LLVM Tutorial: How to Implement Loop-Invariant Code Motion Optimization

A developer tutorial on DEV Community walks through implementing Loop-Invariant Code Motion (LICM), a classic compiler optimization, using LLVM's pass infrastructure. LICM works by identifying computations inside a loop that produce the same result on every iteration and moving them outside the loop to avoid redundant recalculation. The tutorial builds on earlier articles covering LLVM analysis and transformation passes, including dead code elimination and constant propagation. The implementation centers on two key components: a run method that uses LLVM's LoopInfo analysis to locate loops and their preheaders, and an isLoopInvariant predicate that checks whether an instruction's operands are defined outside the loop. The author emphasizes practical, simplified code to demonstrate LLVM's facilities rather than providing a production-ready solution.