Simple 'hello world' example in LLVM IRBuilder

hello_world_llvm.cpp

// % clang++ $(llvm-config --cxxflags --ldflags --system-libs --libs core) -o hello_world_llvm hello_world_llvm.cpp
// % ./hello_world_llvm
// hello world

#include <llvm/ExecutionEngine/ExecutionEngine.h>
#include <llvm/ExecutionEngine/GenericValue.h>
#include <llvm/IR/IRBuilder.h>
#include <stdio.h>

using namespace llvm;

int main() {
    auto context = std::make_unique<LLVMContext>();
    IRBuilder<> builder(*context);

    auto module = std::make_unique<Module>("hello", *context);

    // build a 'main' function
    auto i32 = builder.getInt32Ty();
    auto prototype = FunctionType::get(i32, false);
    Function *main_fn = Function::Create(prototype, Function::ExternalLinkage, "main", module.get());
    BasicBlock *body = BasicBlock::Create(*context, "body", main_fn);
    builder.SetInsertPoint(body);

    // use libc's printf function
    auto i8p = builder.getInt8PtrTy();
    auto printf_prototype = FunctionType::get(i8p, true);
    auto printf_fn = Function::Create(printf_prototype, Function::ExternalLinkage, "printf", module.get());

    // call printf with our string
    auto format_str = builder.CreateGlobalStringPtr("hello world\n");
    builder.CreateCall(printf_fn, { format_str });

    // return 0 from main
    auto ret = ConstantInt::get(i32, 0);
    builder.CreateRet(ret);

    // if you want to print the LLVM IR:
    //module->print(llvm::outs(), nullptr);

    // execute it!
    ExecutionEngine *executionEngine = EngineBuilder(std::move(module)).setEngineKind(llvm::EngineKind::Interpreter).create();
    Function *main = executionEngine->FindFunctionNamed(StringRef("main"));
    auto result = executionEngine->runFunction(main, {});

    // return the result
    return result.IntVal.getLimitedValue();
}