Chapter 4. Basic IR Transformations

Until now, we have seen how the IR is independent of its target and how it can be used to generate code for a specific backend. To generate efficient code for the backend, we optimize the IR generated by the frontend by running a series of analysis and transformation passes using the LLVM pass manager. We must note that most of the optimizations that happen in a compiler take place on the IR, one of the reasons being that the IR is retargetable and the same set of optimizations would be valid for a number of targets. It reduces the effort of writing the same optimization for every target. There are some target-specific optimizations too; they happen at the selection DAG level, which we will see later. Another reason for IR being the target of optimization is that LLVM IR is in SSA form, which means every variable is assigned only once and every new assignment to a variable is a new variable itself. One very visible benefit of this representation...