mold 2.33.0 is a new release of the high-speed linker. It includes the following new features and bug fixes.

New features

• mold gained a new linker flag --separate-debug-file to bundle debug info sections into a separate file instead of putting them into a main output file. You can optionally specify a filename in the form of --separate-debug-file=<filename>. By default, a debug info file is created in the same directory as the main output file with the .dbg extension. mold embeds the debug file's filename into the main output file so that gdb can automatically follow the link to find debug info when debugging the main output file.

  The main objective of this flag is to speed up the mold linker even more. By default, mold creates a separate debug file in the background after creating a main output file, so that you can start running the executable as soon as possible while mold is still working on linking its debug info sections. For example, linking clang with debug info normally takes ~1.70s on a Threadripper 7980X machine, while it takes only ~0.52s with --separate-debug-info. Shaving off a full second in quick edit-rebuild-run cycles should improve programmers' productivity. If you do not want mold to work in the background, pass the --no-detach option. (596ffa959acde21435b9626432a5f2ae151e49df)

• mold now supports the --no-allow-shlib-undefined flag. If the option is given, mold checks if all undefined symbols are resolved not only for input object files but also for shared libraries passed to the linker. To use the feature, you need to pass all shared libraries, including transitively dependent ones, to the linker so that the linker can resolve all symbols that are available at runtime. (3001f02b55aa80123e77a4551df519ef5e031cdb)

• mold gained the --dynamic-list-data flag for the sake of compatibility with GNU ld. If the flag is given, all data symbols are exported as dynamic symbols. (dd8d9714cb14213766372fe39ecb2e8c1fe4533f)

• [x86-64] -z x86-64-v2, -z x86-64-v3, -z x86-64-v4 flags are supported. (5606087776fcf90d949c838602cd75743256b96a)

Bug fixes and compatibility improvements

• [x86-64] Recent x86-64 processors support Intel CET to protect control flow integrity. When the feature is enabled, the instruction that is executed immediately after an indirect branch must be endbr64 or a CPU fault will raise. In other words, it restricts the locations where the control can transfer to with indirect branches. Doing that makes ROP attacks harder to conduct.

  A problem with that is the compiler needs to conservatively emit an endbr64 at the beginning of each global function because the compiler doesn't know whether or not the function's address is taken in other translation units. As a result, the resulting binary contains more endbr64s than necessary, weakening the protection.

  mold supports the -z rewrite-endbr option to conduct a whole program analysis and rewrite endbr64 with nop if a function's address is not actually taken within the program. Previously, mold didn't take section symbols into account when conducting the analysis, which resulted in culling some endbr64s that must not be removed. Now, the bug has been fixed. We confirmed that mold can build itself with -z rewrite-endbr, and the resulting mold executable works fine with Intel CET. (ed7eec5969078f7420a9329c792ad3f8942f3fc0)

• mold now creates a .eh_frame section even if it's empty. (14a4b05950a142ce4e22048dacb9166cb8686757)

• [LoongArch] The following relocations are now supported: R_LARCH_TLS_LE_HI20_R, R_LARCH_TLS_LE_ADD_R, R_LARCH_TLS_LE_LO12_R, R_LARCH_CALL36, R_LARCH_RELAX (36e5b4b9492a9bd7a00127fd3eda9ab72dbc5c09, 98a7cff0c5b9d8b43eafefda9b659ae85158ea75, 2c6f379d690c9df3af0f321d7057eb6749788d4e)

• [LoongArch] Some relaxations that reduce the section size are now supported. (74b359fe62e9b912529cbd697a12e80e80695029, 121f917884deec51cc693dfb9f385884130c0a67)

• [LoongArch] Range extension thunk support has been removed in favor of R_LARCH_CALL36 relocations. (47c092aac0dc64d3addfc4819212ecd5a93a8c1f)

Acknowledgements

mold is an open-source project, and we accept donations via GitHub Sponsors and OpenCollective. We thank everyone who sponsors our project. In particular, we'd like to acknowledge the following organizations and people who have sponsored $32/mo or more during this release cycle:

• Bloomberg
• Cybozu, Inc.
• Ahrefs
• Signal Slot Inc.
• Mercury
• G-Research-OSS
• Jinkyu Yi
• kraptor23
• Richard Zak
• Wei Wu
• daquexian
• CubeSoft, Inc.
• Josh Triplett
• Foxie Solutions
• nomeaning
• Igor Matuszewski
• Dougall Johnson