Questions
- Is there an expectation that keypaths significantly slower than functions or straight access with a closure?
- If so, is there anyway to mitigate these discrepancies while still using keypaths?
Context
I'm noticing anywhere between an increased factor of 2-10+ when using keypaths as functions or using dynamic member lookup.
Program 1: Key paths as functions
import AppKit
func arrayClosureTest(_ size: UInt) {
let xs = (0 ... size).map(String.init)
let t0 = CACurrentMediaTime()
let ys = xs.map { $0.count }
let t1 = CACurrentMediaTime()
print("closure test:\n\t\t\(t1 - t0)")
}
func arrayKeyPathTest(_ size: UInt) {
let xs = (0 ... size).map(String.init)
let t0 = CACurrentMediaTime()
let ys = xs.map(\.count)
let t1 = CACurrentMediaTime()
print("key path test:\n\t\t\(t1 - t0)")
}
func main() {
print(String(repeating: "=", count: 50))
let sizes: [UInt] = [1_000, 10_000, 100_000, 1_000_000]
for size in sizes {
arrayClosureTest(size)
arrayKeyPathTest(size)
print(String(repeating: "=", count: 50))
}
}
yields with no optimizations (-O0).
==================================================
closure test:
9.460499859414995e-05
key path test:
0.0006224889948498458
==================================================
closure test:
0.0005262159975245595
key path test:
0.006581138004548848
==================================================
closure test:
0.0057080870028585196
key path test:
0.06060750799952075
==================================================
closure test:
0.05970345200330485
key path test:
0.5920746110059554
==================================================
If I change the optimizations settings to Fastest, Smallest (-Os), then the output is
==================================================
closure test:
0.0001036890025716275
key path test:
0.0008002639951882884
==================================================
closure test:
0.0005304780061123893
key path test:
0.006536139000672847
==================================================
closure test:
0.00698990399541799
key path test:
0.06180560700886417
==================================================
closure test:
0.060244746011449024
key path test:
0.5942129169998225
==================================================
If I change the optimizations settings to Fastest (-Ofast), then the output is
==================================================
closure test:
8.602200250606984e-05
key path test:
0.0006292640027822927
==================================================
closure test:
0.0005449989985208958
key path test:
0.006529158999910578
==================================================
closure test:
0.005752074997872114
key path test:
0.05974736200005282
==================================================
closure test:
0.06013945199083537
key path test:
0.5992421360133449
==================================================
Program 2: @dynamicallyMemberLookup with key paths
import AppKit
@dynamicMemberLookup
struct Identity<A> {
let value: A
subscript<B>(dynamicMember keyPath: KeyPath<A, B>) -> B{
value[keyPath: keyPath]
}
}
func straightAccessTest(_ size: UInt) {
let xs = (0 ... size).map(Identity.init)
let t0 = CACurrentMediaTime()
let ys = xs.map { $0.value.hashValue }
let t1 = CACurrentMediaTime()
print("access test:\n\t\t\(t1 - t0)")
}
func dynamicMemberLookupTest(_ size: UInt) {
let xs = (0 ... size).map(Identity.init)
let t0 = CACurrentMediaTime()
let ys = xs.map { $0.hashValue }
let t1 = CACurrentMediaTime()
print("dynamic member test:\n\t\t\(t1 - t0)")
}
func main() {
print(String(repeating: "=", count: 50))
let sizes: [UInt] = [1_000, 10_000, 100_000, 1_000_000]
for size in sizes {
straightAccessTest(size)
dynamicMemberLookupTest(size)
print(String(repeating: "=", count: 50))
}
}
The output for -O0, -Os and -Ofast were pretty much the same for all three. Here is -Ofast's output:
==================================================
access test:
3.349900362081826e-05
dynamic member test:
0.0005792470037704334
==================================================
access test:
0.00019683199934661388
dynamic member test:
0.005823437997605652
==================================================
access test:
0.0014278379967436194
dynamic member test:
0.057225974000175484
==================================================
access test:
0.015988421000656672
dynamic member test:
0.5222387870016973
==================================================