nudt-compiler-cpp/src/ir/passes/Mem2Reg.cpp

#include "ir/PassManager.h"
#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#include <stack>
#include <algorithm>
#include <queue>
#include <functional>

namespace ir {

// Predeclaration of rebuild CFG helper
void RebuildCFG(Function* func);

bool RunMem2Reg(Function* func, Context& ctx) {
  // 1. Build dominator tree
  DominatorTree dom_tree(func);
  dom_tree.Run();

  // 2. Identify promotable allocas
  std::vector<AllocaInst*> promotable_allocas;
  for (const auto& bbPtr : func->GetBlocks()) {
    for (const auto& instPtr : bbPtr->GetInstructions()) {
      if (instPtr->GetOpcode() == Opcode::Alloca) {
        auto* alloca = static_cast<AllocaInst*>(instPtr.get());
        // Alloca of scalar type: i32 or float (pointers to i32/float in minimum IR)
        if (alloca->GetType()->IsPtrInt32() || alloca->GetType()->IsPtrFloat()) {
          // Verify all uses are load/store
          bool promotable = true;
          for (const auto& use : alloca->GetUses()) {
            auto* user = use.GetUser();
            auto* inst_user = dynamic_cast<Instruction*>(user);
            if (!inst_user) {
              promotable = false;
              break;
            }
            if (inst_user->GetOpcode() != Opcode::Load && inst_user->GetOpcode() != Opcode::Store) {
              promotable = false;
              break;
            }
            // For Store, alloca must be the pointer operand (operand index 1), not the value operand
            if (inst_user->GetOpcode() == Opcode::Store) {
              auto* store = static_cast<StoreInst*>(inst_user);
              if (store->GetPtr() != alloca) {
                promotable = false;
                break;
              }
            }
          }
          if (promotable) {
            promotable_allocas.push_back(alloca);
          }
        }
      }
    }
  }

  if (promotable_allocas.empty()) {
    return false;
  }

  // 3. For each alloca, find definition blocks and place Phi nodes
  // Maps each basic block and alloca to the inserted Phi instruction
  std::unordered_map<BasicBlock*, std::unordered_map<AllocaInst*, PhiInst*>> phi_nodes;
  std::unordered_set<Instruction*> instructions_to_erase;

  for (auto* alloca : promotable_allocas) {
    std::vector<BasicBlock*> def_blocks;
    for (const auto& use : alloca->GetUses()) {
      auto* inst = dynamic_cast<Instruction*>(use.GetUser());
      if (inst && inst->GetOpcode() == Opcode::Store) {
        def_blocks.push_back(inst->GetParent());
      }
    }

    // DF-based Phi placement
    std::queue<BasicBlock*> worklist;
    std::unordered_set<BasicBlock*> added;
    std::unordered_set<BasicBlock*> def_set(def_blocks.begin(), def_blocks.end());

    for (auto* bb : def_blocks) {
      worklist.push(bb);
      added.insert(bb);
    }

    while (!worklist.empty()) {
      auto* x = worklist.front();
      worklist.pop();

      for (auto* y : dom_tree.GetDominanceFrontier(x)) {
        if (added.find(y) == added.end()) {
          // Place Phi node in Y
          std::shared_ptr<Type> ty = alloca->GetType()->IsPtrFloat() ? Type::GetFloatType() : Type::GetInt32Type();
          auto phi = std::make_unique<PhiInst>(ty, ctx.NextTemp());
          auto* phi_ptr = phi.get();

          // Insert Phi at the start of block Y
          y->InsertInstructionAtBegin(std::move(phi));
          phi_nodes[y][alloca] = phi_ptr;

          added.insert(y);
          if (def_set.find(y) == def_set.end()) {
            worklist.push(y);
          }
        }
      }
    }
  }

  // 4. Rename variables using DFS traversal of dominator tree
  std::unordered_map<AllocaInst*, std::vector<Value*>> current_def;
  
