Open Radar

 
Summary:
didSet introduces a slight performance hit most of the time, but when you’re assigning a variable of Dictionary type it gets pathologically slow.


Steps to Reproduce:
1. Compile and run this code with -O or -Ounchecked:

struct S {
	var v: [Int: ()] = [:] { didSet {} }
}

var s = S()

for i in 0..<100000 {
	s.v[i] = ()
}


Expected Results:
I expected it to return more or less interactively; n here is relatively small.


Actual Results:
Resonance:Desktop $ xcrun swiftc -Ounchecked didSet-performance.swift && time ./didSet-performance

real	3m3.603s
user	3m3.359s
sys	0m0.189s


Regression:
Contrast without didSet:
Resonance:Desktop $ xcrun swiftc -Ounchecked didSet-performance.swift && time ./didSet-performance

real	0m0.027s
user	0m0.022s
sys	0m0.004s

Or with an array and didSet:
Resonance:Desktop $ xcrun swiftc -Ounchecked didSet-performance.swift && time ./didSet-performance

real	0m2.912s
user	0m2.879s
sys	0m0.030s

Or with an array but without didSet:
Resonance:Desktop $ xcrun swiftc -Ounchecked didSet-performance.swift && time ./didSet-performance

real	0m0.008s
user	0m0.003s
sys	0m0.003s


Notes:
N/A