nudt-compiler-cpp/src/ir/passes/IfConversion.cpp

// IfConversion: 将简单 if-else diamond 转换为算术 select
// - 扫描 CondBr→T→Br→M 且 F==M 的 diamond 模式
// - 安全检查：T 必须只有单一前驱（B），仅允许纯算术指令（禁 Div/Mod/浮点）
// - 将 phi 转换为 fv + (tv-fv)*zext(cond)
// - 配合 CFGSimplify 清理空块，使循环体变为单 BB → 可被 LoopUnroll 展开

#include "ir/IR.h"

#include <unordered_set>
#include <vector>

namespace ir {

namespace {

static Value* UnwrapCondition(Value* cond) {
  for (int pass = 0; pass < 2; ++pass) {
    auto* outer = dynamic_cast<BinaryInst*>(cond);
    if (!outer || outer->GetOpcode() != Opcode::Ne) break;
    auto* rc = dynamic_cast<ConstantInt*>(outer->GetRhs());
    if (!rc || rc->GetValue() != 0) break;
    auto* zext = dynamic_cast<CastInst*>(outer->GetLhs());
    if (!zext || zext->GetOpcode() != Opcode::ZExt) break;
    cond = zext->GetOperandValue();
  }
  return cond;
}

static BasicBlock* GetOnlyBrTarget(BasicBlock* bb) {
  const auto& insts = bb->GetInstructions();
  if (insts.empty()) return nullptr;
  auto* br = dynamic_cast<BranchInst*>(insts.back().get());
  return br ? br->GetTarget() : nullptr;
}

static std::vector<BasicBlock*> ComputePredecessors(
    BasicBlock* bb, const std::vector<std::unique_ptr<BasicBlock>>& all_blocks) {
  std::vector<BasicBlock*> preds;
  for (const auto& other : all_blocks) {
    if (other.get() == bb) continue;
    const auto& insts = other->GetInstructions();
    if (insts.empty()) continue;
    auto* term = insts.back().get();
    if (auto* br = dynamic_cast<BranchInst*>(term)) {
      if (br->GetTarget() == bb) preds.push_back(other.get());
    } else if (auto* cbr = dynamic_cast<CondBranchInst*>(term)) {
      if (cbr->GetTrueTarget() == bb || cbr->GetFalseTarget() == bb)
        preds.push_back(other.get());
    }
  }
  return preds;
}

static bool IsSimpleBlock(BasicBlock* bb) {
  for (const auto& inst : bb->GetInstructions()) {
    switch (inst->GetOpcode()) {
      case Opcode::Add: case Opcode::Sub: case Opcode::Mul:
      case Opcode::And: case Opcode::Or:
      case Opcode::Eq:  case Opcode::Ne:  case Opcode::Lt:
      case Opcode::Le:  case Opcode::Gt:  case Opcode::Ge:
      case Opcode::ZExt:
      case Opcode::Br:
        continue;
      default:
        return false;
    }
  }
  return true;
}

static Value* GetPhiValueFrom(PhiInst* phi, BasicBlock* bb) {
  for (size_t i = 0; i < phi->GetNumOperands(); i += 2) {
    if (dynamic_cast<BasicBlock*>(phi->GetOperand(i + 1)) == bb)
      return phi->GetOperand(i);
  }
  return nullptr;
}

static void RemovePhiEntriesFrom(PhiInst* phi, BasicBlock* bb) {
  std::vector<std::pair<Value*, Value*>> keep;
  for (size_t i = 0; i < phi->GetNumOperands(); i += 2) {
    auto* pred = dynamic_cast<BasicBlock*>(phi->GetOperand(i + 1));
    if (pred != bb)
      keep.push_back({phi->GetOperand(i), phi->GetOperand(i + 1)});
  }
  if (keep.size() * 2 != phi->GetNumOperands()) {
    phi->ClearOperands();
    for (auto& [val, pred] : keep) {
      phi->AddOperand(val);
      phi->AddOperand(pred);
    }
  }
}

