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 as fast as concatenating input object files with cat command. It may sound like an impossible goal, but I believe it's not entirely impossible because of the following two reasons:

1. cat is a simple single-threaded program which isn't the fastest one as a file copy command. My linker can use multiple threads to copy file contents more efficiently to save time to do extra work.

2. Copying file contents is I/O-bounded, and many CPU cores should be available during file copy. We can use them to do extra work while copying file contents.

Concretely speaking, I want to use the linker to link a Chromium executable (~1.8 GiB in size) just in 1 second. 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 12x performance bump over lld. Compared to GNU gold, it's more than 50x.

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:

Background

• Even though lld has significantly improved the situation, linking is still one of the slowest steps in a build. It is especially annoying when I changed one line of code and had to wait for a few seconds or even more for a linker to complete. It should be instantaneous. There's a need for a faster linker.

• The number of cores on a PC has increased a lot lately, and this trend is expected to continue. However, the existing linkers can't take the advantage of that because they don't scale well for more cores. I have a 64-core/128-thread machine, so my goal is to create a linker that uses the CPU nicely. mold should be much faster than other linkers on 4 or 8-core machines too, though.

• It looks to me that the designs of the existing linkers are somewhat similar, and I believe there are a lot of drastically different designs that haven't been explored yet. Develoeprs generally don't care about linkers as long as they work correctly, and they don't even think about creating a new one. So there may be lots of low hanging fruits there in this area.

Basic design

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

• 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.

• After the first invocation of the linker, the linker should not exit but instead become a daemon to keep parsed input files in memory. Subsequent linker invocations for the same output file make the linker daemon to reload updated input files, and then the daemon calls fork(2) to create a subprocess and let it do the actual linking.

• 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 files in the first half of the process. The key factor of success would be to design nice data structures 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 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 of all archives 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.

• Many linkers support incremental linking, but I think that's a hack to work around the slowness of regular linking. I want to focus on making the regular linking extremely fast.

Compatibility

• 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 projects 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.

• mold emits Linux executables and runs only on Linux. I won't avoid Unix-ism when writing code (e.g. I'll probably use fork(2)). I don't want to think about portability until mold becomes a thing that's worth to be ported.

Details

• If we aim to the 1 second 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.

• At least on Linux, it looks like the filesystem's performance to allocate new blocks to a new file is the limiting factor when creating a new large file and filling its contents using mmap. If you already have a large file on a filesystem, writing to it is much faster than creating a new fresh file and writing to it. Based on this observation, mold should overwrite to an existing executable file if exists. My quick benchmark showed that I could save 300 milliseconds when creating a 2 GiB output file.

  Linux doesn't allow to open an executable for writing if it is running (you'll get "text busy" error if you attempt). mold should fall back to the usual way if it fails to open an output file.

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

• A .build-id, a unique ID embedded to an output file, is usually computed by applying a cryptographic hash function (e.g. SHA-1) to an output file. But it adds an extra time for linking because a linker has to compute a SHA-1 checksum after the actual linking is done. We should instead compute a SHA-1 for the tuple of (all input files, command line options, linker version) as a build-id, as it should uniquely identify the output.

• 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.

Size of the problem

When linking Chrome, a linker reads 3,430,966,844 bytes of data in total. The data contains the following items:

Data item	Number
Object files	30,723
Public undefined symbols	1,428,149
Mergeable strings	1,579,996
Comdat groups	9,914,510
Regular sections (*1)	10,345,314
Public defined symbols	10,512,135
Symbols	23,953,607
Sections	27,543,225
Relocations against SHF_ALLOC sections	39,496,375
Relocations	62,024,719

(*1) Sections that have to be copied from input object files to an output file. Sections that contain relocations or symbols are for example excluded.