LLVM code generation with C++ API: How to handle OpenMP calls

Question

I'm working on a compiler for a small language. Inside the compiler, I'm using the LLVM C++ API to generate llvm code, similar to the LLVM Kaleidoscope tutorial. So I'm using TheModule, TheContext, BasicBlocks, and calls to Builder.Create...().

I can currently generate valid llvm code for arithmetic, control flow, and methods. However, I would also like my small language to support very simple OpenMP pragmas. For example,

#pragma omp parallel
{
  print "Hello World"
}

I've tried writing a similar program in C++,

#include <iostream>
int main() {
  #pragma omp parallel
  {
    std::cout << "Hi";
  }
}

and generating llvm using clang++ -S -emit-llvm file.cpp -fopenmp. Along with the rest of the code, this generates the following lines which seem to implement the OpenMP functionality:

declare void @__kmpc_fork_call(%ident_t*, i32, void (i32*, i32*, ...)*, ...)
define internal void @.omp_outlined.(...)

From researching these statements, I found the Clang OpenMP API that contains calls like

OMPParallelDirective * OMPParallelDirective::Create(...)

I'm guessing this is what the Clang compiler uses to generate the statements above. However, it seems to be separate from the LLVM C++ API, as it doesn't reference TheContext, TheModule, etc...

So my question: Is there any way to leverage the Clang OpenMP API calls with my LLVM C++ API calls to generate the kmpc__fork_call and @.omp_outlined IR needed for parallel computation?

I did try decompiling the llvm generated from the C++ code back into LLVM C++ API code using llc -march=cpp file.bc ... but was unsuccessful.

Answer 1

The API you found operate on clang AST and are hardly usable outside clang. In fact, there are no OpenMP constructs at the LLVM IR level - everything is already lowered down to runtime calls, etc.

So, you'd really need to implement codegeneration for OpenMP by yourself emitting runtime calls as necessary (and per your language semantics).