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The following document is a gathering of various tips and tricks for writing high-performance Swift code. The intended audience of this document is compiler and standard library developers.
Some of the tips in this document can help improve the quality of your Swift program and make your code less error prone and more readable. Explicitly marking final-classes and class-protocols are two obvious examples. However some of the tips described in this document are unprincipled, twisted and come to solve a specific temporary limitation of the compiler or the language. Many of the recommendations in this document come with trade offs for things like program runtime, binary size, code readability, etc.
The first thing one should always do is to enable optimization. Swift provides three different optimization levels:
-Onone: This is meant for normal development. It performs minimal optimizations and preserves all debug info.-O: This is meant for most production code. The compiler performs aggressive optimizations that can drastically change the type and amount of emitted code. Debug information will be emitted but will be lossy.-Ounchecked: This is a special optimization mode meant for specific libraries or applications where one is willing to trade safety for performance. The compiler will remove all overflow checks as well as some implicit type checks. This is not intended to be used in general since it may result in undetected memory safety issues and integer overflows. Only use this if you have carefully reviewed that your code is safe with respect to integer overflow and type casts.
In the Xcode UI, one can modify the current optimization level as follows:
...
By default Swift compiles each file individually. This allows Xcode to
compile multiple files in parallel very quickly. However, compiling each file
separately prevents certain compiler optimizations. Swift can also compile
the entire program as if it were one file and optimize the program as if it
were a single compilation unit. This mode is enabled using the command
line flag -whole-module-optimization. Programs that are compiled in
this mode will most likely take longer to compile, but may run faster.
This mode can be enabled using the Xcode build setting 'Whole Module Optimization'.
Swift by default is a very dynamic language like Objective-C. Unlike Objective C, Swift gives the programmer the ability to improve runtime performance when necessary by removing or reducing this dynamicism. This section goes through several examples of language constructs that can be used to perform such an operation.
Classes use dynamic dispatch for methods and property accesses by default. Thus
in the following code snippet, a.aProperty, a.doSomething() and
a.doSomethingElse() will all be invoked via dynamic dispatch:
class A {
var aProperty: [Int]
func doSomething() { ... }
dynamic doSomethingElse() { ... }
}
class B : A {
override var aProperty {
get { ... }
set { ... }
}
override func doSomething() { ... }
}
func usingAnA(a: A) {
a.doSomething()
a.aProperty = ...
}
In Swift, dynamic dispatch defaults to indirect invocation through a vtable
[1]. If one attaches the dynamic keyword to the declaration, Swift will
emit calls via Objective-C message send instead. In both cases this is slower
than a direct function call because it prevents many compiler optimizations [2]
in addition to the overhead of performing the indirect call itself. In
performance critical code, one often will want to restrict this dynamic
behavior.
The final keyword is a restriction on a declaration of a class, a method, or
a property such that the declaration cannot be overridden. This implies that the
compiler can emit direct function calls instead of indirect calls. For instance
in the following C.array1 and D.array1 will be accessed directly
[3]. In contrast, D.array2 will be called via a vtable:
final class C {
// No declarations in class 'C' can be overridden.
var array1: [Int]
func doSomething() { ... }
}
class D {
final var array1 [Int] // 'array1' cannot be overridden by a computed property.
var array2: [Int] // 'array2' *can* be overridden by a computed property.
}
func usingC(c: C) {
c.array1[i] = ... // Can directly access C.array without going through dynamic dispatch.
c.doSomething() = ... // Can directly call C.doSomething without going through virtual dispatch.
}
func usingD(d: D) {
d.array1[i] = ... // Can directly access D.array1 without going through dynamic dispatch.
d.array2[i] = ... // Will access D.array2 through dynamic dispatch.
}
Applying the private keyword to a declaration restricts the visibility of
the declaration to the file in which it is declared. This allows the compiler to
be able to ascertain all other potentially overriding declarations. Thus the
absence of any such declarations enables the compiler to infer the final
keyword automatically and remove indirect calls for methods and field accesses
accordingly. For instance in the following, e.doSomething() and
f.myPrivateVar, will be able to be accessed directly assuming E, F
do not have any overriding declarations in the same file:
private class E {
func doSomething() { ... }
}
class F {
private var myPrivateVar : Int
}
func usingE(e: E) {
e.doSomething() // There is no sub class in the file that declares this class.
// The compiler can remove virtual calls to doSomething()
// and directly call A's doSomething method.
}
func usingF(f: F) -> Int {
return f.myPrivateVar
}
An important feature provided by the Swift standard library are the generic containers Array and Dictionary. This section will explain how to use these types in a performant manner.
