The code size of iOS apps is critical due to the size limit of apple store. More and more iOS apps are written in Swift. The Swift programming language provides many new language features such as protocol to facilitate software development. In order to support the implementation of these new features, the existing Swift compiler has to generate the protocol related code and data. However, these generated code and data may not be used in the whole project. For example, some protocol definition is used one module as public and not really consumed by any other modules. Preliminary experiment shows the size of some commercial iOS app can be potentially reduced by 9% through aggressive dead code elimination. Those unused code and data cannot be eliminated easily by the compiler optimizations since they are recorded in llvm.used data structure. In addition, the generated code and data might be implicitly used by Swift runtime library. This calls for a smarter, much more advanced static analysis and novel additions to the classic dead code elimination technique. We introduce a novel building pipeline that eliminates the protocol from swift class by leveraging the link time optimization in existing LLVM compiler. In this framework, the swift files are first compiled as LLVM bitcode files and the llvm-link is used to merge all the LLVM bitcode files as one bitcode file. A new LLVM optimization is proposed to eliminate the protocol conformance related variables from the LLVM.used array in this bitcode file. It enables more opportunities for link time optimization to transform global variables into local variables and then identify the dead local variables. The following dead code elimination is extended to update the protocol conformance tables as well as LLVM.used array. The experiment shows that this novel approach reduces the code size of some commercial iOS app by 2%.
