Adopting the Parallel DWARF linker in dsymutil

3 days ago

Apple platforms optimize the compile-link-debug cycle by leaving debug info in object files with a debug map, requiring dsymutil for self-contained bundles.
dsymutil acts as an optimizing linker using the One Definition Rule to deduplicate types, essential for large C++ projects to stay within Mach-O's 4GB limit.
The classic dsymutil is single-threaded due to memory constraints, while a parallel linker exists but faces challenges with qualification and determinism.
A semantic DWARF diffing tool was prototyped to compare outputs from classic and parallel linkers, identifying differences without binary identity.
Determinism in the parallel linker was achieved by prioritizing compile units based on link order, ensuring reproducible builds.
To switch the default to the parallel linker, qualification involves passing existing tests, using the LLDB test suite, and manual diffing on large projects.
Performance improvement: dsymutil on clang takes about 40 seconds with the parallel linker vs. 3 minutes with the classic linker.

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