ConstantFolding.cpp

#include "ConstantFolding.hpp"

using namespace llvm;

PreservedAnalyses
ConstantFolding::run(Module& mod, ModuleAnalysisManager& mam)
{
  int constFoldTimes = 0;

  // 遍历所有函数
  for (auto& func : mod) {
    // 遍历每个函数的基本块
    for (auto& bb : func) {
      std::vector<Instruction*> instToErase;
      // 遍历每个基本块的指令
      for (auto& inst : bb) {
        // 判断当前指令是否是二元运算指令
        if (auto binOp = dyn_cast<BinaryOperator>(&inst)) {
          // 获取二元运算指令的左右操作数，并尝试转换为常整数
          Value* lhs = binOp->getOperand(0);
          Value* rhs = binOp->getOperand(1);
          auto constLhs = dyn_cast<ConstantInt>(lhs);
          auto constRhs = dyn_cast<ConstantInt>(rhs);
          switch (binOp->getOpcode()) {
            case Instruction::Add: {
              // 若左右操作数均为整数常量，则进行常量折叠与use替换
              if (constLhs && constRhs) {
                binOp->replaceAllUsesWith(ConstantInt::getSigned(
                  binOp->getType(),
                  constLhs->getSExtValue() + constRhs->getSExtValue()));
                instToErase.push_back(binOp);
                ++constFoldTimes;
              }
              break;
            }
            case Instruction::Sub: {
              if (constLhs && constRhs) {
                binOp->replaceAllUsesWith(ConstantInt::getSigned(
                  binOp->getType(),
                  constLhs->getSExtValue() - constRhs->getSExtValue()));
                instToErase.push_back(binOp);
                ++constFoldTimes;
              }
              break;
            }
            case Instruction::Mul: {
              if (constLhs && constRhs) {
                binOp->replaceAllUsesWith(ConstantInt::getSigned(
                  binOp->getType(),
                  constLhs->getSExtValue() * constRhs->getSExtValue()));
                instToErase.push_back(binOp);
                ++constFoldTimes;
              }
              break;
            }
            case Instruction::UDiv:
            case Instruction::SDiv: {
              if (constLhs && constRhs) {
                binOp->replaceAllUsesWith(ConstantInt::getSigned(
                  binOp->getType(),
                  constLhs->getSExtValue() / constRhs->getSExtValue()));
                instToErase.push_back(binOp);
                ++constFoldTimes;
              }
              break;
            }
            default:
              break;
          }
        }
      }
      // 统一删除被折叠为常量的指令
      for (auto& i : instToErase)
        i->eraseFromParent();
    }
  }

  mOut << "ConstantFolding running...\nTo eliminate " << constFoldTimes
       << " instructions\n";
  return PreservedAnalyses::all();
}