Lowering MIR to a Codegen IR

Now that we have a list of symbols to generate from the collector, we need to generate some sort of codegen IR. In this chapter, we will assume LLVM IR, since that's what rustc usually uses. The actual monomorphization is performed as we go, while we do the translation.

Recall that the backend is started by rustc_codegen_ssa::base::codegen_crate. Eventually, this reaches rustc_codegen_ssa::mir::codegen_mir, which does the lowering from MIR to LLVM IR.

The code is split into modules which handle particular MIR primitives:

• rustc_codegen_ssa::mir::block will deal with translating blocks and their terminators. The most complicated and also the most interesting thing this module does is generating code for function calls, including the necessary unwinding handling IR.
• rustc_codegen_ssa::mir::statement translates MIR statements.
• rustc_codegen_ssa::mir::operand translates MIR operands.
• rustc_codegen_ssa::mir::place translates MIR place references.
• rustc_codegen_ssa::mir::rvalue translates MIR r-values.

Before a function is translated a number of simple and primitive analysis passes will run to help us generate simpler and more efficient LLVM IR. An example of such an analysis pass would be figuring out which variables are SSA-like, so that we can translate them to SSA directly rather than relying on LLVM's mem2reg for those variables. The analysis can be found in rustc_codegen_ssa::mir::analyze.

Usually a single MIR basic block will map to a LLVM basic block, with very few exceptions: intrinsic or function calls and less basic MIR statements like assert can result in multiple basic blocks. This is a perfect lede into the non-portable LLVM-specific part of the code generation. Intrinsic generation is fairly easy to understand as it involves very few abstraction levels in between and can be found in rustc_codegen_llvm::intrinsic.

Everything else will use the builder interface. This is the code that gets called in the rustc_codegen_ssa::mir::* modules discussed above.

TODO: discuss how constants are generated