Use lld by default on x64 Ubuntu 20.04 LTS (original) (raw)

This is a metabug, constraining the unbound scope of #39915.

What is lld

A linker that's part of the llvm project, which is desirable for two reasons:

• it's very friendly to cross compilation (hence its emphasis for embedded targets)
• it's very fast (often runs in half the time as Gold -- linking can take several minutes for big projects (rustc, servo, etc.) and linking can be a huge % of the compile with incremental builds, so halving this runtime is a Big Deal.)

Rust currently ships its own copy of lld which it calls rust-lld. This is used by default to link bare-metal targets and wasm.

Goal

The goal of this metabug is to use rust-lld by default on a major x64 linux distro. I have arbitrarily chosen Ubuntu 20.04 LTS as our target. With all likelihood, this will incidentally get it working/enabled on many other linux distros, BSDs, and other similar platforms, but I am purposefully constraining the scope to one distro for the sake of focusing the effort.

My understanding is that the ELF backend is quite well maintained. Quoting lld's own landing page:

We are currently working closely with the FreeBSD project to make LLD default system linker in future versions of the operating system, so we are serious about addressing compatibility issues. As of February 2017, LLD is able to link the entire FreeBSD/amd64 base system including the kernel. With a few work-in-progress patches it can link approximately 95% of the ports collection on AMD64. For the details, see FreeBSD quarterly status report.

Hopefully this has continued to improve, and using lld will be a slam dunk. Although I specify that we should use rust-lld, it may be necessary/desirable to detect and use system copies of lld.

Blocking Issues

• Support feeding rust-lld into gcc (and clang) #71519 - support making gcc use rust-lld
• Coverage is not generated when using lld as linker #71233 - coverage information not generated when using lld