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The following is a gathering of various tips and tricks for writing performance sensitive Swift code.
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 overridding 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 overridding 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 can not 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.
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)
}
| [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. |