mold/README.md

mold: A Modern Linker

This is a repository of a linker I'm currently developing as an independent project for my Masters degree.

My goal is to make a linker that is almost as fast as concatenating object files with cat command. Concretely speaking, I want to use the linker to link a Chromium executable (about 1.8 GiB in size) just in 2 seconds. LLVM's lld, the fastest open-source linker which I originally created a few years ago takes about 12 seconds to link Chromium on my machine. So the goal is 6x performance bump over lld. I don't know if I can ever achieve that, but it's worth a try. I need to create something anyway to earn units to graduate, and I want to (at least try to) create something useful.

I have quite a few new ideas as to how to achieve that speedup, though they are still just random unproved thoughts which need to be implemented and tested with benchmarks. Here is a brain dump:

• In order to achieve a cat-like performance, the most important thing is to fix the layout of an output file as soon as possible, so that we can start copying actual data from input object files to an output executable/shared library file.

• The number of cores on a PC has increased a lot lately, and this trend is expected to continue. We'll see many more cores on our development machines. We should design the linker in such a way that it scales well with many more cores. I have a 64-core/128-thread machine, so my goal is to use that CPU nicely.

• Copying data from input files to an output file is I/O-bounded, so there should be room for doing computationally-intensive tasks while copying data from one file to another.

• We should focus on shortening the link time for the usual modify-build-debug development cycle because the linker speed matters most in that situation. It is OK to take a little bit more time for cold build because for cold build, the time taken by the compiler is a dominant factor anyway.

• After the first invocation of the linker, the linker should not exit but instead become a daemon to keep parsed input files in memory. The daemonized linker keeps an eye on the build directories using inotify(2), and as soon as a new file is created or an exiting file is updated, it reloads a file to memory.

• Daemonizing alone wouldn't make the linker magically faster. We need to split the linker into two in such a way that the latter half of the process finishes as quickly as possible by speculatively parsing and preprocessing input file in the first half of the process. The key factor of success would be to design nice data structures to pass data between the first half and the second half that allows us to offload as much processing as possible from the second to the first half.

• One of the most time-consuming stage among linker stages is symbol resolution. To resolve symbols, we basically have to throw all symbol strings into a hash table to match undefined symbols with defined symbols. But this can be done in the daemon rather than after the actual command line is specified using string interning.

• Object files may contain a special section called a mergeable string section. The section contains lots of null-terminated strings, and the linker is expected to gather all mergeable string sections and merge their contents. So, if two object files contain the same string literal, for example, the resulting output will contain a single merged string. This step is time-consuming, but string merging can be done in the daemon using string interning.

• Static archives (.a files) contain object files, but the static archive's string table contains only defined symbols of member object files and lacks other types of symbols. That makes static archives unsuitable for speculative parsing. The daemon should ignore the string table of static archive and directly read all member object files to get the whole picture of all possible input files.

• If there's a relocation that uses a GOT of a symbol, then we have to create a GOT entry for that symbol. Otherwise, we shouldn't. That means we need to scan all relocation tables to fix the length and the contents of a .got section. This is perhaps time-consuming, but we can do that while copying data from input files to an output file. After the data copy is done, we can attach a .got section at the end of the output file.

• If we aim to the 2 seconds goal for Chromium, every millisecond counts. We can't ignore the latency of process exit. If we mmap a lot of files, _exit(2) is not instantaneous but takes a few hundred milliseconds because the kernel has to clean up a lot of resources. As a workaround, we should organize the linker command as two processes; the first process forks the second process, and the second process does the actual work. As soon as the second process writes a result file to a filesystem, it notifies the first process, and the first process exits. The second process can take time to exit, because it is not an interactive process.

• GNU ld, GNU gold and LLVM lld support essentially the same set of command line options and features. mold doesn't have to be completely compatible with them. As long as it can be used for linking large user-land programs, I'm fine with that. It is OK to leave some command line options unimplemented; if mold is blazingly fast, other project would still be happy to adopt it by modifying their projects' build files.

• I don't want to support the linker script language in mold because it's so complicated and inevitably slows down the linker. User-land programs rarely use linker scripts, so it shouldn't be a roadblock for most projects.

• Intel Threading Building Blocks (TBB) is a good library for parallel execution and has several concurrent containers. We are particularly interested in using parallel_for_each and concurrent_hash_map.

• The output from the linker should be deterministic for the sake of build reproducibility. This might add a little bit of overhead to the linker, but that shouldn't be too much.