compiler backend include:

• WebAssembly Support: Direct compilation to WebAssembly
• More Native Targets: Support for various native platforms
• Interoperability Enhancements: Better interop with target languages
• Performance Optimizations: Target-specific optimizations
• Cross-Compilation: Single-command compilation to multiple targets
• Advanced Optimizations: Target-specific performance improvements

References

• JavaScript AST is inspired by the ESTree Spec
• JVM codegen draws from Scala 3 compiler techniques
• LLVM-based compilation follows the LLVM Language Reference