Add post on cranelift backend (#720)

* Add post on cranelift backend

- Don't recommend building in-tree. It's slow.
- Show incremental times
- Say why incremental builds were slower with cg_clif than LLVM

* Update date to the 15th

Author: jyn514

posts/inside-rust/2020-11-15-Using-rustc_codegen_cranelift.md

@@ -0,0 +1,135 @@
+---
+layout: post
+title: "Using rustc_codegen_cranelift for debug builds"
+author: Joshua Nelson
+team: The Compiler Team <https://www.rust-lang.org/governance/teams/compiler>
+---
+
+## What is `rustc_codegen_cranelift`?
+
+[`rustc_codegen_cranelift`], or just `cg_clif` for short, is a new experimental
+codegen backend for the Rust compiler. The existing backend is LLVM, which is very
+good at producing fast, highly optimized code, but is not very good at
+compiling code quickly. `cg_clif`, which uses the [Cranelift] project, would
+provide a fast backend which greatly improves compile times, at the cost of
+performing very few optimizations. This is a great fit for debug builds, and the hope is
+that `cg_clif` will eventually be the default backend in debug mode.
+
+## What is the progress of using `rustc_codegen_cranelift` for debug builds?
+
+There has been a [Major Change Proposal][MCP] open for some time for making
+`cg_clif` part of the main Rust repository. Recently, [the MCP was
+accepted][compiler-team#270] and the compiler team [merged][#77975]
+`rustc_cranelift_codegen` [into the main Rust git repository][#77975].
+`cg_clif` is not yet distributed with `rustup`, but this means you can now
+build it from source in-tree!
+
+## How do I use `rustc_codegen_cranelift`?
+
+In this section, I'll walk through step-by-step how to build the new backend from source, then use it on your own projects. All code is copy/paste-able, and each step is explained.
+
+First, let's build `cg_clif` from source.
+
+```sh
+$ git clone https://github.com/bjorn3/rustc_codegen_cranelift.git
+$ ./prepare.sh
+$ ./build.sh
+```
+
+Now, we can start using it to compile a project. For demonstration purposes,
+I'll be be using `cargo`, but you can use any Rust project supported by
+`cg_clif`.
+
+```
+$ cd ..
+$ git clone https://github.com/rust-lang/cargo/
+$ cd cargo
+$ ../rustc_codegen_cranelift/build/cargo.sh build
+...
+    Finished dev [unoptimized + debuginfo] target(s) in 49.93s
+```
+
+It works! For comparison, let's see how long the equivalent LLVM backend would
+take.
+
+```sh
+$ rustup install nightly-2020-10-31
+$ cargo +nightly-2020-10-31 build
+...
+    Finished dev [unoptimized + debuginfo] target(s) in 54.64s
+```
+
+LLVM takes a full 5 seconds longer for a full build. Next, let's try incremental builds:
+
+```
+$ git apply <<EOF
+diff --git a/src/cargo/lib.rs b/src/cargo/lib.rs
+index bccb41121..703afa754 100644
+--- a/src/cargo/lib.rs
++++ b/src/cargo/lib.rs
+@@ -36,8 +36,8 @@ use anyhow::Error;
+ use log::debug;
+ use std::fmt;
+ 
+-pub use crate::util::errors::{InternalError, VerboseError};
+ pub use crate::util::{CargoResult, CliError, CliResult, Config};
++pub use crate::util::errors::{InternalError, VerboseError};
+ 
+ pub const CARGO_ENV: &str = "CARGO";
+EOF
+$ ../rustc_codegen_cranelift/build/cargo.sh build
+    Finished dev [unoptimized + debuginfo] target(s) in 7.98s
+$ cargo +nightly-2020-10-31 build
+   Compiling cargo v0.50.0 (/home/joshua/cargo)
+    Finished dev [unoptimized + debuginfo] target(s) in 5.48s
+```
+
+LLVM is actually *faster* here: `serde_derive` took longer to run under cranelift, since it wasn't as optimized. Under cranelift it takes ~14% percent of the time, while under LLVM it takes less than 3%.
+
+## Building in-tree
+
+This section is mostly for compiler hackers, but feel free to follow along even
+if you're just interested! The reason this isn't the recommended way to build
+`cg_clif` is because the Rust compiler takes a very long time to build.
+
+First, download the Rust repository.
+
+```console
+git clone https://github.com/rust-lang/rust
+```
+
+Now, let's set up the build system to use `cg_clif`.
+
+```text
+$ cat > config.toml <<EOF
+[rust]
+codegen-backends = ["cranelift"]
+EOF
+```
+
+Finally, let's run the build. This can take a long time, over a half-hour in some cases.
+
+```console
+./x.py build
+```
+
+## How can I help?
+
+You don't need to be a compiler developer to help improve `cg_clif`!  The best
+way you can help is by testing `cg_clif` on different Rust crates across the
+ecosystem.  Just while writing this article, I found [two][#1102]
+[bugs][#1101], so there's plenty of work left to be done. Please report any bugs you find
+to the [`rustc_codegen_cranelift` git repository][issue].
+
+In the future, we hope to distribute `cg_clif` with Rustup, and if it matures sufficiently, eventually make it the default backend for debug builds.
+
+[`rustc_codegen_cranelift`]: https://github.com/bjorn3/rustc_codegen_cranelift
+[Cranelift]: https://github.com/bytecodealliance/wasmtime/tree/main/cranelift#cranelift-code-generator
+[#77975]: https://github.com/rust-lang/rust/pull/77975
+[MCP]: https://forge.rust-lang.org/compiler/mcp.html
+[compiler-team#270]: https://github.com/rust-lang/compiler-team/issues/270
+[`rustc-dev-guide`]: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html#creating-a-rustup-toolchain
+[git worktree]: https://rustc-dev-guide.rust-lang.org/building/suggested.html#working-on-multiple-branches-at-the-same-time
+[#1102]: https://github.com/bjorn3/rustc_codegen_cranelift/issues/1102
+[#1101]: https://github.com/bjorn3/rustc_codegen_cranelift/issues/1101
+[issue]: https://github.com/bjorn3/rustc_codegen_cranelift/issues/new