Swift Compiler

The main Swift repository contains the source code for the Swift compiler and standard library, as well as related components such as SourceKit (for IDE integration), the Swift regression test suite, and implementation-level documentation.

The Swift driver repository contains a new implementation of the Swift compiler’s “driver”, which aims to be a more extensible, maintainable, and robust drop-in replacement for the existing compiler driver.

Compiler Architecture

As a whole, the Swift compiler is principally responsible for translating Swift source code into efficient, executable machine code. However, the Swift compiler front-end also supports a number of other tools, including IDE integration with syntax coloring, code completion, and other conveniences. This document provides a high-level description of the major components of the Swift compiler:

  • Parsing: The parser is a simple, recursive-descent parser (implemented in lib/Parse) with an integrated, hand-coded lexer. The parser is responsible for generating an Abstract Syntax Tree (AST) without any semantic or type information, and emits warnings or errors for grammatical problems with the input source.

  • Semantic analysis: Semantic analysis (implemented in lib/Sema) is responsible for taking the parsed AST and transforming it into a well-formed, fully-type-checked form of the AST, emitting warnings or errors for semantic problems in the source code. Semantic analysis includes type inference and, on success, indicates that it is safe to generate code from the resulting, type-checked AST.

  • Clang importer: The Clang importer (implemented in lib/ClangImporter) imports Clang modules and maps the C or Objective-C APIs they export into their corresponding Swift APIs. The resulting imported ASTs can be referred to by semantic analysis.

  • SIL generation: The Swift Intermediate Language (SIL) is a high-level, Swift-specific intermediate language suitable for further analysis and optimization of Swift code. The SIL generation phase (implemented in lib/SILGen) lowers the type-checked AST into so-called “raw” SIL. The design of SIL is described in docs/SIL.rst.

  • SIL guaranteed transformations: The SIL guaranteed transformations (implemented in lib/SILOptimizer/Mandatory) perform additional dataflow diagnostics that affect the correctness of a program (such as a use of uninitialized variables). The end result of these transformations is “canonical” SIL.

  • SIL optimizations: The SIL optimizations (implemented in lib/SILOptimizer/Analysis, lib/SILOptimizer/ARC, lib/SILOptimizer/LoopTransforms, and lib/SILOptimizer/Transforms) perform additional high-level, Swift-specific optimizations to the program, including (for example) Automatic Reference Counting optimizations, devirtualization, and generic specialization.

  • LLVM IR generation: IR generation (implemented in lib/IRGen) lowers SIL to LLVM IR, at which point LLVM can continue to optimize it and generate machine code.