LLVM C++ API bitcode loading and execution - llvm

Question

I am looking for a full sample that covers usage of the LLVM C++ API, particularly loading a function from a bitcode (not a typo, they call it bitcode) file, running it and get the results. I have studied this blog post and I am trying to port it to C++ but I am struggling to understand how to create the various instances needed, particularly the execution engine. I am using clang -c -emit-llvm file.c to compile a C file to a .bc LLVM bitcode file. The command clang -S -emit-llvm file.c also works and generates a textual .ll file. The function parseIRFile seems to be able to load both.
This is what I have so far:
LLVMContext context;
SMDiagnostic error;
unique_ptr<Module> mod = parseIRFile(StringRef(pathToLlOrBcFile), error, context);
I does not have to use JIT, I am fine with the basic interpreter for now; but I wish to make it work with MCJIT or whatever it's called later on.
Thanks to #arnt for noticing that I was actually using the IR text format; I changed the Makefile and the C++ app to reflect the fact that both .ll and .bc can be parsed by the same function.

Answer 1

I am using llvm-devel.x86_64 9.0.1-5.fc31 on Fedora 31. Full code below.
main.cc (This is the C++ app that loads the LLVM bitcode)
#include <iostream>
#include <llvm/IR/Module.h>
#include <llvm/IRReader/IRReader.h>
#include <llvm/Support/SourceMgr.h>
#include <llvm/ExecutionEngine/ExecutionEngine.h>
#include <llvm/ExecutionEngine/GenericValue.h>
using std::unique_ptr;
using std::cout;
using std::endl;
using llvm::Module;
using llvm::SMDiagnostic;
using llvm::LLVMContext;
using llvm::parseIRFile;
using llvm::StringRef;
using llvm::ExecutionEngine;
using llvm::EngineBuilder;
using llvm::ArrayRef;
using llvm::GenericValue;
using llvm::Function;
int main(int argc, char const *argv[]) {
LLVMContext context;
SMDiagnostic error;
unique_ptr<Module> mod = parseIRFile(StringRef("hosted.bc" /* .ll files also work */), error, context);
ExecutionEngine *executionEngine = EngineBuilder(std::move(mod)).setEngineKind(llvm::EngineKind::Interpreter).create();
Function *add = executionEngine->FindFunctionNamed(StringRef("add"));
GenericValue param1, param2;
param1.FloatVal = 5.5;
param2.FloatVal = 2.7;
GenericValue params[] = { param1, param2 };
ArrayRef<GenericValue> args = ArrayRef<GenericValue>(params, 2);
GenericValue result = executionEngine->runFunction(add, args);
cout << param1.FloatVal << " + " << param2.FloatVal << " = " << result.FloatVal << endl;
}
hosted.c (This is a C app that I compile into an .bc file with clang)
float add(float a, float b) {
return a + b;
}
Makefile (Used to compile the native app and the LLVM bytecode to be hosted in it)
app.o: main.cc
g++ main.cc -lLLVM -o app.o
hosted.bc: hosted.c
clang -c -emit-llvm hosted.c
clean:
rm app.o
rm hosted.bc
.PHONY: clean
Output (Compiling and running)
[dario#localhost llvm-cpp-first]$ make hosted.bc && make && ./app.o
clang -c -emit-llvm hosted.c
g++ main.cc -lLLVM -o app.o
5.5 + 2.7 = 8.2