Implementing a new pass
A pass can perform arbitrary complex transformations on the LLVM IR. To illustrate the mechanics of adding a new pass, we add a pass that performs a simple instrumentation.
To investigate the performance of a program, it is interesting to know how often functions are called, and how long they run. One way to collect this data is to insert counters into each function. This process is called instrumentation. We will write a simple instrumentation pass that inserts a special function call at the entry of each function and each exit point. These functions collect the timing information and write it into a file. As a result, we can create a very basic profiler that we’ll name the poor person’s profiler, or in short, ppprofiler. We will develop the new pass so that it can be used as a standalone plugin or added as a plugin to the LLVM source tree. After that, we’ll look at how the passes that come with LLVM are integrated into the framework...