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[DebuggingTheCompiler.md] Show how to use -Rmodule-loading and SWIFT_FORCE_MODULE_LOADING to debug module/swiftinterface loading issues. #86133

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[DebuggingTheCompiler.md] Show how to use -Rmodule-loading and SWIFT_FORCE_MODULE_LOADING to debug module/swiftinterface loading issues. #86133

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29 changes: 29 additions & 0 deletions docs/DebuggingTheCompiler.md
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@ benefit of all Swift developers.
- [Reducing SIL test cases using bug_reducer](#reducing-sil-test-cases-using-bug_reducer)
- [Disabling PCH Verification](#disabling-pch-verification)
- [Diagnosing LSAN Failures in the Compiler](#diagnosing-lsan-failures-in-the-compiler)
- [Diagnosing the modules and swiftinterface files that the Compiler is loading](#diagnosing-the-modules-and-swiftinterface-files-that-the-compiler-is-loading)
- [Debugging the Compiler Build](#debugging-the-compiler-build)
- [Build Dry Run](#build-dry-run)
- [Debugging the Compiler Driver](#debugging-the-compiler-driver-build)
Expand Down Expand Up @@ -937,6 +938,34 @@ $ apt update
$ apt install vim
```

## Diagnosing the modules and swiftinterface files that the Compiler is loading

In order to determine which swiftinterface files or modules a compiler is
loading, one can pass in the `-Rmodule-loading` flag to the compiler. This will
cause the compiler to emit diagnostics that show where it is loading modules
from. E.x.:

```
<unknown>:0: remark: 'Swift' has a required transitive dependency on 'SwiftShims'
<unknown>:0: remark: loaded module 'SwiftShims'; source: '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk/usr/lib/swift/shims/module.modulemap', loaded: '2HUHAYXMA6V6X/SwiftShims-2VJU34GCOR4TK.pcm'
<unknown>:0: remark: loaded module 'Swift'; source: '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk/usr/lib/swift/Swift.swiftmodule/arm64e-apple-macos.swiftinterface', loaded: '$MODULE_CACHE_PATH/Swift-145L0LE2COJMJ.swiftmodule'
<unknown>:0: remark: '_StringProcessing' has a required transitive dependency on 'Swift'
<unknown>:0: remark: loaded module '_StringProcessing'; source: '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk/usr/lib/swift/_StringProcessing.swiftmodule/arm64e-apple-macos.swiftinterface', loaded: '$MODULE_CACHE_PATH/_StringProcessing-GDUC793JZ4FG.swiftmodule'
<unknown>:0: remark: loaded module '_SwiftConcurrencyShims'; source: '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk/usr/lib/swift/shims/module.modulemap', loaded: '$MODULE_CACHE_PATH/2HUHAYXMA6V6X/_SwiftConcurrencyShims-2VJU34GCOR4TK.pcm'
<unknown>:0: remark: '_Concurrency' has a required transitive dependency on 'Swift'
<unknown>:0: remark: '_Concurrency' has a required transitive dependency on 'SwiftShims'
<unknown>:0: remark: loaded module '_Concurrency'; source: '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk/usr/lib/swift/_Concurrency.swiftmodule/arm64e-apple-macos.swiftinterface', loaded: '$MODULE_CACHE_PATH/_Concurrency-2F0RT3BEWY3IN.swiftmodule'
```

One can also control whether or not the compiler prefers the interface or
serialized version of a module by using the environment variable
`SWIFT_FORCE_MODULE_LOADING`. The supported options are:

* prefer-interface
* prefer-serialized
* only-interface
* only-serialized

# Debugging the Compiler Build

## Build Dry Run
Expand Down