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<h1 id="lowering-mir-to-a-codegen-ir"><a class="header" href="#lowering-mir-to-a-codegen-ir">Lowering MIR to a Codegen IR</a></h1>
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<p>Now that we have a list of symbols to generate from the collector, we need to
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generate some sort of codegen IR. In this chapter, we will assume LLVM IR,
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since that's what rustc usually uses. The actual monomorphization is performed
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as we go, while we do the translation.</p>
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<p>Recall that the backend is started by
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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>. Eventually, this reaches
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/fn.codegen_mir.html"><code>rustc_codegen_ssa::mir::codegen_mir</code></a>, which does the lowering from
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MIR to LLVM IR.</p>
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<p>The code is split into modules which handle particular MIR primitives:</p>
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<ul>
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<li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/block/index.html"><code>rustc_codegen_ssa::mir::block</code></a> will deal with translating
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blocks and their terminators. The most complicated and also the most
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interesting thing this module does is generating code for function calls,
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including the necessary unwinding handling IR.</li>
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<li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/statement/index.html"><code>rustc_codegen_ssa::mir::statement</code></a> translates MIR statements.</li>
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<li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/operand/index.html"><code>rustc_codegen_ssa::mir::operand</code></a> translates MIR operands.</li>
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<li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/place/index.html"><code>rustc_codegen_ssa::mir::place</code></a> translates MIR place references.</li>
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<li><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/rvalue/index.html"><code>rustc_codegen_ssa::mir::rvalue</code></a> translates MIR r-values.</li>
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</ul>
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<p>Before a function is translated a number of simple and primitive analysis
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passes will run to help us generate simpler and more efficient LLVM IR. An
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example of such an analysis pass would be figuring out which variables are
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SSA-like, so that we can translate them to SSA directly rather than relying on
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LLVM's <code>mem2reg</code> for those variables. The analysis can be found in
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/analyze/index.html"><code>rustc_codegen_ssa::mir::analyze</code></a>.</p>
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<p>Usually a single MIR basic block will map to a LLVM basic block, with very few
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exceptions: intrinsic or function calls and less basic MIR statements like
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<code>assert</code> can result in multiple basic blocks. This is a perfect lede into the
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non-portable LLVM-specific part of the code generation. Intrinsic generation is
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fairly easy to understand as it involves very few abstraction levels in between
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and can be found in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/intrinsic/index.html"><code>rustc_codegen_llvm::intrinsic</code></a>.</p>
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<p>Everything else will use the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/builder/index.html">builder interface</a>. This is the code that gets
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called in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/index.html"><code>rustc_codegen_ssa::mir::*</code></a> modules discussed above.</p>
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<blockquote>
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<p>TODO: discuss how constants are generated</p>
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</blockquote>
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