Printing diagnostic messages

In software development, there are many ways to diagnose a bug—for instance, using a debugger, inserting a sanitizer into your program (to catch invalid memory access, for example), or simply using one of the simplest yet most effective ways: adding print statements. While the last option doesn't sound really smart, it is actually pretty useful in many cases where other options cannot unleash their full potential (for example, release mode binaries with poor debug information quality or multithread programs).

LLVM provides a small utility that not only helps you to print out debug messages but also filters which messages to show. Let's say we have an LLVM Pass, SimpleMulOpt, which replaces multiplication by power-of-two constants with left-shifting operations (which is what we did in the last section of the previous chapter, Processing LLVM IR). Here is part of its run method:

PreservedAnalyses
SimpleMulOpt::run(Function &F, FunctionAnalysisManager...