mold 2.33.0

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. (596ffa9)

• 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. (3001f02)

• 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. (dd8d971)

• [x86-64] -z x86-64-v2, -z x86-64-v3, -z x86-64-v4 flags are supported. (5606087)

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. (ed7eec5)

• mold now creates a .eh_frame section even if it's empty. (14a4b05)

• [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 (36e5b4b, 98a7cff, 2c6f379)

• [LoongArch] Some relaxations that reduce the section size are now supported. (74b359f, 121f917)

• [LoongArch] Range extension thunk support has been removed in favor of R_LARCH_CALL36 relocations. (47c092a)

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

mold 2.32.1

mold 2.32.1 is a maintenance release of the high-speed linker.

Bug fixes and compatibility improvements

• Previously, shared libraries specified with --as-needed were not considered as "needed" if they were referenced only by weak undefined symbols. Such weak symbols were converted to absolute symbols at address zero at link-time. Although this behavior was not technically wrong, it caused a significant issue in a rare occasion (#1286). Now, weak undefined symbols retain --as-needed shared libraries. (06b5926)
• [RISC-V] RISC-V object files contain ISA strings in the .riscv.attributes section. Previously, we had reported valid ISA strings containing digits as errors. The issue has now been resolved. (841a186)
• [RISC-V] We no longer write dynamic relocation addends to relocated places because it caused static position-independent executables to crash on process startup in some environments. In other words, --no-apply-dynamic-relocs is enabled by default.
• LTO now works on MinGW. (50bf031)

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:

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

mold 2.32.0

mold 2.32.0 is a new release of the high-speed linker. It includes the following new features and bug fixes:

New features

• mold supports a feature called Identical Code Folding, or ICF. As the name suggests, ICF finds identical functions and merges them to reduce the size of an output file. This is especially effective for template-heavy C++ programs since templates tend to be instantiated to the same machine code for different types. For example, std::vector<int> is likely to be instantiated to the same code as std::vector<unsigned>. We've made an improvement to our ICF algorithm so that the --icf feature is ~50% faster than the previous version. (fa8e95a)
• The -z rodynamic option is now supported for compatibility with LLVM lld. With the option, mold places the .dynamic section into a read-only segment. (9a233df)

Bug fixes and compatibility improvements

• Previously, mold behaved differently compared to other linkers if both -z defs and --undefined=ignore-in-object-files were given (#1270). Now, they override each other so that the mold's behavior is compatible with others. (8cd85aa)
• Previously, --dependency-file mistakenly recorded response files as dependencies (#1258). This bug has been fixed. (4281f45)
• There was a bug that mold corrupted debug info section contents when the --relocatable option was given (#1265). This issue has been fixed. (08b0a16)
• [PPC64] The R_PPC64_TPREL16_LO_DS relocation type is supported. (a8cd2e8)
• [ARM64, PPC64, LoongArch] mold 2.31.0 or earlier may have failed with an assertion failure when creating a large output file (#1224). This issue has been resolved. (c7c8583)

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:

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

mold 2.31.0

mold 2.31.0 is a new release of the high-speed linker. It includes the following new features and bug fixes:

New features

• mold is now up to 10% faster when linking very large, debug info-enabled executables such as Blender (~1.8 GiB) or Clang (~3.8 GiB), thanks to several improvements we've made to the string merging algorithm. (53ebcd8, d714301, 40f6b17, c9faf3d)
• -z start-stop-visibility=hidden is now supported so that linker-synthesized __start_<section-name> and __stop_<section-name> symbols can be completely hidden from other ELF modules. Previously, only -z start-stop-visibility=protected was supported. (99a5b15)
• -Bsymbolic-non-weak and -Bsymbolic-non-weak-functions options are now supported for compatibility with LLVM lld. Just like lld, these options control which symbols are exported as dynamic symbols. -Bsymbolic-non-weak makes the linker to export only weak symbols, whereas -Bsymbolic-non-weak-functions makes it to export only weak function symbols. (7d17aa8)

