为什么连续遍历一个Structs数组比一个类数组更快？

Question

为什么连续遍历一个Structs数组比一个类数组更快？

Epi*_*yte 5 arrays optimization performance struct swift

我正在开发一个使用Swift的游戏,我有一个静态的位置数据数组,我用它来在游戏循环中进行处理.我最初使用一个Structs数组来保存这些数据,但我决定切换到类,所以我可以使用引用.但是在进行更改和分析之后,我注意到CPU在处理此数据的方法上花费的时间比在使用Structs时花费的时间多得多.

所以我决定创建一个简单的测试来看看发生了什么.

final class SomeClass {}
struct SomeStruct {}

let classes = [
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
]

let structs = [
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
]

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func test1() {
    for i in 0...10000000 {
        for s in classes {}
    }
}

func test2() {
    for i in 0...10000000 {
        for s in structs {}
    }
}

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Test1需要15.4722579717636 s而Test2仅需0.276068031787872 s.通过结构阵列连续迭代的速度提高了56倍.所以我的问题是,这是为什么？我正在寻找一个详细的答案.如果我不得不猜测,我会说结构本身按顺序存储在内存中,而类只存储为地址.所以他们每次都需要被解除引用.但话说回来,每次都不需要复制结构吗？

旁注:两个数组都很小,但我不断迭代它们.如果我将代码更改为迭代一次但是使数组非常大,如下所示:

for i in 0...10000000 {
   structs.append(SomeStruct())
   classes.append(SomeClass())
}
func test1() {
    for s in classes {}
}

func test2() {
    for s in structs {}
}

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然后我得到以下结果:Test1需要0.841085016727448 s而Test2需要0.00960797071456909 s.结构需要快88倍.

我正在使用OS X版本构建,并设置了优化级别 Fastest,Smallest [-Os]

编辑

根据要求,我编辑了这个问题,以包含一个测试,其中结构和类不再是空的.它们使用我在游戏中使用的相同属性.仍然没有什么区别.结构仍然快得多,我不知道为什么.希望有人能提供答案.

import Foundation

final class StructTest {
    let surfaceFrames = [
        SurfaceFrame(a: SurfacePoint(x: 0, y: 410), b: SurfacePoint(x: 0, y: 400), c: SurfacePoint(x: 875, y: 410), surfaceID: 0, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 880, y: 304), b: SurfacePoint(x: 880, y: 294), c: SurfacePoint(x: 962, y: 304), surfaceID: 1, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 787, y: 138), b: SurfacePoint(x: 791, y: 129), c: SurfacePoint(x: 1031, y: 248), surfaceID: 2, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 523, y: 138), b: SurfacePoint(x: 523, y: 128), c: SurfacePoint(x: 806, y: 144), surfaceID: 3, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1020, y: 243), b: SurfacePoint(x: 1020, y: 233), c: SurfacePoint(x: 1607, y: 241), surfaceID: 4, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1649, y: 304), b: SurfacePoint(x: 1649, y: 294), c: SurfacePoint(x: 1731, y: 305), surfaceID: 5, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1599, y: 240), b: SurfacePoint(x: 1595, y: 231), c: SurfacePoint(x: 1852, y: 128), surfaceID: 6, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1807, y: 141), b: SurfacePoint(x: 1807, y: 131), c: SurfacePoint(x: 2082, y: 138), surfaceID: 7, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 976, y: 413), b: SurfacePoint(x: 976, y: 403), c: SurfacePoint(x: 1643, y: 411), surfaceID: 8, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1732, y: 410), b: SurfacePoint(x: 1732, y: 400), c: SurfacePoint(x: 2557, y: 410), surfaceID: 9, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 2130, y: 490), b: SurfacePoint(x: 2138, y: 498), c: SurfacePoint(x: 2109, y: 512), surfaceID: 10, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1598, y: 828), b: SurfacePoint(x: 1597, y: 818), c: SurfacePoint(x: 1826, y: 823), surfaceID: 11, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 715, y: 826), b: SurfacePoint(x: 715, y: 816), c: SurfacePoint(x: 953, y: 826), surfaceID: 12, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 840, y: 943), b: SurfacePoint(x: 840, y: 933), c: SurfacePoint(x: 920, y: 943), surfaceID: 13, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1005, y: 1011), b: SurfacePoint(x: 1005, y: 1001), c: SurfacePoint(x: 1558, y: 1011), surfaceID: 14, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1639, y: 943), b: SurfacePoint(x: 1639, y: 933), c: SurfacePoint(x: 1722, y: 942), surfaceID: 15, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1589, y: 825), b: SurfacePoint(x: 1589, y: 815), c: SurfacePoint(x: 1829, y: 825), surfaceID: 16, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 0, y: 0), b: SurfacePoint(x: 1, y: 1), c: SurfacePoint(x: 2, y: 2), surfaceID: 17, dynamic:true)
    ]

