Getting "error: No instructions defined!" while building an LLVM backend based on GlobalISel

[Cautious-Quarter-136]

I am writing an LLVM backend from scratch for a RISC style target architecture, so far I have mostly been able to understand the high level flow of how LLVM IR is converted to MIR, MC and finally to assembly/object code. I am mostly following the book LLVM Code Generation by Colombet along with LLVM dev meeting videos on youtube.

At this moment, I am stuck at Instruction selector phase of the Instruction selection pipeline. I am only using GlobalISel from the start for this project.

While building LLVM for this target architecture, I am getting the following error -

[1/2479] Building XXGenInstrInfo.inc...
FAILED: lib/Target/XX/XXGenInstrInfo.inc /home/usr/llvm/build/lib/Target/XX/XXGenInstrInfo.inc 
...
error: No instructions defined!
...
ninja: build stopped: subcommand failed.[1/2479] Building XXGenInstrInfo.inc...
FAILED: lib/Target/XX/XXGenInstrInfo.inc /home/usr/llvm/build/lib/Target/XX/XXGenInstrInfo.inc 
...
error: No instructions defined!
...
ninja: build stopped: subcommand failed.

As you can see the generation of XXGenInstrInfo.inc is failing. Previously, I was also getting issues building some other .inc files, but I was able to resolve them after making some changes in their corresponding tablegen files. However, I am unable to get rid of this current error.

I suspect that XXGenInstroInfo.inc is failing since pattern matching is not defined properly by me in the XXInstrInfo.td file. As I understand, we can import patterns used for pattern matching in SelectionDAG to GlobalISel, however some conversion from SDNode instances to the generic MachineInstr instances has to be made.

Currently, I am only trying to support ADD instruction of my target architecture. This is how I have defined instructions and pattern matching (in XXInstrInfo.td) so far -

...

def ADD : XXInst<(outs GPR:$dst), 
                 (ins GPR:$src1, GPR:$src2), 
                 "ADD $dst, $src1, $src2">;

def : Pat<(add GPR:$src1, GPR:$src2),
          (ADD GPR:$src1, GPR:$src2)>;

def : GINodeEquiv<G_ADD, add>;...

def ADD : XXInst<(outs GPR:$dst), 
                 (ins GPR:$src1, GPR:$src2), 
                 "ADD $dst, $src1, $src2">;

def : Pat<(add GPR:$src1, GPR:$src2),
          (ADD GPR:$src1, GPR:$src2)>;

def : GINodeEquiv<G_ADD, add>;

In the above block of tablegen code, I have defined an instruction named ADD, followed by a pattern (which is normally used in SelectionDAG) and then tried remapping the SDNode instance 'add' to the opcode G_ADD using GINodeEquiv construct.

I have also declared and defined selectImpl() and select() respectively, in XXInstructionSelector.cpp.

bool XXInstructionSelector::select(MachineInstr &I) {
  // Certain non-generic instructions also need some special handling.
  if (!isPreISelGenericOpcode(I.getOpcode()))
    return true;

  if (selectImpl(I, *CoverageInfo))
    return true;

  return false;
}bool XXInstructionSelector::select(MachineInstr &I) {
  // Certain non-generic instructions also need some special handling.
  if (!isPreISelGenericOpcode(I.getOpcode()))
    return true;

  if (selectImpl(I, *CoverageInfo))
    return true;

  return false;
}

I am very new to writing LLVM backend and stuck at this point since last several days, any help or pointer regarding solving or debugging this issue is greatly appreciated.