Bug fixes and compatibility improvements

• Previously, if a linker script contains a newline character in the beginning four bytes of a file, it was not recognized as a linker script by mold. Now, mold allows newlines at the beginning of a file. (ea054cc)
• Under rare circumstances, the INPUT linker script command may have found a different file than GNU ld would. Now, mold's behavior aligns with GNU ld's. (163975d)
• Previously, the --repro option produced corrupted tar files. Now the bug has been fixed. (32c4a09)
• mold generally guarantees that its output is reproducible, meaning that if you run the linker with the exact same command line options and input files, the output is guaranteed to be bit-for-bit identical to the previous outputs. However, under rare circumstances, it might produce different output due to a bug. It's reported that this nondeterminism caused random crashes for some programs (#1247). This bug has been fixed. (6463a7c)
• mold no longer sets the address of the .text section as the entry point address if --entry option is not given, just like LLVM lld. (020b1a7)
• [RISC-V] __global_pointer$ symbol is now exported from executables as required by the processor-specific ABI. (3df7c8e)
• [ARM32] --long-plt option is now recognized as known option by mold. mold ignores the option, though, because the PLTs generated by our linker is always long. (d432e98)

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:

• Cybozu, Inc.
• Signal Slot Inc.
• Mercury
• G-Research-OSS
• Jinkyu Yi
• kraptor23
• a_p_u_r_o
• Rodrigo Salazar
• Wei Wu
• daquexian
• Matt Godbolt
• Dougall Johnson
• CubeSoft, Inc.
• Josh Triplett
• Robey Pointer
• nomeaning
• David Wells
• Foxie Solutions
• Yihao Ke
• darkdragon-001
• Nazar Mokrynskyi
• Martin Liška
• Richard Zak
• Igor Matuszewski
• Santiago Pastorino
• Hunter Kvalevog

mold 2.30.0

mold 2.30.0 is a maintenance release of the high-speed linker. It includes the following minor bug fixes:

• We have increased the version number from 2.4.1 to 2.30.0, even though this release contains only minor bug fixes. This change was made to prevent GNU libtool from mistaking mold 2.4.1 for GNU ld 2.4.1, which led it to incorrectly conclude that our linker was an outdated version of the GNU linker. Bumping up the version number to align with GNU ld may not be the most elegant solution, but it is a practical approach to resolve the compatibility issue with GNU libtool. (c7f6a91)
• Previously, mold may have inserted an unnecessary gap before the .bss section in an output file, thereby creating an extra segment for it. While not technically incorrect, it was certainly unnecessary. mold 2.30.0 eliminates this unnecessary on-disk gap for .bss. (c395da1)
• Previously, under rare circumstances, mold might fail with the "ConcurrentMap is full" error message if --gdb-index was used. This bug has been resolved. (c60d1d0)
• Previously, mold might generate an excessive number of "ignoring .llvm_addrsig section without sh_link" warnings. These warnings are now suppressed. (51f871f)
• Sections with unknown section types are now reported as errors. (d21207c)
• [PPC32] A crash bug related to --gc-sections has been fixed. (8eae0a3)

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:

• Signal Slot Inc.
• Mercury
• G-Research-OSS
• kraptor23
• Cybozu, Inc.
• Jinkyu Yi
• Matt Godbolt
• Robey Pointer
• David Wells
• Brett Slatkin
• Dougall Johnson
• CubeSoft, Inc.
• Rodrigo Salazar
• Yihao Ke
• Richard Zak
• Wei Wu
• daquexian
• Santiago Pastorino
• Josh Triplett
• nomeaning

mold 2.4.1

mold 2.4.1 is a maintenance release of the high-speed linker. It contains the following minor bug fixes.

