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| <h1 id="from-mir-to-binaries"><a class="header" href="#from-mir-to-binaries">From MIR to binaries</a></h1> |
| <p>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.</p> |
| <p>So far, |
| we've shown how the compiler can take raw source code in text format |
| and transform it into <a href="./mir/index.html">MIR</a>. |
| 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.</p> |
| <blockquote> |
| <p>NOTE: This part of a compiler is often called the <em>backend</em>. |
| 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".</p> |
| </blockquote> |
| <p>So what do we need to do?</p> |
| <ol> |
| <li>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 <em>monomorphization</em>, |
| so the process of collecting all the concrete types is called <em>monomorphization collection</em>.</li> |
| <li>Next, we need to actually lower the MIR to a codegen IR |
| (usually LLVM IR) for each concrete type we collected.</li> |
| <li>Finally, we need to invoke the codegen backend, |
| which runs a bunch of optimization passes, |
| generates executable code, |
| and links together an executable binary.</li> |
| </ol> |
| <p>The code for codegen is actually a bit complex due to a few factors:</p> |
| <ul> |
| <li>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.</li> |
| <li>Codegen happens asynchronously in another thread for performance.</li> |
| <li>The actual codegen is done by a third-party library (either of the 3 backends).</li> |
| </ul> |
| <p>Generally, the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/index.html"><code>rustc_codegen_ssa</code></a> crate contains backend-agnostic code, |
| while the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/index.html"><code>rustc_codegen_llvm</code></a> crate contains code specific to LLVM codegen.</p> |
| <p>At a very high level, the entry point is |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/base/fn.codegen_crate.html"><code>rustc_codegen_ssa::base::codegen_crate</code></a>. |
| This function starts the process discussed in the rest of this chapter.</p> |
| |
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