[SR-6252] JSONDecoder is unnecessarily slow

[bjhomer]

Previous ID	SR-6252
Radar	rdar://problem/35301452
Original Reporter	@bjhomer
Type	Improvement

Attachment: Download

Additional Detail from JIRA

Votes	1
Component/s	Foundation, Standard Library
Labels	Improvement, Codable
Assignee	@itaiferber
Priority	Medium

md5: eb4b493ba98c649f2cca2b70447bb532

Issue Description:

When converting a large JSON file, Swift's JSONDecoder type takes far longer than similar functionality from 3rd-party libraries. In investigating this, I've found that nearly all the time is spent in swift_dynamicCast(), converting from NSDictionary and [String: Any]. Since JSONSerialization produces NSDictionary objects, this conversion happens often.

In one case, involving a 7MB JSON file, I was able to cut the decoding time from ~1s to ~0.25s simply by making JSONDecoder work with NSDictionary directly, rather than converting to the Swift-native type. Since the underlying storage is already an NSDictionary for the moment, this seems like a natural optimization.

I'm attaching a test application that includes the changes to JSONDecoder. The JSON file is pulled from https://github.com/bwhiteley/JSONShootout.

[belkadan]

cc @itaiferber

[itaiferber]

@swift-ci Create

[bjhomer]

Note that in the attached sample project, the ~0.25s times are when built with optimizations. Without optimizations, it's closer to ~0.45s. I believe that as attached, it's running without optimizations.