add a bit on llvm

Author: mark-i-m

src/codegen.md

@@ -1 +1,48 @@
-# Generating LLVM IR
+# Code generation
+
+Code generation or "codegen" is the part of the compiler that actually generates
+an executable binary. rustc uses LLVM for code generation.
+
+## What is LLVM?
+
+All of the preceeding 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.
+
+Like most compilers, rustc is composed of a "frontend" and a "backend". The
+"frontend" is responsible for taking raw source code, checking it for
+correctness, and getting it into a format `X` from which we can generate
+executable machine code. The "backend" then takes that format `X` and produces
+(possibly optimized) executable machine code for some platform. All of the
+previous chapters deal with rustc's frontend.
+
+rustc's backend is [LLVM](https://llvm.org), "a collection of modular and
+reusable compiler and toolchain technologies". In particular, the LLVM project
+contains a pluggable compiler backend (also called "LLVM"), which is used by
+many compiler projects, including the `clang` C compiler and our beloved
+`rustc`.
+
+LLVM's "format `X`" is called LLVM IR. It is basically assembly code with
+additional low-level types and annotations added. These annotations are helpful
+for doing optimizations on the LLVM IR and outputed machine code. The end result
+of all this is (at long last) something executable (e.g. an ELF object or wasm).
+
+There are a few benefits to using LLVM:
+
+- We don't have to write a whole compiler backend. This reduces implementation
+  and maintainance burden.
+- We benefit from the large suite of advanced optimizations that the LLVM
+  project has been collecting.
+- We automatically can compile Rust to any of the platforms for which LLVM has
+  support. For example, as soon as LLVM added support for wasm, voila! rustc,
+  clang, and a bunch of other languages were able to compile to wasm! (Well,
+  there was some extra stuff to be done, but we were 90% there anyway).
+- We and other compiler projects benefit from each other. For example, when the
+  [Spectre and Meltdown security vulnerabilities][spectre] were discovered, only LLVM
+  needed to be patched.
+
+[spectre]: https://meltdownattack.com/
+
+## Generating LLVM IR
+
+TODO