Declare a Function to use in FunctionPass with New Pass Manager using llvm::PassInfoMixin in LLVM 15

Question

I'm currently working with LLVM 15.0.0 and facing a challenge while using the New Pass Manager and llvm::PassInfoMixin. My goal is to insert calls to a specific function within a FunctionPass, but this function is not declared in the Module. I understand that in the context of a FunctionPass, directly modifying the module (such as adding a function declaration) is not allowed (docs).

With the legacy pass manager, I could use doInitialization and doFinalization for such module-level changes, but these are not available with the New Pass Manager (unless I'm missing something?). I am looking for guidance on how to properly declare a function in the module before executing my function pass. (The function definition itself will be available during linking.)

Here's a brief outline of my current approach:

class MyFunctionPass : public llvm::PassInfoMixin<MyFunctionPass> {
public:
    llvm::PreservedAnalyses run(llvm::Function &F, llvm::FunctionAnalysisManager &AM) {
        // Try to find if declaration exists
        llvm::Module *M = F.getParent();
        llvm::Function *funcToCall = M->getFunction("externalFunctionName");

        if (!funcToCall) {
            // Function is not declared, need to declare it here or handle this case
            // Using M would not work. Modifying the Module here is not allowed
        }

        // Insert calls to funcToCall in F
        // ...

        return llvm::PreservedAnalyses::all();
    }
};

I'm calling the pass dynamically with opt:

opt -load-pass-plugin=MyFunctionPass.so -passes='myfunctionpass' -disable-output testcase.ll

I understand that the New Pass Manager encourages separation of concerns and that a FunctionPass should not modify the module structure. However, I'm unsure how to proceed with declaring the function elsewhere or if there's an alternative approach I could take within the constraints of the New Pass Manager.

Possible workarounds

1. I'm not sure if there a way to declare a function in the module in preparation for a FunctionPass using llvm::PassInfoMixin. For example, maybe using the constructor.
2. I could use a separate ModulePass for this and somehow ensure it runs before my FunctionPass. (Not sure how to make it required.)

Are there any recommended practices or patterns in LLVM 15 for handling such scenarios?

Any insights, code examples, or references to relevant documentation would be greatly appreciated.

Answer 1

The key here is perhaps the reasons why you should not add functions in a FunctionPass:

• The caller of a FunctionPass iterates over a list of functions and adding a function changes that list. Changing a list while iterating over it is a notorious bug magnet. People make incorrect assumptions about whether the newly created function will be processed in the same loop, accidentally create infinite loops, etc.
• LLVM tries hard to provide passes with a simple, reliable environment. Changing the set of functions to work on while working on them creates complexity for other passes.

In this case I'd do it, though. You're creating a single external function while processing the first function, and removing none. That's not the kind of modification the LLVM authors want to ban.

Function passes don't operate on external functions so it's a noop for the other passes (and in a way for the pass manager too). Also it doesn't matter whether the pass manager iterates over the new function or not, so your change makes no unsafe assumptions. But make sure to keep the called function external ;)