    func run() {
        let startTime = CFAbsoluteTimeGetCurrent()
        for  i in 0 ... 10000000 {
            for s in surfaceFrames {

            }
        }
        let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
        println("Time elapsed \(timeElapsed) s")
    }
}



struct SurfacePoint {
    var x,y: Int
}
struct SurfaceFrame {
    let a,b,c :SurfacePoint
    let surfaceID: Int
    let dynamic: Bool
}

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import Foundation

final class ClassTest {
    let surfaceFrames = [
        SurfaceFrame(a: SurfacePoint(x: 0, y: 410), b: SurfacePoint(x: 0, y: 400), c: SurfacePoint(x: 875, y: 410), surfaceID: 0, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 880, y: 304), b: SurfacePoint(x: 880, y: 294), c: SurfacePoint(x: 962, y: 304), surfaceID: 1, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 787, y: 138), b: SurfacePoint(x: 791, y: 129), c: SurfacePoint(x: 1031, y: 248), surfaceID: 2, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 523, y: 138), b: SurfacePoint(x: 523, y: 128), c: SurfacePoint(x: 806, y: 144), surfaceID: 3, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1020, y: 243), b: SurfacePoint(x: 1020, y: 233), c: SurfacePoint(x: 1607, y: 241), surfaceID: 4, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1649, y: 304), b: SurfacePoint(x: 1649, y: 294), c: SurfacePoint(x: 1731, y: 305), surfaceID: 5, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1599, y: 240), b: SurfacePoint(x: 1595, y: 231), c: SurfacePoint(x: 1852, y: 128), surfaceID: 6, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1807, y: 141), b: SurfacePoint(x: 1807, y: 131), c: SurfacePoint(x: 2082, y: 138), surfaceID: 7, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 976, y: 413), b: SurfacePoint(x: 976, y: 403), c: SurfacePoint(x: 1643, y: 411), surfaceID: 8, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1732, y: 410), b: SurfacePoint(x: 1732, y: 400), c: SurfacePoint(x: 2557, y: 410), surfaceID: 9, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 2130, y: 490), b: SurfacePoint(x: 2138, y: 498), c: SurfacePoint(x: 2109, y: 512), surfaceID: 10, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1598, y: 828), b: SurfacePoint(x: 1597, y: 818), c: SurfacePoint(x: 1826, y: 823), surfaceID: 11, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 715, y: 826), b: SurfacePoint(x: 715, y: 816), c: SurfacePoint(x: 953, y: 826), surfaceID: 12, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 840, y: 943), b: SurfacePoint(x: 840, y: 933), c: SurfacePoint(x: 920, y: 943), surfaceID: 13, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1005, y: 1011), b: SurfacePoint(x: 1005, y: 1001), c: SurfacePoint(x: 1558, y: 1011), surfaceID: 14, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1639, y: 943), b: SurfacePoint(x: 1639, y: 933), c: SurfacePoint(x: 1722, y: 942), surfaceID: 15, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 1589, y: 825), b: SurfacePoint(x: 1589, y: 815), c: SurfacePoint(x: 1829, y: 825), surfaceID: 16, dynamic:false),
        SurfaceFrame(a: SurfacePoint(x: 0, y: 0), b: SurfacePoint(x: 1, y: 1), c: SurfacePoint(x: 2, y: 2), surfaceID: 17, dynamic:true)
    ]

    func run() {
        let startTime = CFAbsoluteTimeGetCurrent()
        for  i in 0 ... 10000000 {
            for s in surfaceFrames {