static void SetPhiEntry(PhiInst* phi, BasicBlock* bb, Value* val) {
  for (size_t i = 0; i < phi->GetNumOperands(); i += 2) {
    if (dynamic_cast<BasicBlock*>(phi->GetOperand(i + 1)) == bb) {
      phi->SetOperand(i, val);
      return;
    }
  }
  phi->AddOperand(val);
  phi->AddOperand(bb);
}

static bool TryConvertOneDiamond(BasicBlock* B, BasicBlock* T, BasicBlock* M,
                                  Value* cond_i1, Context& ctx,
                                  const std::vector<std::unique_ptr<BasicBlock>>& all_blocks) {
  if (!IsSimpleBlock(T)) return false;
  if (GetOnlyBrTarget(T) != M) return false;
  auto t_preds = ComputePredecessors(T, all_blocks);
  if (t_preds.size() != 1 || t_preds[0] != B) return false;

  struct PhiEntry { PhiInst* phi; Value* val_t; Value* val_f; };
  std::vector<PhiEntry> to_convert;
  for (const auto& inst : M->GetInstructions()) {
    auto* phi = dynamic_cast<PhiInst*>(inst.get());
    if (!phi) break;
    Value* val_t = GetPhiValueFrom(phi, T);
    if (!val_t) continue;
    Value* val_f = GetPhiValueFrom(phi, B);
    if (!val_f) {
      for (size_t i = 0; i < phi->GetNumOperands(); i += 2) {
        auto* pred = dynamic_cast<BasicBlock*>(phi->GetOperand(i + 1));
        if (pred != T) { val_f = phi->GetOperand(i); break; }
      }
    }
    if (!val_f) continue;
    to_convert.push_back({phi, val_t, val_f});
  }
  if (to_convert.empty()) return false;

  auto* cbr = B->GetInstructions().back().get();
  B->TakeInstruction(cbr);

  auto& t_insts = const_cast<std::vector<std::unique_ptr<Instruction>>&>(T->GetInstructions());
  std::vector<Instruction*> t_to_move;
  for (const auto& inst : t_insts)
    if (inst->GetOpcode() != Opcode::Br)
      t_to_move.push_back(inst.get());
  for (auto* inst : t_to_move) {
    auto taken = T->TakeInstruction(inst);
    B->InsertInstructionBeforeTerminator(std::move(taken));
  }
  if (!T->GetInstructions().empty())
    T->TakeInstruction(T->GetInstructions().back().get());

  for (auto& [phi, val_t, val_f] : to_convert) {
    if (val_t == val_f) {
      RemovePhiEntriesFrom(phi, T);
      SetPhiEntry(phi, B, val_f);
      continue;
    }
    auto* zext = B->Append<CastInst>(Opcode::ZExt, Type::GetInt32Type(), cond_i1, ctx.NextTemp());
    auto* diff = B->Append<BinaryInst>(Opcode::Sub, Type::GetInt32Type(), val_t, val_f, ctx.NextTemp());
    auto* masked = B->Append<BinaryInst>(Opcode::Mul, Type::GetInt32Type(), diff, zext, ctx.NextTemp());
    auto* select_val = B->Append<BinaryInst>(Opcode::Add, Type::GetInt32Type(), val_f, masked, ctx.NextTemp());
    RemovePhiEntriesFrom(phi, T);
    SetPhiEntry(phi, B, select_val);
  }

