diff --git a/src/mir/GreedyAlloc.cpp b/src/mir/GreedyAlloc.cpp
index 803c9e38..eef77987 100644
--- a/src/mir/GreedyAlloc.cpp
+++ b/src/mir/GreedyAlloc.cpp
@@ -52,8 +52,10 @@ struct SpillWeightCmp
         const auto& lb = intervals[b];
         if (la.generation != lb.generation)
             return la.generation > lb.generation;
+        // 新 vreg 按 FirstUsePos 升序：确保处理顺序与指令顺序一致，
+        // 这样当 vreg B 被分配时，更早的 vreg A 已在 reg_assignments_ 中
         if (la.deferred_count == 0 && lb.deferred_count == 0)
-            return la.Length() > lb.Length();
+            return la.FirstUsePos() > lb.FirstUsePos();
         return la.spill_weight < lb.spill_weight;
     }
 };
@@ -532,6 +534,7 @@ static void AllocateRegistersForFunction(MachineFunction &function)
     // ---- 阶段 0：活跃分析 + 预处理 ----
     auto raw = ComputeInstLiveness(function);
     auto intervals = EnhanceIntervals(raw, function);
+        intervals.reserve(function.GetNumVRegs() * 16);
 
     auto &blocks = function.GetBlocks();
     std::vector<int> pos_to_block;
@@ -549,7 +552,7 @@ static void AllocateRegistersForFunction(MachineFunction &function)
     auto block_depth = AnalyzeLoopDepth(function);
     ComputeSpillWeights(intervals, block_depth, pos_to_block);
     PropagateCopyHints(intervals, function);
-    intervals.reserve(function.GetNumVRegs() * 4);
+    intervals.reserve(function.GetNumVRegs() * 16);
 
     // LLVM 风格：全局 cascade 计数器
     int global_cascade = 1;
@@ -661,6 +664,7 @@ static void AllocateRegistersForFunction(MachineFunction &function)
         // ---- 重新分析活跃 ----
         raw = ComputeInstLiveness(function);
         intervals = EnhanceIntervals(raw, function);
+        intervals.reserve(function.GetNumVRegs() * 16);
 
         // ---- 重建位置映射 ----
         pos_to_block.clear();