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@@ -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.