• mold 2.4.0 or prior may promote weak dynamic symbols to strong ones under a rare circumstance, which caused "undefined symbol" error at runtime. The bug has been fixed. (50bdf39)
• Previously, if two or more VERSION clauses in a version script match to the same symbol, the first one took precedence. This was incompatible with GNU ld, which gives the last one the highest priority, causing a Qt library link failure. This compatibility issue has been resolved. (e1e16bf)
• By default, we demangle symbols in error messages so that they are easier to read. Previously, Rust symbols could accidentally be demangled as C++ symbols. Now, mold attempts to demangle symbols as Rust ones only for object files created by rustc. (ea9864b)
• [RISC-V] mold now relaxes a GOT-load instruction sequence into a direct address materialization if the symbol address is known at link time. This relaxation eliminates one memory load and slightly improves the linked program's performance. (2ccaa81)
• [PowerPC64 ELFv2] GCC may emit references to _savegpr0_*, _restgpr0_*, _savegpr1_* and _restgpr1_* symbols for the -Os command line option to optimize the output for code size. These symbols are not defined by any object file and expected to be synthesized by the linker. mold didn't use to synthesize these symbols, and therefore object files created with -Os sometimes failed due to missing symbol errors. Now, mold synthesizes these symbols. (d4ff48a)
• [PowerPC64] R_PPC64_DTPREL16_LO_DS relocation type has now been supported. (6d8e6af)
• [Illumos] On Illumos OS, absolute symbols in DSOs need to be resolved at runtime because the dynamic linker treats such symbols in a special manner. Previously, mold directly used absolute symbol addresses at link-time and did not place them into the dynamic symbol table. That optimization has been removed for compatibility with Illumos. (bed5b17, 7f8d77d)

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:

• Cybozu, Inc.
• Wei Wu
• Signal Slot Inc.
• Mercury
• G-Research-OSS
• Jinkyu Yi
• kraptor23
• kyle-elliott
• Emerge Tools
• nomeaning
• daquexian
• Matt Godbolt
• Brett Slatkin
• Dougall Johnson
• Santiago Pastorino
• CubeSoft, Inc.
• Josh Triplett
• David Wells
• Martin Liška
• Richard Zak

mold 2.4.0

mold 2.4.0 is a new release of the high-speed linker.

New features

• mold gained the --spare-program-headers=<number> option, which adds a specified number of spare entries at the end of the program header. The option aims to make post-processing tools to add program header entries very easily. Note that sorting program header entries after adding new ones may be necessary to meet the constraints of the ELF file format. For details, see the elf(5) man page. (eb6c213)
• mold's -z rewrite-endbr option rewrites superflous endbr64 instructions with nop as a countermeasure against control-flow highjacking attacks. Previously, this worked exclusively with object files compiled with -ffunction-sections, requiring each function to be compiled into a separate section. Starting from this release, -z rewrite-endbr works on object files compiled without it. In other words, mold is now capable of rewriting endbr64 instructions even if the instruction is not at the beginning of a section. (3cb8a52)

Bug fixes and compatibility improvements

• Previously, mold couldn't handle object files containing multiple .eh_frame sections. The .eh_frame is a section containing data for exception handling. Usually, an object file contains only one .eh_frame which describes how to handle exceptions for all text sections in the same file. However, on rare conditions, it seems ld -r creates an object file containing multiple .eh_frame sections. mold is now able to handle such object files. (f4c5a8a)
• mold -run <command> is an easy way to run the given command with a virtual environment in which the ld command is replaced with mold. The feature is implemented using LD_PRELOAD to hook fork(2)-family functions. Before this release, some invocations of ld were not intercepted correctly because we missed the posix_spawnp(2) function. Now, the function is intercepted just like other fork(2)-family functions. (3fd1cec)
• mold used to produce a non-working executable on a rare occasion when all thread-local variables lacked an initial value and the read-only data required alignment equal to or greater than the page size. This bug has been resolved. (de7d37e)
• Previously, mold might assign a different symbol version to a symbol compared to GNU ld if it matches both a wildcard pattern and an exact pattern in a version script. Our behavior is now compatible with that of GNU ld. (0fdbace)
• [x86-64, i686] Recent versions of LLVM emit a machine code sequence for TLSDESC thread-local variables that differs from GCC's, and mold previously mis-optimized this sequence, leading to crashes in the linked programs. In other words, if you are using LLVM/Clang and compile object files with -mtls-dialect=gnu2, mold might mis-optimize the output file. Now, the bug has been fixed. (000ce0e)

