we got 3 (#1447)

Author: tshepang

src/backend/codegen.md

@@ -1,13 +1,16 @@
 # Code generation
 
-Code generation or "codegen" is the part of the compiler that actually
-generates an executable binary. Usually, rustc uses LLVM for code generation;
-there is also support for [Cranelift]. The key is that rustc doesn't implement
-codegen itself. It's worth noting, though, that in the Rust source code, many
-parts of the backend have `codegen` in their names (there are no hard
-boundaries).
-
-[Cranelift]: https://github.com/bytecodealliance/wasmtime/tree/HEAD/cranelift
+Code generation (or "codegen") is the part of the compiler
+that actually generates an executable binary.
+Usually, rustc uses LLVM for code generation,
+bu there is also support for [Cranelift] and [GCC].
+The key is that rustc doesn't implement codegen itself.
+It's worth noting, though, that in the Rust source code,
+many parts of the backend have `codegen` in their names
+(there are no hard boundaries).
+
+[Cranelift]: https://github.com/bytecodealliance/wasmtime/tree/main/cranelift
+[GCC]: https://github.com/rust-lang/rustc_codegen_gcc
 
 > NOTE: If you are looking for hints on how to debug code generation bugs,
 > please see [this section of the debugging chapter][debugging].

src/part-5-intro.md

@@ -1,54 +1,57 @@
 # From MIR to Binaries
 
-All of the preceding chapters of this guide have one thing in common: we never
-generated any executable machine code at all! With this chapter, all of that
-changes.
+All of the preceding chapters of this guide have one thing in common:
+we never generated any executable machine code at all!
+With this chapter, all of that changes.
 
-So far, we've shown how the compiler can take raw source code in text format
-and transform it into [MIR]. We have also shown how the compiler does various
-analyses on the code to detect things like type or lifetime errors. Now, we
-will finally take the MIR and produce some executable machine code.
+So far,
+we've shown how the compiler can take raw source code in text format
+and transform it into [MIR].
+We have also shown how the compiler does various
+analyses on the code to detect things like type or lifetime errors.
+Now, we will finally take the MIR and produce some executable machine code.
 
 [MIR]: ./mir/index.md
 
-> NOTE: This part of a compiler is often called the _backend_. The term is a bit
-> overloaded because in the compiler source, it usually refers to the "codegen
-> backend" (i.e. LLVM or Cranelift). Usually, when you see the word "backend"
-> in this part, we are referring to the "codegen backend".
+> NOTE: This part of a compiler is often called the _backend_.
+> The term is a bit overloaded because in the compiler source,
+> it usually refers to the "codegen backend" (i.e. LLVM, Cranelift, or GCC).
+> Usually, when you see the word "backend"  in this part,
+> we are referring to the "codegen backend".
 
 So what do we need to do?
 
-0. First, we need to collect the set of things to generate code for. In
-   particular, we need to find out which concrete types to substitute for
-   generic ones, since we need to generate code for the concrete types.
-   Generating code for the concrete types (i.e. emitting a copy of the code for
-   each concrete type) is called _monomorphization_, so the process of
-   collecting all the concrete types is called _monomorphization collection_.
+0. First, we need to collect the set of things to generate code for.
+   In particular,
+   we need to find out which concrete types to substitute for generic ones,
+   since we need to generate code for the concrete types.
+   Generating code for the concrete types
+   (i.e. emitting a copy of the code for each concrete type) is called _monomorphization_,
+   so the process of collecting all the concrete types is called _monomorphization collection_.
 1. Next, we need to actually lower the MIR to a codegen IR
    (usually LLVM IR) for each concrete type we collected.
-2. Finally, we need to invoke LLVM or Cranelift, which runs a bunch of
-   optimization passes, generates executable code, and links together an
-   executable binary.
+2. Finally, we need to invoke the codegen backend,
+   which runs a bunch of optimization passes,
+   generates executable code,
+   and links together an executable binary.
 
 [codegen1]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/base/fn.codegen_crate.html
 
 The code for codegen is actually a bit complex due to a few factors:
 
-- Support for multiple codegen backends (LLVM and Cranelift). We try to share as much
-  backend code between them as possible, so a lot of it is generic over the
-  codegen implementation. This means that there are often a lot of layers of
-  abstraction.
+- Support for multiple codegen backends (LLVM, Cranelift, and GCC).
+  We try to share as much backend code between them as possible,
+  so a lot of it is generic over the codegen implementation.
+  This means that there are often a lot of layers of abstraction.
 - Codegen happens asynchronously in another thread for performance.
-- The actual codegen is done by a third-party library (either LLVM or Cranelift).
+- The actual codegen is done by a third-party library (either of the 3 backends).
 
-Generally, the [`rustc_codegen_ssa`][ssa] crate contains backend-agnostic code
-(i.e. independent of LLVM or Cranelift), while the [`rustc_codegen_llvm`][llvm]
-crate contains code specific to LLVM codegen.
+Generally, the [`rustc_codegen_ssa`][ssa] crate contains backend-agnostic code,
+while the [`rustc_codegen_llvm`][llvm] crate contains code specific to LLVM codegen.
 
 [ssa]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/index.html
 [llvm]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/index.html
 
 At a very high level, the entry point is
-[`rustc_codegen_ssa::base::codegen_crate`][codegen1]. This function starts the
-process discussed in the rest of this chapter.
-
+[`rustc_codegen_ssa::base::codegen_crate`][codegen1].
+This function starts the process discussed in the rest of this chapter.