Cold-start regression in v9 caused by Swift runtime metadata / protocol conformance overhead

[minori-fh]

Platform

iOS

Environment

Other

Installed

Swift Package Manager

Other Error Monitoring Solution

No

Other Error Monitoring Solution Name

No response

Version

9.2.0 / 9.5.0

Xcode Version

16.x

Did it work on previous versions?

No response

Steps to Reproduce

1. Integrate sentry-cocoa v8.58.2 into a large Swift iOS app
2. Measure cold-start launch time with Instruments / Headspin
3. Upgrade to sentry-cocoa v9.x
4. Repeat cold-start measurements
5. Compare swift_conformsToProtocol samples and startup duration

Expected Result

Cold-start performance remains similar between v8 and v9.

Actual Result

Upgrading to v9 introduces a 40–65% cold-start regression.
swift_conformsToProtocol increases from ~0.2% → ~11.6% during launch, correlated with larger __swift5_proto / Swift metadata sections and hot-path use of String(describing:).

Are you willing to submit a PR?

No response

[linear-code]

COCOA-1426

[noahsmartin]

I don't think this would be an issue in production, only when debugging locally. This swift forums thread explains it in details: https://forums.swift.org/t/pitch-speed-up-protocol-conformance-checks-on-first-launch-using-bloom-filters/70225/29

TLDR: There is a cache on device that makes protocol conformance lookups O(1) instead of O(n). This cache is disabled in many local development environments (including simulator and when using environment variables commonly added by instruments). However, in production the cache gets used and significantly speeds up app launch. At one point the cache was not used for the first app launch after downloading an app, that may have been fixed in newer versions of dyld. You can get a more accurate measurement of production performance with a tool like ettrace: https://github.com/EmergeTools/ETTrace

I also have a gave a talk at Swift Leeds with more detail on this here: https://www.youtube.com/watch?v=1sMJAO8kwyE

The slowdowns when the dyld cache is not used are also only relevant for apps with many protocol conformances. The amount of conformances in the Sentry SDK would likely be trivial compared to the total number in an app. A large app may have 10s of thousands of these conformances. More details on how to count them can be found in this blog post: https://www.emergetools.com/blog/posts/how-order-files-speed-up-protocols

Despite all of this, there should still be no reason to use String(describing:) and it can't hurt to remove those calls from the SDK