            }
        }
        let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
        println("Time elapsed \(timeElapsed) s")
    }
}



struct SurfacePoint {
    var x,y: Int
}
final class SurfaceFrame {
    let a,b,c :SurfacePoint
    let surfaceID: Int
    let dynamic: Bool

    init(a: SurfacePoint, b: SurfacePoint, c: SurfacePoint, surfaceID: Int, dynamic: Bool) {
        self.a = a
        self.b = b
        self.c = c
        self.surfaceID = surfaceID
        self.dynamic = dynamic
    }
}

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在此测试中,类使用了14.5261079668999 s,而使用结构的测试仅花费了0.310304999351501 s.结构快了47倍.

Answer 1

Epi*_*yte 2

正如 Martin R 所建议的，我对这两个测试进行了分析，实际上，保留/释放调用使得遍历类数组比遍历结构数组慢得多。需要明确的是，这是我运行的测试。

import Foundation

final class SomeClass {}

struct SomeStruct {}

var classes = [
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
    SomeClass(),
]
var structs = [
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
    SomeStruct(),
]

let startTime = CFAbsoluteTimeGetCurrent()
/*
structTest()
classTest()
*/
let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
println("Time elapsed \(timeElapsed) s")

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func structTest() {
    for i in 0 ... 1000000 {
        for e in structs {}
    }
}

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func classTest() {
    for i in 0 ... 1000000 {
        for e in classes {}
    }
}

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以下是使用仪器对两项测试进行分析的图片。只需将运行时间相加就可以看到，类测试在每次迭代期间几乎将所有时间都花在了保留/释放上。我很想知道 Swift 2.0 如何处理这个问题。

结构体在此输入图像描述

课程在此输入图像描述

因此，出于好奇，我想如果我可以通过直接在数组上执行指针算术来绕过保留/释放调用，会发生什么（旁注：我建议您永远不要在真正的应用程序中这样做）。所以我创建了最后一个测试。然而，在这个测试中，我不会多次迭代数组，而是创建一个大数组并迭代一次，因为无论如何这是大部分开销发生的地方。我还决定访问此测试中的属性，以减少优化中的歧义。

这是最终测试的结果：

对大型 Struct 数组进行一次迭代：1.00037097930908 s
对大型 Class 数组进行一次迭代：11.3165299892426 s
使用指针算术对大型 Struct 数组进行一次迭代：0.773443996906281 s
使用指针算术对大型 Class 数组进行一次迭代：2.81995397806168 s

下面是测试的代码。

final class SomeClass {
    var a: Int
    init(a: Int) {
        self.a = a
    }
}
struct SomeStruct {
    var a: Int
    init(a: Int) {
        self.a = a
    }
}

var classes: [SomeClass] = []
var structs: [SomeStruct] = []

var total: Int = 0

for i in 0 ... 100000000 {
    classes.append(SomeClass(a:i))
    structs.append(SomeStruct(a:i))
}

let startTime = CFAbsoluteTimeGetCurrent()
/*structTest()
classTest()
structTestPointer()
classTestPointer()*/
let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
println("Time elapsed \(timeElapsed) s")

func structTest() {
    for someStruct in structs {
        let a = someStruct.a
        total = total &+ a
    }
}

func structTestPointer() {
    var pointer = UnsafePointer<SomeStruct>(structs)
    for j in 0 ..< structs.count {
        let someStruct = pointer.memory
        let a = someStruct.a
        total = total &+ a
        pointer++
    }
}

func classTest() {
    for someClass in classes {
        let a = someClass.a
        total = total &+ a
    }
}

func classTestPointer() {
    var pointer = UnsafePointer<SomeClass>(classes)
    for j in 0 ..< classes.count {
        let someClass = pointer.memory
        let a = someClass.a
        total = total &+ a
        pointer++
    }
}

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