How to register a new LLVM backend?

Question

I'm developing a very basic new LLVM backend for a RISC machine (named Risco), based on the existing Sparc backend and this tutorial. To register the backend, I've used the following.

• At RiscoTargetMachine.cpp:

  extern "C" void LLVMInitializeRiscoTarget()
  {
      // Register the target.
      RegisterTargetMachine<RiscoSimulatorTargetMachine> X(TheRiscoTarget);
      RegisterAsmInfo<RiscoMCAsmInfo> Y(TheRiscoTarget);
  }
  

• At Risco.td:

  def : Processor<"simulator", NoItineraries, [FeatureA]>;
  
  def Risco : Target {
          // Pull in Instruction Info:
          let InstructionSet = RiscoInstrInfo;
  }
  

• At TargetInfo/RiscoTargetInfo.cpp:

  Target llvm::TheRiscoTarget;
  
  extern "C" void LLVMInitializeRiscoTargetInfo() {
          RegisterTarget<> X(TheRiscoTarget, "risco", "Risco");
  }
  

• At the top level LLVM configure script:

  # Added Risco to the TARGETS_TO_BUILD variable at line 4965 (from svn trunk):
  all) TARGETS_TO_BUILD="X86 Sparc PowerPC Alpha ARM Mips CellSPU PIC16 XCore MSP430 SystemZ Blackfin CBackend CppBackend MBlaze PTX Risco" ;;
  

After build, llc -version doesn't show the new target. Even llc -march=risco test.ll says it's an invalid target. What am I missing?

PS: Currently, I'm including the new target as a folder inside llvm/lib/Target. How can I change that so I can build the target separately, and load it dynamic with llc -load?

Answer 1

The default first template parameter for RegisterTarget is Triple::InvalidArch. Try this:

extern "C" void LLVMInitializeRiscoTargetInfo() {
    RegisterTarget<Triple::UnknownArch> X(TheRiscoTarget, "risco", "Risco");
}

You might also need to register an assembly printer for your backend in RiscoAsmPrinter.cpp:

extern "C" void LLVMInitializeRiscoAsmPrinter() {
    RegisterAsmPrinter<RiscoAsmPrinter> X(TheRiscoTarget);
}

I'm not quite sure what you mean by the last bit. My Makefile has LOADABLE_MODULE=1 and builds the target as a shared object in the lib folder. In order to see the Risco target in the list of registered targets, I would run something like ./bin/llc -load ./lib/libLLVMRisco.so -version assuming you are on Linux.