feat(backend): move coalescing via color preference and phi cycle breaking\n\nCollects move_preferences from MovReg instructions and uses them\nduring color selection to prefer the same physical register for\nmove-related virtual registers. Detects and breaks cycles in move\npreference chains to ensure correctness.

src/mir/RegAlloc.cpp

@@ -642,7 +642,8 @@ namespace mir
         int caller_saved_threshold,
         const std::unordered_map<int, int> &interval_length,
         const std::unordered_map<int, int> &ref_count,
-        const std::set<int> &rematerializable_vregs)
+        const std::set<int> &rematerializable_vregs,
+        const std::unordered_map<int, int> &move_preferences)
     {
       const int K = static_cast<int>(allocatable_regs.size());
       GraphColoringResult result;
@@ -788,8 +789,24 @@ namespace mir
         }
 
         int assigned_color = -1;
+        // 尝试使用 move 偏好颜色（如果偏好变量已分配且无冲突）
+        auto pref_it = move_preferences.find(v);
+        if (pref_it != move_preferences.end())
+        {
+          int pref_v = pref_it->second;
+          auto col_it = colored.find(pref_v);
+          if (col_it != colored.end())
+          {
+            int pref_color = col_it->second;
+            if (used_colors.find(pref_color) == used_colors.end())
+              assigned_color = pref_color;
+          }
+        }
+
         // 第一遍：优先选 caller-saved 颜色 (c < caller_saved_threshold)
-        for (int c : allocatable_regs)
+        if (assigned_color < 0)
+        {
+          for (int c : allocatable_regs)
         {
           if (c >= caller_saved_threshold) break;
           if (used_colors.find(c) == used_colors.end())
@@ -811,6 +828,7 @@ namespace mir
             }
           }
         }
+        }
 
         if (assigned_color >= 0)
         {
@@ -1104,6 +1122,56 @@ namespace mir
             rematerializable_vregs.insert(pair.first);
         }
 
+        // 收集 MovReg 的 move 偏好映射
+        std::unordered_map<int, int> move_preferences;
+        for (auto &block : function.GetBlocks())
+        {
+          for (auto &inst : block->GetInstructions())
+          {
+            if (inst.GetOpcode() == Opcode::MovReg)
+            {
+              const auto &ops = inst.GetOperands();
+              if (ops.size() >= 2 &&
+                  ops[0].GetKind() == Operand::Kind::VReg &&
+                  ops[1].GetKind() == Operand::Kind::VReg)
+              {
+                int def_vreg = ops[0].GetVRegId();
+                int use_vreg = ops[1].GetVRegId();
+                if (function.GetVRegClass(def_vreg) == function.GetVRegClass(use_vreg))
+                  move_preferences[def_vreg] = use_vreg;
+              }
+            }
+          }
+        }
+
+        // 检测并打破 move 偏好循环
+        {
+          std::unordered_set<int> visited;
+          for (auto &pair : move_preferences)
+          {
+            int cur = pair.first;
+            if (visited.count(cur)) continue;
+            std::unordered_set<int> path;
+            std::vector<int> chain;
+            int node = cur;
+            while (node != 0 && !visited.count(node))
+            {
+              visited.insert(node);
+              path.insert(node);
+              chain.push_back(node);
+              auto it = move_preferences.find(node);
+              if (it == move_preferences.end()) break;
+              int next = it->second;
+              if (path.count(next))
+              {
+                move_preferences.erase(chain.back());
+                break;
+              }
+              node = next;
+            }
+          }
+        }
+
         std::vector<int> gp_alloc(GP_ALLOCATABLE, GP_ALLOCATABLE + GP_NUM_ALLOCATABLE);
         std::vector<int> fp_alloc(FP_ALLOCATABLE, FP_ALLOCATABLE + FP_NUM_ALLOCATABLE);
 
@@ -1113,10 +1181,10 @@ namespace mir
 
         auto gp_result = ColorGraph(gp_graph, gp_alloc, function, 19,
                                     liveness.interval_length, liveness.ref_count,
-                                    rematerializable_vregs);
+                                    rematerializable_vregs, move_preferences);
         auto fp_result = ColorGraph(fp_graph, fp_alloc, function, 16,
                                     liveness.interval_length, liveness.ref_count,
-                                    rematerializable_vregs);
+                                    rematerializable_vregs, move_preferences);
 
         if (gp_result.spilled.empty() && fp_result.spilled.empty())
         {
@@ -1255,6 +1323,56 @@ namespace mir
           rematerializable_vregs.insert(pair.first);
       }
 
+      // 收集 MovReg 的 move 偏好映射
+      std::unordered_map<int, int> move_preferences;
+      for (auto &block : function.GetBlocks())
+      {
+        for (auto &inst : block->GetInstructions())
+        {
+          if (inst.GetOpcode() == Opcode::MovReg)
+          {
+            const auto &ops = inst.GetOperands();
+            if (ops.size() >= 2 &&
+                ops[0].GetKind() == Operand::Kind::VReg &&
+                ops[1].GetKind() == Operand::Kind::VReg)
+            {
+              int def_vreg = ops[0].GetVRegId();
+              int use_vreg = ops[1].GetVRegId();
+              if (function.GetVRegClass(def_vreg) == function.GetVRegClass(use_vreg))
+                move_preferences[def_vreg] = use_vreg;
+            }
+          }
+        }
+      }
+
+      // 检测并打破 move 偏好循环
+      {
+        std::unordered_set<int> visited;
+        for (auto &pair : move_preferences)
+        {
+          int cur = pair.first;
+          if (visited.count(cur)) continue;
+          std::unordered_set<int> path;
+          std::vector<int> chain;
+          int node = cur;
+          while (node != 0 && !visited.count(node))
+          {
+            visited.insert(node);
+            path.insert(node);
+            chain.push_back(node);
+            auto it = move_preferences.find(node);
+            if (it == move_preferences.end()) break;
+            int next = it->second;
+            if (path.count(next))
+            {
+              move_preferences.erase(chain.back());
+              break;
+            }
+            node = next;
+          }
+        }
+      }
+
       std::vector<int> gp_alloc(GP_ALLOCATABLE, GP_ALLOCATABLE + GP_NUM_ALLOCATABLE);
       std::vector<int> fp_alloc(FP_ALLOCATABLE, FP_ALLOCATABLE + FP_NUM_ALLOCATABLE);
       InterferenceGraph gp_graph, fp_graph;
@@ -1262,10 +1380,10 @@ namespace mir
       BuildInterferenceForFP(function, liveness.block_liveness, fp_alloc, fp_graph);
       auto gp_result = ColorGraph(gp_graph, gp_alloc, function, 19,
                                   liveness.interval_length, liveness.ref_count,
-                                  rematerializable_vregs);
+                                  rematerializable_vregs, move_preferences);
       auto fp_result = ColorGraph(fp_graph, fp_alloc, function, 16,
                                   liveness.interval_length, liveness.ref_count,
-                                  rematerializable_vregs);
+                                  rematerializable_vregs, move_preferences);
       std::set<int> all_spilled = gp_result.spilled;
       for (int v : fp_result.spilled)
         all_spilled.insert(v);