[iOS][macOS] Swift code is compiled -Onone in all build modes #191616

The engine never passes an optimization flag to swiftc. Since swiftc defaults to -Onone, every Swift source in the Darwin embedders is compiled unoptimized in profile and release as well as debug, and assert() / assertionFailure() remain live in shipping binaries.

I wrote most of the original Swift build config myself in #167530 and I'm 99.9% sure this was an oversight on my part; nowhere in that patch did I mention optimisation. I remember getting derailed by some tricky issues in clang module support with libc++. It looks like the swiftflags half of the optimisation configs was simply never filled in. //build/config/compiler:optimize and :no_optimize set cflags and ldflags, but not swiftflags. The only swiftflags assignments in the build are:

• build/config/darwin/BUILD.gn:54: -sdk, -target, -swift-version 5
• build/config/compiler/BUILD.gn:70: swiftflags = []
• build/config/compiler/BUILD.gn:163: -sanitize=thread under is_tsan
• build/config/compiler/BUILD.gn:1119: -g under config("symbols")

which means the flags are identical in all build variants. Looking at the generated ninja files:

ios_debug_unopt : -sdk <sdk> -target arm64-apple-ios15.0 -swift-version 5 -g …
ios_profile     : -sdk <sdk> -target arm64-apple-ios15.0 -swift-version 5 -g
ios_release     : -sdk <sdk> -target arm64-apple-ios15.0 -swift-version 5 -g
host_release    : -sdk <sdk> -target arm64-apple-macos12.0.0 -swift-version 5 -g …

swiftc.py does pass --whole-module-optimization, which sounds like optimisation is enabled, but it's not. WMO is a separate feature that controls how the module is partitioned across frontend invocations. The compiler optimisation level is still -Onone.

The following commits later touched swiftflags and none of them mention optimisation either, though the asan/ubsan patch was, like this, me going back noticing I'd left configs out:

• Rename apple -> darwin across our buildroot #168558
• Enable Swift testing in the iOS embedder #185712
• [ios,macos] Update swiftc.py flags to match swiftc #189174
• iOS,macOS: add tsan and ubsan support for Swift #190497

What needs to happen

Add swiftflags to the optimization configs alongside the existing cflags. -Onone in :no_optimize, and in :optimize either -O or -Osize. is_ios already picks -Os for C/C++ to favour size over speed, so -Osize is probably the right choice for Swift.

Whether we should be optimising for size over speed in 2026 is a separate issue we should investigate.