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:

• Cybozu, Inc.
• Bryant Biggs
• Signal Slot Inc.
• Mercury
• G-Research-OSS
• Jinkyu Yi
• Emerge Tools
• kyle-elliott
• kraptor23
• David Wells
• Kiril Mihaylov
• daquexian
• Brett Slatkin
• Dougall Johnson
• Santiago Pastorino
• CubeSoft, Inc.
• Josh Triplett
• Rahul Butani
• Kyle Lacy

mold 2.3.3

mold 2.3.3 contains the following bug fixes:

• --dynamic-list has different semantics for executables and DSOs. Previously, mold implemented only the semantics for executables, causing issues with libraries such as musl that used this option. mold now handles the option for DSOs correctly. (da3f5dd)
• Old object files often contain .ctors and .dtors sections, which hold function pointers for initializing and finalizing processes, respectively. Their roles have been superseded by .init_array and .fini_array on most targets. mold worked functioned correctly as long as input object files consistently use the old or the new sections. However, mixing object files that contain both types of initializers/finalizers resulted in some functions not being executed. This issue has been fixed. (3f88964)
• --defsym can cause the linker to crash if a given symbol is not defined. The crash bug has been fixed. (ff3d54d)
• [POWER10] On rare occasions, pointers statically initialized to functions could be left as null pointers. This bug has been fixed. (31c3b53)

Additionally, our dist.sh script that we use to create binary packages attached to the release notes pages is now reproducible. That means the script always creates bit-for-bit identical output for the same git commit, irrespective of the OS versions or environments in which it's run. This property is very useful as a countermeasure against supply chain attacks. You can now verify that the binaries we distribute are indeed built from the released version of source files by rebuilding the binaries yourself and comparing the outputs.

mold 2.3.2

mold 2.3.2 contains the following bug fixes.

• mold no longer emits dynamic relocations against the text segment for GNU ifunc symbols. Previously, mold emitted such relocations for position-dependent executables. (4cdfc7e)
• mold no longer reports the "REL-type relocation table is not supported for this target" error and instead ignore incompatible relocation tables. LLVM generates such non-conforming relocation tables for the .llvm.call-graph-profile section. This change was made for compatibility. (3791900)
• mold now pads unused gaps in the text segment with interrupt or NOP instructions, instead of leaving them filled with zeros. This alteration does not change the program's semantics but prevents disassemblers from interpreting the spaces between functions as valid instructions. (c86a59a)
• mold now creates the .mold-lock file for MOLD_JOBS not in the home directory but in $XDG_RUNTIME_DIR, which is usually /var/user/<uid>. (39cdf61)
• [ARM32] There was an issue preventing mold from being built on an ARMv8 64-bit ARM processor with an ARM32 userland, such as the 32-bit Raspberry Pi OS running on a Raspberry Pi 4. This build issue has been resolved. (02ead29)
• [LoongArch] mold can now handle R_LARCH_PCALA_LO12 relocation for the jirl instruction. (d3188e3)

mold 2.3.1

mold 2.3.1 contains the following bug fixes.

• [ARM32, ARM64, PowerPC, LoongArch] mold 2.3.0 would crash when handling large output files. This was due to a bug in the code that creates range extension thunks. This issue has now been resolved. (7be1b66)
• [LoongArch] mold is now capable of handling relocations generated for the -mcmodel=extreme flag. (4bd80ec)