  // Helper for generating default value
  auto get_default_value = [&](AllocaInst* alloca) -> Value* {
    if (alloca->GetType()->IsPtrFloat()) {
      return ctx.GetConstFloat(0.0f);
    } else {
      return ctx.GetConstInt(0);
    }
  };

  // Traversal stack for DFS: stores (block, parent_block)
  struct TraversalNode {
    BasicBlock* bb;
    size_t child_idx;
  };

  std::stack<BasicBlock*> visit_stack;
  std::unordered_map<BasicBlock*, std::vector<std::pair<AllocaInst*, size_t>>> pushed_defs;

  // DFS function
  std::function<void(BasicBlock*)> rename_dfs = [&](BasicBlock* bb) {
    auto& pushes = pushed_defs[bb];

    // Push Phis in this block to current_def
    auto phi_it = phi_nodes.find(bb);
    if (phi_it != phi_nodes.end()) {
      for (const auto& pair : phi_it->second) {
        auto* alloca = pair.first;
        auto* phi = pair.second;
        current_def[alloca].push_back(phi);
        pushes.push_back({alloca, 1});
      }
    }

    // Process loads and stores
    for (const auto& instPtr : bb->GetInstructions()) {
      auto* inst = instPtr.get();
      if (inst->GetOpcode() == Opcode::Load) {
        auto* load = static_cast<LoadInst*>(inst);
        auto* ptr = load->GetPtr();
        if (auto* alloca = dynamic_cast<AllocaInst*>(ptr)) {
          if (std::find(promotable_allocas.begin(), promotable_allocas.end(), alloca) != promotable_allocas.end()) {
            auto& defs = current_def[alloca];
            Value* val = defs.empty() ? get_default_value(alloca) : defs.back();
            load->ReplaceAllUsesWith(val);
            instructions_to_erase.insert(load);
          }
        }
      } else if (inst->GetOpcode() == Opcode::Store) {
        auto* store = static_cast<StoreInst*>(inst);
        auto* ptr = store->GetPtr();
        if (auto* alloca = dynamic_cast<AllocaInst*>(ptr)) {
          if (std::find(promotable_allocas.begin(), promotable_allocas.end(), alloca) != promotable_allocas.end()) {
            current_def[alloca].push_back(store->GetValue());
            pushes.push_back({alloca, 1});
            instructions_to_erase.insert(store);
          }
        }
      }
    }

    // Fill Phi incoming values for CFG successors
    for (auto* succ : bb->GetSuccessors()) {
      auto succ_phi_it = phi_nodes.find(succ);
      if (succ_phi_it != phi_nodes.end()) {
        for (const auto& pair : succ_phi_it->second) {
          auto* alloca = pair.first;
          auto* phi = pair.second;
          auto& defs = current_def[alloca];
          Value* val = defs.empty() ? get_default_value(alloca) : defs.back();
          phi->AddIncoming(val, bb);
        }
      }
    }

    // Recurse to dominator tree children
    for (auto* child : dom_tree.GetDominatedBlocks(bb)) {
      rename_dfs(child);
    }

    // Pop definitions pushed in this block
    for (const auto& push : pushes) {
      auto* alloca = push.first;
      for (size_t k = 0; k < push.second; ++k) {
        if (!current_def[alloca].empty()) {
          current_def[alloca].pop_back();
        }
      }
    }
  };

  if (!func->GetBlocks().empty()) {
    rename_dfs(func->GetEntry());
  }

  // 5. Clean up loads, stores and allocas
  for (auto* alloca : promotable_allocas) {
    instructions_to_erase.insert(alloca);
  }

  for (const auto& bbPtr : func->GetBlocks()) {
    std::vector<Instruction*> to_remove;
    for (const auto& instPtr : bbPtr->GetInstructions()) {
      if (instructions_to_erase.find(instPtr.get()) != instructions_to_erase.end()) {
        to_remove.push_back(instPtr.get());
      }
    }
    for (auto* inst : to_remove) {
      bbPtr->EraseInstruction(inst);
    }
  }

  return true;
}

} // namespace ir