How to easily add declarations to the LLVM module?

Question

I'm writing my own language in LLVM and I'm using external C functions from std and custom. I'm now adding declarations using C++ classes for LLVM IR. Like this:

void register_malloc(llvm::Module *module) {
    std::vector<llvm::Type*> arg_types;
    arg_types.push_back(Type::getInt32Ty(getGlobalContext()));

    FunctionType* type = FunctionType::get(
            Type::getInt8PtrTy(getGlobalContext()), arg_types, false);

    Function *func = Function::Create(
                type, llvm::Function::ExternalLinkage,
                llvm::Twine("malloc"),
                module
           );
    func->setCallingConv(llvm::CallingConv::C);
}

void register_printf(llvm::Module *module) {
    std::vector<llvm::Type*> printf_arg_types;
    printf_arg_types.push_back(llvm::Type::getInt8PtrTy(getGlobalContext()));

    llvm::FunctionType* printf_type =
        llvm::FunctionType::get(
            llvm::Type::getInt32Ty(getGlobalContext()), printf_arg_types, true);

    llvm::Function *func = llvm::Function::Create(
                printf_type, llvm::Function::ExternalLinkage,
                llvm::Twine("printf"),
                module
           );
    func->setCallingConv(llvm::CallingConv::C);
}

I'm gonna define tens of external functions, is there some easy way to define them, and how?

I think about "including" C header(or LLVM IR file .ll) to the module. But I couldn't find any example how to do this...

i guess there is no include; for a little bit convenience, take a look at my previous answer to another question, specially for printf_prototype. — Hongxu Chen
– Hongxu Chen, Commented Sep 24, 2015 at 5:50
You can use clang -S -emit-llvm to "compile" headers and then use CPPBackend to generate C++ code that creates these definitions. — arrowd
– arrowd, Commented Sep 24, 2015 at 7:01
@arrowd I was thinking about something like that. How do I add/load declarations from LLVM file to the module? — kravemir
– kravemir, Commented Sep 24, 2015 at 10:12

arrowd · Accepted Answer · 2015-09-28 05:41:44Z

2

Create an empty C source and include every header you need, then compile it to LLVM IR with clang -S -emit-llvm. This source would contain declarations for every function from headers. Now use llc -march=cpp out.ll and it will produce C++ source that calls LLVM API to generate given IR. You can copy-paste this code into your program.

Make sure you have cpp backend enabled during LLVM build.

answered Sep 28, 2015 at 5:41

arrowd

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2 Comments

yujaiyu Over a year ago

Is there a way to do that within a Module pass instead of copy and pasting the code at the beginning of the file? I have a pass that needs to inject an external function call.

arrowd Over a year ago

As there is no cpp backend now, the only way left is to compile to bitcode and load it in the pass using parseIRFile.

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