265 lines
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265 lines
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<h1 class="menu-title">Rust Compiler Development Guide</h1>
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<main>
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<h1 id="from-mir-to-binaries"><a class="header" href="#from-mir-to-binaries">From MIR to binaries</a></h1>
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<p>All of the preceding chapters of this guide have one thing in common:
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we never generated any executable machine code at all!
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With this chapter, all of that changes.</p>
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<p>So far,
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we've shown how the compiler can take raw source code in text format
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and transform it into <a href="./mir/index.html">MIR</a>.
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We have also shown how the compiler does various
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analyses on the code to detect things like type or lifetime errors.
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Now, we will finally take the MIR and produce some executable machine code.</p>
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<blockquote>
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<p>NOTE: This part of a compiler is often called the <em>backend</em>.
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The term is a bit overloaded because in the compiler source,
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it usually refers to the "codegen backend" (i.e. LLVM, Cranelift, or GCC).
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Usually, when you see the word "backend" in this part,
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we are referring to the "codegen backend".</p>
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</blockquote>
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<p>So what do we need to do?</p>
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<ol>
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<li>First, we need to collect the set of things to generate code for.
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In particular,
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we need to find out which concrete types to substitute for generic ones,
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since we need to generate code for the concrete types.
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Generating code for the concrete types
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(i.e. emitting a copy of the code for each concrete type) is called <em>monomorphization</em>,
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so the process of collecting all the concrete types is called <em>monomorphization collection</em>.</li>
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<li>Next, we need to actually lower the MIR to a codegen IR
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(usually LLVM IR) for each concrete type we collected.</li>
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<li>Finally, we need to invoke the codegen backend,
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which runs a bunch of optimization passes,
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generates executable code,
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and links together an executable binary.</li>
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</ol>
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<p>The code for codegen is actually a bit complex due to a few factors:</p>
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<ul>
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<li>Support for multiple codegen backends (LLVM, Cranelift, and GCC).
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We try to share as much backend code between them as possible,
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so a lot of it is generic over the codegen implementation.
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This means that there are often a lot of layers of abstraction.</li>
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<li>Codegen happens asynchronously in another thread for performance.</li>
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<li>The actual codegen is done by a third-party library (either of the 3 backends).</li>
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</ul>
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<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,
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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>
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<p>At a very high level, the entry point is
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<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>.
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This function starts the process discussed in the rest of this chapter.</p>
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