llvm backend pass demo learning from ChatGPT

• can you write a short llvm backend optimizer?
• Certainly! Here’s an example of a simple LLVM backend optimizer that performs a basic optimization pass called “Constant Folding.” This optimization evaluates and replaces constant expressions with their computed values.

#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Value.h"

using namespace llvm;

struct ConstantFoldingPass : public PassInfoMixin<ConstantFoldingPass> {
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &) {
    bool Changed = false;

    for (BasicBlock &BB : F) {
      for (Instruction &I : BB) {
        if (auto *BinOp = dyn_cast<BinaryOperator>(&I)) {
          if (ConstantInt *C1 = dyn_cast<ConstantInt>(BinOp->getOperand(0))) {
            if (ConstantInt *C2 = dyn_cast<ConstantInt>(BinOp->getOperand(1))) {
              ConstantInt *Folded = nullptr;

              switch (BinOp->getOpcode()) {
                case Instruction::Add:
                  Folded = ConstantInt::get(C1->getType(), C1->getSExtValue() + C2->getSExtValue());
                  break;
                case Instruction::Sub:
                  Folded = ConstantInt::get(C1->getType(), C1->getSExtValue() - C2->getSExtValue());
                  break;
                case Instruction::Mul:
                  Folded = ConstantInt::get(C1->getType(), C1->getSExtValue() * C2->getSExtValue());
                  break;
                default:
                  break;
              }

              if (Folded) {
                BinOp->replaceAllUsesWith(Folded);
                Changed = true;
              }
            }
          }
        }
      }
    }

    return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
  }
};

extern "C" PassPluginLibraryInfo LLVM_ATTRIBUTE_WEAK
llvmGetPassPluginInfo() {
  return {
      LLVM_PLUGIN_API_VERSION, "ConstantFoldingPass", "v0.1",
      [](PassBuilder &PB) {
        PB.registerPipelineParsingCallback([](StringRef Name, FunctionPassManager &FPM,
                                              ArrayRef<PassBuilder::PipelineElement>) {
          if (Name == "constant-folding") {
            FPM.addPass(ConstantFoldingPass());
            return true;
          }
          return false;
        });
      }};
}

To build and use this optimizer, you’ll need to compile it as an LLVM plugin using the LLVM build system. Once compiled, you can use the optimizer in an LLVM pipeline by specifying the pass name “constant-folding.” For example:

opt -passes=constant-folding -S <input.ll> -o <output.ll>

This optimizer specifically targets binary operations (add, sub, and mul) with constant integer operands and replaces them with the computed constant value. Note that this is a simple demonstration, and a real-world optimizer would typically handle a wider range of instructions and optimizations.