Like on most other platforms, Chromium uses the LLD linker on macOS. This is a recent change. If things go well, it will ship in m93, but we might discover showstopper bugs that delay things a bit.
Chromium uses LLD as linker on all platforms, except when targeting iOS. LLD is faster than other ELF linkers (ELF is the executable file format used on most OSs, including Linux, Android, Chrome OS, Fuchsia), and it's faster than other COFF linkers (the executable file format on Windows).
LLD is currently 3-4x as fast as ld64, the macOS system linker, at linking Chromium Framework in symbol_level=0 release builds, despite ld64 being already fast.
LLD has advantages unrelated to speed, too:
It's developed in the LLVM repository, and we ship it in our clang package. We can fix issues upstream and quickly deploy fixed versions, instead of having to wait for Xcode releases (which is where ld64 ships).
For the same reason, it has a much simpler LTO setup: Clang and LLD both link in the same LLVM libraries that are built at the same revision, and compiler and linker bitcodes are interopable for that reason. With ld64, the LTO code has to be built as a plugin that's loaded by the linker.
LLD/Mach-O supports “LLVM-y” features that the ELF and COFF LLDs support as well, such as thin archives, colored diagnostics, and response files (ld64 supports this too as of Xcode 12, but we had to wait many years for it, and it's currently too crashy to be usable).
While LLD for ELF, LLD for COFF, and LLD for MachO are mostly independent codebases, they all use LLVM libraries. That gives them similar behavior. Using LLD unifies the build across platforms somewhat.
For that reason, we moved to LLD for macOS builds (not for iOS builds, that's intentionally not in scope at the moment).
Just like the LLD ELF port tries to be commandline-compatible with other ELF linkers and the LLD COFF port tries to be commandline-compatible with the Visual Studio linker link.exe, the LLD Mach-O port tries to be commandline-compatible with ld64. This means LLD accepts different flags on different platforms.
LLD is used by default in all build configurations except for PGO profile collection builds (bug). All tests on all bots are passing, both Intel and Arm.
There are a few open issues:
If you want to work on LLD, follow this paragraph.
For simple cases, LLD's --reproduce=foo.tar flag / LLD_REPRODUCE=foo.tar env var is sufficient.
See “Note to self:” here for making a repro file that involved the full app and framework bundles.