Debug Swift debugging with LLDB

Transcript

1. Debug Swift debugging with LLDB JJ Lin
2. Issues: Cannot debug framework source code
3. Issues: Cannot debug framework source code
4. • When the compiler compiles a function, it generates machine code.
5. • Debug info in object file: For the debugger to map the address in the executable to the source code and line number
6. • For archiving distribution, debug info will be put into .dSYM bundles by dsymutil.
7. DWARF & dSYM • For debug at run-time • Crash log symbolication
8. DWARF • Debugging With Attribute Record Formats • A debugging file format used by many compilers and debuggers to support source-level debugging
9. $ dwarfdump Framework1.framework.dSYM/Contents/Resources/DWARF/Framework1
10. DWARF & dSYM • DWARF ◦ Keep debug symbols in the executable. • DWARF with dSYM File ◦ dSYM: debug SYMbol ◦ Strips debug symbols from the executable, and places them into a separate dSYM file. ◦ Reduce binary size, but slower
11. Issues: Cannot debug framework source code • So how to debug framework? ◦ sample: TerminalInterface
12. (lldb) image list • `image` is an abbreviation for `target modules` • List all the modules currently loaded ◦ executable ◦ dependent shared library • Could help to verify that dSYM is actually found • The UI framework is called TerminalInterface, and LLDB did find the framework and debug info.
13. What if… dSYM is not loaded? • (lldb) add-dsym {dSYM}
14. (lldb) image lookup -va {memory-address} • Look up information within executable and dependent shared library images.
15. • The source file is pointed to build server, not local machine (lldb) image lookup -va {memory-address}
16. (lldb) image lookup -r -n {symbol name}
17. Redirect source map • Since the source file is pointed to build server, not local machine • We can redirect the source map • (lldb) settings set target.source-map prefix new • (lldb) settings show target.source-map • Re-run the project, then the source code could be inspected ✅
18. Redirect source map - LLDB Init File • Handy tips: Define per-project LLDB init file for commands
19. Redirect source map - LLDB Init File • Handy tips: Define per-project LLDB init file for commands
20. • (lldb) po //failed Issues: Cannot debug framework source code
21. (lldb) v • but v works • Xcode variable view • = (lldb) frame variable • = (lldb) v
22. LLDB is debugger, and also a compiler
24. If you are interested… • Type Metadata ◦ The Swift runtime keeps a metadata record for every type used in a program, including every instantiation of generic types. ◦ These metadata records can be used by reflection and debugger tools to discover information about types. • Reflection ◦ Reflection in Swift allows us to inspect and manipulate arbitrary values at runtime. • https://github.com/apple/swift/blob/main/docs/ABI/TypeMetadata.rst
26. • Solution: Archive the framework again, with the missing module
27. Some Tips
28. What if… cannot find dSYM? • Xcode Settings - Generate Debug Symbols ◦ Default: Yes for both Debug and Release
29. What if… symbol not found? • Xcode Settings - Debug Information Level ◦ Toggles the amount of debug information emitted when debug symbols are enabled. ◦ Option - Compiler default ▪ Generate source level debug information ◦ Option - Generate line number tables only ▪ Allows to obtain stack traces with function names, file names and line numbers ▪ No other data (e.g. description of local variables, or function parameters).
33. Reference • https://developer.apple.com/videos/play/wwdc2022/110370/ • https://developer.apple.com/videos/play/wwdc2021/10211 • https://developer.apple.com/documentation/xcode/build-settings-reference • https://clang.llvm.org/docs/UsersManual.html#controlling-size-of-debug-information • https://github.com/apple/swift/blob/main/docs/ABI/TypeMetadata.rst
34. Thank you