llvm dce

[lfeng14]

• 主体逻辑: 如果发现某一条指令是出现操作结果没有被使用的情况，就删除这条指令，并且记录这条指令的操作数，将它们加入到WorkList中，查看它们是否也是dead的，如果是，就继续执行清除。

  static bool eliminateDeadCode(Function &F, TargetLibraryInfo *TLI) {
    bool MadeChange = false;
    SmallSetVector<Instruction *, 16> WorkList;
    // Iterate over the original function, only adding insts to the worklist
    // if they actually need to be revisited. This avoids having to pre-init
    // the worklist with the entire function's worth of instructions.
    for (Instruction &I : llvm::make_early_inc_range(instructions(F))) {
      // We're visiting this instruction now, so make sure it's not in the
      // worklist from an earlier visit.
      if (!WorkList.count(&I))
        MadeChange |= DCEInstruction(&I, WorkList, TLI);
    }
  
    while (!WorkList.empty()) {
      Instruction *I = WorkList.pop_back_val();
      MadeChange |= DCEInstruction(I, WorkList, TLI);
    }
    return MadeChange;
  }
  

  static bool DCEInstruction(Instruction *I,
                             SmallSetVector<Instruction *, 16> &WorkList,
                             const TargetLibraryInfo *TLI) {
    if (isInstructionTriviallyDead(I, TLI)) {
      if (!DebugCounter::shouldExecute(DCECounter))
        return false;
  
      salvageDebugInfo(*I);
      salvageKnowledge(I);
  
      // Null out all of the instruction's operands to see if any operand becomes
      // dead as we go.
      for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
        Value *OpV = I->getOperand(i);
        I->setOperand(i, nullptr);
  
        if (!OpV->use_empty() || I == OpV)
          continue;
  
        // If the operand is an instruction that became dead as we nulled out the
        // operand, and if it is 'trivially' dead, delete it in a future loop
        // iteration.
        if (Instruction *OpI = dyn_cast<Instruction>(OpV))
          if (isInstructionTriviallyDead(OpI, TLI))
            WorkList.insert(OpI);
      }
  
      I->eraseFromParent();
      ++DCEEliminated;
      return true;
    }
    return false;
  }
  

参考

• https://www.ziyue.cafe/posts/dce-llvm-pass-analysis/