The Swift programming language is built on LLVM and uses LLVM IR and the LLVM backend for code generation, but it also contains a new high-level IR called SIL to model the semantics of the language (and perform optimizations) at a higher level. In this talk, we discuss the motivations and applications of SIL, including high-level semantic analyses and transformations such as flow-dependent diagnostics, devirtualization, specialization, reference counting optimization, and TBAA, and we compare SIL's design with that of LLVM IR.