Merge pull request #269 from rust-lang/wesleywiser-patch-1

Add meeting notes for the cranelift backend design meeting

Author: nikomatsakis

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`.