Quick Hacks: Swift Intermediate Language

The Swift compiler's secret sauce

Welcome to Quick Hacks, my advanced tips series exclusive to paid subscribers. You’ll also gain exclusive early access to my long-form articles.

The Swift Compiler is a mysterious multi-headed hydra. It takes your code through a many-step journey from hello world to machine code:

• First, it parses your code into tokens on an abstract syntax tree. It performs semantic analysis on this data structure in real-time, checking types and emitting compiler warnings.

• Next, it generates SIL, Swift Intermediate Language, and optimises the result.

• Finally, it’s converted into LLVM Intermediate Representation; which allows it to be handed off to the LLVM toolchain. This performs more optimisations and allows the resulting to machine code to be compiled for any chip architecture.

The Swift Intermediate Language generation step, and subsequent optimisations, makes Swift special, and goes a long way towards its eponymous speed.

The SIL optimisations might include:

• Performing devirtualisation — this replaces table dispatch with faster static dispatch.

• Specialisation — Generating specific versions of generic functions to further avoid dynamic dispatch.

• Dead code elimination to prevent code that’s never executed from bloating the binary.

• Inlining code to avoid the stack pointer jumping to a different function; reducing this small amount of overhead.