In Swift, types can be divided into two different categories: value types (structs, enums, tuples) and reference types (classes). A key distinction is that value types cannot be included inside an NSArray. Thus when using value types, the optimizer can remove most of the overhead in Array that is necessary to handle the possibility of the array being backed an NSArray.
Additionally, In contrast to reference types, value types only need reference counting if they contain, recursively, a reference type. By using value types without reference types, one can avoid additional retain, release traffic inside Array.
// Don't use a class here.
struct PhonebookEntry {
var name : String
var number : [Int]
}
var a : [PhonebookEntry]
Keep in mind that there is a trade-off between using large value types and using reference types. In certain cases, the overhead of copying and moving around large value types will outweigh the cost of removing the bridging and retain/release overhead.
If you need an array of reference types and the array does not need to be bridged to NSArray, use ContiguousArray instead of Array:
class C { ... }
var a: ContiguousArray<C> = [C(...), C(...), ..., C(...)]
All standard library containers in Swift are value types that use COW
(copy-on-write) [4] to perform copies instead of explicit copies. In many cases
this allows the compiler to elide unnecessary copies by retaining the container
instead of performing a deep copy. This is done by only copying the underlying
container if the reference count of the container is greater than 1 and the
container is mutated. For instance in the following, no copying will occur when
d is assigned to c, but when d undergoes structural mutation by
appending 2, d will be copied and then 2 will be appended to d:
var c: [Int] = [ ... ] var d = c // No copy will occur here. d.append(2) // A copy *does* occur here.
Sometimes COW can introduce additional unexpected copies if the user is not careful. An example of this is attempting to perform mutation via object-reassignment in functions. In Swift, all parameters are passed in at +1, i.e. the parameters are retained before a callsite, and then are released at the end of the callee. This means that if one writes a function like the following:
func append_one(a: [Int]) -> [Int] {
a.append(1)
return a
}
var a = [1, 2, 3]
a = append_one(a)
a may be copied [5] despite the version of a without one appended to it
has no uses after append_one due to the assignment. This can be avoided
through the usage of inout parameters:
func append_one_in_place(inout a: [Int]) {
a.append(1)
}
var a = [1, 2, 3]
append_one_in_place(&a)
Swift eliminates integer overflow bugs by checking for overflow when performing normal arithmetic. These checks are not appropriate in high performance code where one knows that no memory safety issues can result.
In performance-critical code you can elide overflow checks if you know it is safe.
a : [Int]
b : [Int]
c : [Int]
// Precondition: for all a[i], b[i]: a[i] + b[i] does not overflow!
for i in 0 ... n {
c[i] = a[i] &+ b[i]
}
Swift provides a very powerful abstraction mechanism through the use of generic
types. The Swift compiler emits one block of concrete code that can perform
MySwiftFunc<T> for any T. The generated code takes a table of function
pointers and a box containing T as additional parameters. Any differences in
behavior between MySwiftFunc<Int> and MySwiftFunc<String> are accounted
for by passing a different table of function pointers and the size abstraction
provided by the box. An example of generics:
class MySwiftFunc<T> { ... }
MySwiftFunc<Int> X // Will emit code that works with Int...
MySwiftFunc<String> Y // ... as well as String.
When optimizations are enabled, the Swift compiler looks at each invocation of such code and attempts to ascertain the concrete (i.e. non-generic type) used in the invocation. If the generic function's definition is visible to the optimizer and the concrete type is known, the Swift compiler will emit a version of the generic function specialized to the specific type. This process, called specialization, enables the removal of the overhead associated with generics. Some more examples of generics:
class MyStack<T> {
func push(element: T) { ... }
func pop() -> T { ... }
}
func myAlgorithm(a: [T], length: Int) { ... }
// The compiler can specialize code of MyStack[Int]
var stackOfInts: MyStack[Int]
// Use stack of ints.
for i in ... {
stack.push(...)
stack.pop(...)
}
var arrayOfInts: [Int]
// The compiler can emit a specialized version of 'myAlgorithm' targeted for
// [Int]' types.
myAlgorithm(arrayOfInts, arrayOfInts.length)
The optimizer can only perform specializations if the definition of the generic declaration is visible in the current Module. This can only occur if the declaration is in the same file as the invocation of the generic. NOTE The standard library is a special case. Definitions in the standard library are visible in all modules and available for specialization.
The compiler can only specialize generic code if the call site and the callee function are located in the same compilation unit. One trick that we can use to allow compiler to optimize the callee function is to write code that performs a type check in the same compilation unit as the callee function. The code behind the type check then re-dispatches the call to the generic function - but this time it has the type information. In the code sample below we've inserted a type check into the function "play_a_game" and made the code run hundreds of times faster.
