Roadmap

1. What is 🐮?
2. Implementing 🐮
3. The Swift Standard Library
4. Builtins
5. The Swift Compiler
6. The Abstract Syntax Tree
7. Swift Intermediate Language
8. LLVM Intermediate Representation
9. The Swift Runtime
10. SwiftShims
11. The isKnownUniquelyReferenced Function
12. Conclusion

What is 🐮?

For those unfamiliar, 🐮 enhances the efficiency of Swift structs by combining the advantages of value semantics with the minimal memory usage typically associated with reference semantics. Structs using 🐮 store their data in a heap memory buffer. When a struct is duplicated, only a lightweight pointer to this data is copied, allowing multiple instances to share the same underlying memory—this represents reference semantics.

Upon modification of the data, value semantics are activated. The struct allocates a new memory buffer, transfers the updated data there, and points to this new buffer, leaving the original memory block intact.

🐮 in the Swift Standard Library

Numerous core types in the Swift Standard Library, such as arrays, sets, and dictionaries, leverage this optimization. When utilizing these structures, developers can benefit from the underlying 🐮 optimization without any additional effort. Additionally, you can implement 🐮 in your own custom types!

Implementing 🐮

Experienced iOS engineers often share insights on how to create your own types that utilize 🐮. Here’s a sample implementation inspired by Apple's Swift Optimization Tips:

final class Ref<T> {

var val: T

init(_ v: T) { val = v }

}

struct Box<T> {

var ref: Ref<T>

init(_ x: T) { ref = Ref(x) }

var value: T {

get { return ref.val }

set {

if !isKnownUniquelyReferenced(&ref) {

ref = Ref(newValue)

return

}

ref.val = newValue

}

}

}

In this example, the struct Box serves as a wrapper for type T, which is contained within an instance of the Ref class. This class functions as a reference type, pointing to a memory buffer holding the data of T.

Getter and Setter

When accessing the data within Box, we retrieve it from the Ref class. However, when modifying this data, the outcome hinges on whether the Box instance has been copied. If there is a single instance of Box, the data can be mutated directly. Conversely, if multiple instances exist, the Box wrapper is duplicated, prompting a new Ref instance to be created with the updated data, while the original references remain unaltered.

isKnownUniquelyReferenced

The function isKnownUniquelyReferenced plays a pivotal role in enabling this optimization. To grasp its functionality, let’s explore its implementation.

The Swift Standard Library

First, we should download the Swift source code and search for isKnownUniquelyReferenced to examine its implementation. We find it located in ManagedBuffer.swift, under the definition of ManagedBufferPointer. Here’s the implementation:

@inlinable

public func isKnownUniquelyReferenced(_ object: inout T) -> Bool {

return _isUnique(&object)

}

Simple yet effective!

Builtins

Let's take a moment to investigate what _isUnique does. It's defined close by in Builtin.swift:

@usableFromInline @_transparent

internal func _isUnique(_ object: inout T) -> Bool {

return Bool(Builtin.isUnique(&object))

}

The documentation from Apple is always enlightening! The Builtin.isUnique function is where the intrigue deepens.

The Swift Compiler

The Swift Compiler processes Swift source files into efficient machine code, following a pipeline with distinct optimization stages.

The compilation process begins with parsing .swift files into an Abstract Syntax Tree (AST), a structured representation that simplifies further processing. Following semantic analysis, the compiler generates Swift Intermediate Language (SIL) from the AST, which is then optimized before being transformed into LLVM Intermediate Representation (LLVM IR).

Our Journey with isKnownUniquelyReferenced

Now that we understand the compilation process, let's return to isKnownUniquelyReferenced. We can explore how the isUnique Builtin function is applied within the AST and how it transitions through SIL and LLVM IR.

The Abstract Syntax Tree

The isUnique method is synthesized in the AST and adds functionality to the Swift Standard Library, allowing it to be invoked like a standard Swift function.

Swift Intermediate Language

After constructing the AST, the Swift Compiler generates Swift Intermediate Language. The journey continues as we locate the SIL Generation library for the isUnique Builtin function.

LLVM Intermediate Representation

Finally, we reach the LLVM IR stage where the isUnique function is translated into low-level instructions, setting the stage for its execution in the Swift Runtime.

The Swift Runtime

The Swift Runtime underpins the execution of Swift programs, managing dynamic dispatch, error handling, and memory operations.

By examining the LLVM IR instruction for isUnique, we find it leads to the Swift Runtime ABI, which ultimately checks the reference count of an object.

Conclusion

What began as a simple quest to understand the isKnownUniquelyReferenced function evolved into a deep exploration of the Swift Standard Library, compiler, and runtime. Each layer of abstraction revealed the intricate workings behind the scenes.

I trust you found this journey enlightening and that it enhances your understanding of 🐮, preparing you for your next iOS interview. If you feel I overlooked any crucial elements or have suggestions for future explorations, please share in the comments!