If a module was first read using the adjacent swiftmodule and then
reloaded using the swiftinterface, we would do an up to date check on
the adjacent module but write out the unit using the swiftinterface.
This would cause the same modules to be indexed repeatedly for the first
invocation using a new SDK. On the next run we would instead raad the
swiftmodule from the cache and thus the out of date check would match
up.
The impact of this varies depending on the size of the module graph in
the initial compilation and the number of jobs started at the same time.
Each SDK dependency is re-indexed *and* reloaded, which is a drain on
both CPU and memory. Thus, if many jobs are initially started and
they're all going down this path, it can cause the system to run out of
memory very quickly.
Resolves rdar://103119964.
Passing the frontend flag -Rmodule-loading makes the compiler emit
remarks with the path of every module loaded. The path for Swift modules
is either the swiftinterface file for modules built with library
evolution or the binary swiftmodule otherwise. The path for clangmodules
is always in the cache which could be improved as it may be less useful.
Here's an extract of the output for a simple SwiftUI app:
<unknown>:0: remark: loaded module from
/Users/xymus/Library/Developer/Xcode/DerivedData/ModuleCache.noindex/2VJP7CNCGWRF0/SwiftShims-18ZF6992O9H75.pcm
<unknown>:0: remark: loaded module from
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.2.sdk/usr/lib/swift/Swift.swiftmodule/x86_64-apple-ios-simulator.swiftinterface
<unknown>:0: remark: loaded module from
/Users/xymus/Library/Developer/Xcode/DerivedData/ModuleCache.noindex/2VJP7CNCGWRF0/os-1HVC6DNXVU37C.pcm
<unknown>:0: remark: loaded module from
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.2.sdk/usr/lib/swift/os.swiftmodule/x86_64-apple-ios-simulator.swiftinterface
<unknown>:0: remark: loaded module from
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.2.sdk/System/Library/Frameworks/SwiftUI.framework/Modules/SwiftUI.swiftmodule/x86_64-apple-ios-simulator.swiftinterface
