mold: A Massively Parallel Linker
2026-08-24 • Operating Systems
Operating Systems
AI summaryⓘ
The authors present mold, a new linker program that speeds up the process of combining compiled code into one file. They found that existing linkers don't fully use all CPU cores because of how tasks are tangled together. Mold separates these tasks and runs them in parallel, making linking much faster, especially for big programs. Tests show mold is several times faster than current popular linkers.
linkerlinkingobject filesparallelismsymbol resolutionarchive processingC++debug binariesUnix/Linuxbuild process
Authors
Rui Ueyama
Abstract
Linking is a critical step in the software build process that combines compiled object files into a single executable or shared library. Despite decades of engineering effort, link times remain a significant bottleneck in the edit-compile-debug cycle, particularly for large C++ programs. Existing linkers exploit limited parallelism, leaving most CPU cores idle during linking. We present mold, a Unix/Linux linker that applies data parallelism systematically across the entire linking pipeline. We first analyze the architectural constraints that prevent existing linkers from scaling, including entangled symbol resolution and archive processing, and then show how a clean-slate design that decouples them overcomes these limitations. On large real-world programs, mold links multi-gigabyte debug binaries in at most a few seconds, and often in under a second. It is 2.4-16.1x faster than the state-of-the-art lld linker, and up to 112x faster than the traditional GNU ld. An ablation study shows that no single optimization dominates; the speedup comes from the cumulative effect of parallelizing all passes.