CODEWITHSUNDEEP
c++llvmllvm-clangllvm-c++-api
user868538
user868538

Reputation: 177

Using C++ classes in LLVM Modules

Based on the Kaleidoscope and Kaleidoscope with MCJIT tutorials, I have code to create a Module and function and call it using MCJIT. The function needs a prototype:

auto ft = llvm::FunctionType::get(llvm::Type::getInt32Ty(Context), argTypes, false);

However, the example only covers Double as parameters and return values (the above uses an int). To do anything advanced, you need to pass things like classes and containers.

How do you use existing C++ classes in the module?

Sure, you can link to any library you want, but you need to declare function prototypes to use them. If the library API has classes, how do you declare them?

What I want is something like this:

auto ft = llvm::FunctionType::get(llvm::Type::getStructTy("class.std::string"), argTypes, false);

where class.std::string has been imported from string.h.

The LLVM API only has primitive types. You can define structs to represent the classes, but this is way too hard to do manually (and not portable).

A way to do it might be to compile the class to bitcode and read it into a module, but I want to avoid temporary files if possible. Also I'm not sure how to extract the type from the module but it should be possible. I tried this on a header file of one of my classes (I renamed the header file to a cpp file otherwise clang would make into a .gch precompiled header) and the result was just a constant... maybe it was optimised out? I tried it on the cpp file and it resulted in 36000 lines of code...

Then I found this page. Instead of using the LLVM API, I should use the Clang API because Clang, as a compiler, can compile the code into a Module. Then I can use the LLVM API with the imported Modules. Is this the right way to go? Any working source code is appreciated because it took forever just to get function calling working (the tutorials are out of date and documentation is scarce).

Upvotes: 4

Views: 735

Answers (1)

GenuinePlaceholder
GenuinePlaceholder

Reputation: 695

The way I would do it is to compile the class to LLVM IR, and then link the two modules. Then, there's two options to extract the type from the module:

First, you can use the llvm::TypeFinder. The way you use it is by creating it, and then calling run() on it with the module as an argument. This code snippet will print out all of the types in the module:

llvm::TypeFinder type_finder;
type_finder.run(module, true);
for (auto t : type_finder) {
  std::cout << t->getName().str() << std::endl;
}

Alternatively, it's possible to use Module's getIdentifiedStructTypes() method and iterate over the resulting vector in the same way as above.

Upvotes: 1

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