Announcement: Mold Linker v3.0.0 — Now Powered by Rust

The high-speed linker, mold, has reached a major milestone with the release of version 3.0.0. As previously hinted at in the 2.42.1 notes, the project has undergone a complete architectural transformation: mold has been rewritten from C++ in Rust.

🎯 Strategic Objectives

The primary goals for the 3.x series are:

• Closing Compatibility Gaps: Specifically improving support for linker scripts to align more closely with GNU ld.
• Distribution Readiness: Preparing the tool for adoption as the standard default linker across various Linux distributions.

Note on Compatibility: Mold 3.0.0 maintains parity with previous versions regarding target architectures, command-line arguments, and general output, with the exception of the specific bug fixes detailed below.

Verification Process

To ensure a seamless transition, the developers employed a rigorous validation strategy:

1. Execution of the full test suite across all supported targets.
2. Comparative analysis of linker output using a vast array of real-world workloads and option permutations.
3. Successful builds of the entire Gentoo package set.

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🛡️ Safety and Stability

The migration to Rust significantly enhances the linker's resilience when handling malformed or corrupted input files.

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🛠️ Build and Installation Guide

The build pipeline has been overhauled. Please follow these updated instructions:

Compilation Steps

• Execute cargo build --release to compile the binary.
• Run ./install-mold.sh to perform the installation.

Environment Variables:

• PREFIX and DESTDIR: Supported by the installation script.
• MOLD_LIBDIR: If libraries are installed in a non-standard path (e.g., /usr/lib64 instead of $PREFIX/lib), this variable must be set during both the build and install phases so mold -run can locate mold-wrapper.so.

Dependencies and Allocation

• Memory Allocation: By default, it statically links mimalloc 3.5.3. To switch to the system's allocator, use: --features system-allocator
• Compression/Hashing: Includes zstd and BLAKE3, and links the system zlib if present. To force the use of the system's zstd via pkg-config, set: ZSTD_SYS_USE_PKG_CONFIG=1

Build System Changes

Note: install-test-deps.sh is now only required for those running the test suite.

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🐛 Bug Fixes & Compatibility Enhancements

💥 Crash Fixes

• Resolved crashes occurring during the creation of statically-linked executables when using --default-symver or version scripts.
• Fixed a corruption/crash issue when the output file was also listed as an input (e.g., mold -r -o foo.o foo.o bar.o).
• Fixed crashes associated with --icf=all when combined with --emit-relocs.
• Fixed crashes in --noinhibit-exec when referencing symbols in discarded COMDAT groups.

⚙️ Logic and Behavioral Improvements

• Section GC: --gc-sections will no longer strip functions designated by --fini or --init.
• Symbol Handling: Common symbols with identical names but varying sizes now adopt the largest size and most stringent alignment (matching lld and GNU ld).
• ICF: --icf=safe no longer folds functions that are exported from shared libraries.
• Versioning:
  • Versioned references (e.g., foo@VER) in shared libraries no longer trigger the inclusion of unversioned foo members from archives.
  • In version scripts, if a symbol appears in multiple nodes, the first occurrence now takes precedence.
• Determinism: Eliminated non-deterministic output in several areas:
  • --repro and --dependency-file.
  • Dynamic relocations.
  • -r when handling numerous COMDAT groups with the same name.

🛠️ Relocatable (-r) and Relocation Fixes

• Relocatable Bugs: Fixed issues where C++ exceptions failed in subsequent input files, symbols were erroneously removed by -s, -x, or -X, and data was dropped by --gc-sections.
• Section Headers: Corrected sh_link for SHF_LINK_ORDER sections (such as .ARM.exidx).
• DWARF: Fixed crashes when using --strip-debug with DWARF type units.
• Address Accuracy: Corrected silent address errors for R_ARM_REL32 and R_X86_64_GOTOFF64 (GOT-relative relocations) against shared library symbols.
• Diagnostics: PC-relative relocation errors in position-independent output now provide clear explanations and remediation steps.

⚠️ Error Handling (Now failing explicitly instead of crashing)

The following scenarios now trigger a formal error:

• --defsym values exceeding $64\text{ bits}$.
• --defsym aliasing a symbol already defined in a shared library.
• Undefined non-weak hidden symbols in shared libraries (or when using --unresolved-symbols=ignore-all).
• Allocated sections that are simultaneously compressed.
• Combining --strip-all with --emit-relocs.

⌨️ CLI and Environment

• GNU Parity: Single-dash options are now parsed identically to GNU ld.
• Argument Parsing: The following flags no longer consume the subsequent argument:
  • --thinlto-index-only
  • --lto-pseudo-probe-for-profiling
  • --dynamic-list-data
• New Options: Added support for --no-color-diagnostics.
• Paths: Fixed relative path handling for --repro, --chroot, and -C.
• Resources: Reserved $8\text{ GiB}$ of virtual address space at startup $\rightarrow$ Removed reservation, enabling compatibility with ulimit -v.
• Environment: MOLD_JOBS now functions correctly even if XDG_RUNTIME_DIR is an empty string.

🏗️ Miscellaneous & Architecture Specifics

• Debug Files: .gnu_debuglink sections are no longer included in files created via --separate-debug-file.
• Symbols: Removed redundant symbols: __start_EHDR, __stop_EHDR, __start_PHDR, and __stop_PHDR.
• Architecture Fixes:
  • [AArch64/PPC32/ARM32]: Fixed range extension thunks for static function calls in different sections (critical for large binaries like Chromium ARM64 debug builds).
  • [AArch64]: Added support for R_AARCH64_GOT_LD_PREL19 and R_AARCH64_TSTBR14.
  • [ARM32]: Expanded support for various relocation types (e.g., R_ARM_...).