//Framework.swift:
protocol Pingable { func ping() -> Self }
protocol Playable { func play() }
extension Int : Pingable {
func ping() -> Int { return self + 1 }
}
class Game<T : Pingable> : Playable {
var t : T
init (_ v : T) {t = v}
func play() {
for _ in 0...100_000_000 { t = t.ping() }
}
}
func play_a_game(game : Playable ) {
// This check allows the optimizer to specialize the
// generic call 'play'
if let z = game as? Game<Int> {
z.play()
} else {
game.play()
}
}
/// -------------- >8
// Application.swift:
play_a_game(Game(10))
In Swift, values keep a unique copy of their data. There are several advantages to using value-types, like ensuring that values have independent state. When we copy values (the effect of assignment, initialization, and argument passing) the program will create a new copy of the value. For some large values these copies could be time consuming and hurt the performance of the program.
Consider the example below that defines a tree using "value" nodes. The tree nodes contain other nodes using a protocol. In computer graphics scenes are often composed from different entities and transformations that can be represented as values, so this example is somewhat realistic.
protocol P {}
struct Node : P {
var left, right : P?
}
struct Tree {
var node : P?
init() { ... }
}
When a tree is copied (passed as an argument, initialized or assigned) the whole tree needs to be copied. In the case of our tree this is an expensive operation that requires many calls to malloc/free and a significant reference counting overhead.
However, we don't really care if the value is copied in memory as long as the semantics of the value remains.
To eliminate the cost of copying large values adopt copy-on-write behavior. The easiest way to implement copy-on-write is to compose existing copy-on-write data structures, such as Array. Swift arrays are values, but the content of the array is not copied around every time the array is passed as an argument because it features copy-on-write traits.
In our Tree example we eliminate the cost of copying the content of the tree by wrapping it in an array. This simple change has a major impact on the performance of our tree data structure, and the cost of passing the array as an argument drops from being O(n), depending on the size of the tree to O(1).
struct Tree : P {
var node : [P?]
init() {
node = [ thing ]
}
}
There are two obvious disadvantages of using Array for COW semantics. The first problem is that Array exposes methods like "append" and "count" that don't make any sense in the context of a value wrapper. These methods can make the use of the reference wrapper awkward. It is possible to work around this problem by creating a wrapper struct that will hide the unused APIs and the optimizer will remove this overhead, but this wrapper will not solve the second problem. The Second problem is that Array has code for ensuring program safety and interaction with Objective-C. Swift checks if indexed accesses fall within the array bounds and when storing a value if the array storage needs to be extended. These runtime checks can slow things down.
An alternative to using Array is to implement a dedicated copy-on-write data structure to replace Array as the value wrapper. The example below shows how to construct such a data structure:
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 (!isUniquelyReferencedNonObjC(&ref)) {
ref = Ref(newValue)
return
}
ref.val = newValue
}
}
}
The type Box can replace the array in the code sample above.
Swift classes are always reference counted. The swift compiler inserts code
that increments the reference count every time the object is accessed.
For example, consider the problem of scanning a linked list that's
implemented using classes. Scanning the list is done by moving a
reference from one node to the next: elem = elem.next. Every time we move
the reference swift will increment the reference count of the next object
and decrement the reference count of the previous object. These reference
count operations are expensive and unavoidable when using Swift classes.
final class Node {
var next: Node?
var data: Int
...
}
In performance-critical code you can use choose to use unmanaged
references. The Unmanaged<T> structure allows developers to disable
automatic reference counting for a specific reference.
var Ref : Unmanaged<Node> = Unmanaged.passUnretained(Head)
while let Next = Ref.takeUnretainedValue().next {
...
Ref = Unmanaged.passUnretained(Next)
}
Swift can limit protocols adoption to classes only. One advantage of marking protocols as class-only is that the compiler can optimize the program based on the knowledge that only classes satisfy a protocol. For example, the ARC memory management system can easily retain (increase the reference count of an object) if it knows that it is dealing with a class. Without this knowledge the compiler has to assume that a struct may satisfy the protocol and it needs to be prepared to retain or release non-trivial structures, which can be expensive.
If it makes sense to limit the adoption of protocols to classes then mark protocols as class-only protocols to get better runtime performance.
protocol Pingable : class { func ping() -> Int }
| [1] | A virtual method table or 'vtable' is a type specific table referenced by instances that contains the addresses of the type's methods. Dynamic dispatch proceeds by first looking up the table from the object and then looking up the method in the table. |
| [2] | This is due to the compiler not knowing the exact function being called. |
| [3] | i.e. a direct load of a class's field or a direct call to a function. |
| [4] | Explain what COW is here. |
| [5] | In certain cases the optimizer is able to via inlining and ARC optimization remove the retain, release causing no copy to occur. |