diff --git a/content/minutes/design-meeting/2020-04-03-cranelift-backend-for-rustc.md b/content/minutes/design-meeting/2020-04-03-cranelift-backend-for-rustc.md new file mode 100644 index 00000000..3eb0a8b0 --- /dev/null +++ b/content/minutes/design-meeting/2020-04-03-cranelift-backend-for-rustc.md @@ -0,0 +1,48 @@ +--- +title: 2020-04-03 Cranelift backend for rustc +type: docs +--- + +# Cranelift backend for rustc + +You can also read the [complete discussion in our zulip-archive](https://zulip-archive.rust-lang.org/131828tcompiler/01241designmeeting20200403compilerteam257.html). + +[Meeting Proposal](https://github.com/rust-lang/compiler-team/issues/257) + +[Pre-meeting details and agenda outline](https://hackmd.io/@bjorn3/HJL5ryFS8) + +## Overview of the Cranelift backend + +[Cranelift](https://github.com/bytecodealliance/wasmtime/tree/master/cranelift) is a code generation backend written in Rust which is under development for use in WebAssembly compilers. +As such, it could potentially improve compilation times in `rustc` because it is optimized for efficiently generating code rather than generating efficient code. + +Over the past ~1.5 years, [@bjorn3](https://github.com/bjorn3) has been working on writing a Cranelift-based code generation backend for `rustc`: [`rustc_codegen_cranelift`](https://github.com/bjorn3/rustc_codegen_cranelift) or `cg_cranelift` for short. +It is currently complete enough to compile many programs and is usually faster at compiling them than the regular LLVM backend. + +The main goal of this project is to build an alternative codegen backend that can be used for faster, non-optimized (debug) builds. + +`rustc` has support for runtime-pluggable backends via the unstable `-Zcodegen-backend=/path/to/dylib` flag. +`cg_cranelift` is developed in a separate repository from the Rust project, so it uses this flag to plug into `rustc`. +`cg_cranelift` also takes advantage of an internal `rustc` library called `rustc_codegen_ssa` which tries to be a backend-agnostic code generation support library but some parts of it are still very LLVM-like. + +## Next steps + +At this time, we want to help increase the support `cg_cranelift` gets but it isn't a top priority for us currently. +The eventual goal seems to be first-class support for `cg_cranelift` alongside our existing LLVM backend but that won't happen for a while so we can afford to take small, measured steps in the meantime. + +A few things we would like to do now: + +- Continue refactoring `rustc_codegen_ssa` to be less LLVM-like and allow `cg_cranelift` to re-use more of the code. + +- Bring in `cg_cranelift` to the `rust` repo (probably via `git-subtree`) and require it to build during CI. + - This is similar to the support we give to tools like clippy or miri. + +- Add a way in our testing infrastructure to mark tests as reliant on a specific codegen backend and annotate existing tests that require LLVM. + +- Add a way to manually run tests with `cg_cranelift` to Rustbuild (aka `x.py`). + +We may also want to do these soon but we need additional discussion: + +- Run tests in `cg_cranelift` mode and notify if they failed (but don't gate CI on it). + +- Form a working group related to `cg_cranelift`.