  B->Append<BranchInst>(Type::GetVoidType(), M);
  return true;
}

static void IfConvertFunction(Function* func, Context& ctx) {
  auto& blocks = const_cast<std::vector<std::unique_ptr<BasicBlock>>&>(func->GetBlocks());
  bool changed = true;
  while (changed) {
    changed = false;
    for (const auto& bb : blocks) {
      const auto& insts = bb->GetInstructions();
      if (insts.empty()) continue;
      auto* cbr = dynamic_cast<CondBranchInst*>(insts.back().get());
      if (!cbr) continue;
      BasicBlock* T = cbr->GetTrueTarget();
      BasicBlock* F = cbr->GetFalseTarget();
      Value* cond = UnwrapCondition(cbr->GetCond());
      if (TryConvertOneDiamond(bb.get(), T, F, cond, ctx, blocks)) {
        changed = true;
        break;
      }
    }
  }
}

static void CleanupRedundantPhis(Function* func) {
  for (const auto& bb : func->GetBlocks()) {
    auto& insts = const_cast<std::vector<std::unique_ptr<Instruction>>&>(bb->GetInstructions());
    for (size_t i = 0; i < insts.size(); ) {
      auto* phi = dynamic_cast<PhiInst*>(insts[i].get());
      if (!phi) break;
      Value* unique_val = nullptr;
      bool all_same = true;
      for (size_t j = 0; j < phi->GetNumOperands(); j += 2) {
        Value* v = phi->GetOperand(j);
        if (!unique_val) unique_val = v;
        else if (unique_val != v) { all_same = false; break; }
      }
      if (all_same && unique_val) {
        phi->ReplaceAllUsesWith(unique_val);
        phi->ClearOperands();
        insts.erase(insts.begin() + i);
        continue;
      }
      ++i;
    }
  }
}

static void MergeSinglePredBlocks(Function* func) {
  auto& blocks = const_cast<std::vector<std::unique_ptr<BasicBlock>>&>(func->GetBlocks());
  bool changed = true;
  while (changed) {
    changed = false;
    for (auto& bb_ptr : blocks) {
      BasicBlock* bb = bb_ptr.get();
      if (bb == func->GetEntry()) continue;
      bool has_phi = false;
      for (const auto& inst : bb->GetInstructions()) {
        if (dynamic_cast<PhiInst*>(inst.get())) { has_phi = true; break; }
      }
      if (has_phi) continue;
      auto preds = ComputePredecessors(bb, blocks);
      if (preds.size() != 1) continue;
      BasicBlock* pred = preds[0];
      if (pred == bb) continue;
      const auto& pred_insts = pred->GetInstructions();
      if (pred_insts.empty()) continue;
      auto* br = dynamic_cast<BranchInst*>(pred_insts.back().get());
      if (!br || br->GetTarget() != bb) continue;
      pred->TakeInstruction(pred_insts.back().get());
      auto& bb_insts = const_cast<std::vector<std::unique_ptr<Instruction>>&>(bb->GetInstructions());
      std::vector<Instruction*> to_move;
      for (auto& inst : bb_insts)
        to_move.push_back(inst.get());
      for (auto* inst : to_move) {
        auto taken = bb->TakeInstruction(inst);
        pred->InsertInstructionBeforeTerminator(std::move(taken));
      }
      for (auto& other : blocks) {
        if (other.get() == bb) continue;
        auto& o_insts = const_cast<std::vector<std::unique_ptr<Instruction>>&>(other->GetInstructions());
        for (auto& inst : o_insts) {
          auto* phi = dynamic_cast<PhiInst*>(inst.get());
          if (!phi) break;
          for (size_t i = 0; i < phi->GetNumOperands(); i += 2) {
            if (dynamic_cast<BasicBlock*>(phi->GetOperand(i + 1)) == bb)
              phi->SetOperand(i + 1, pred);
          }
        }
      }
      changed = true;
      break;
    }
  }
}

}  // namespace

void RunIfConversion(Module& module) {
  for (auto& func : module.GetFunctions()) {
    if (func->IsExternal()) continue;
    IfConvertFunction(func.get(), module.GetContext());
    CleanupRedundantPhis(func.get());
    MergeSinglePredBlocks(func.get());
  }
}

}  // namespace ir