feat(backend): AsmPrinter 帧基址缓存，避免连续栈访问重复计算地址

新增 g_frame_base_offset/g_frame_base_valid 缓存机制：
- PrintStackAccess 尝试复用 x13 中已计算的帧地址
- 相邻访问偏移差在 ldur/stur ±256 或 ldr/str 0~32760 范围内时免重算
- x13 被覆写时（ADRP/EmitAddressFromBase/EmitStackAdjust）自动失效
- 为后续 MIR 层 spill 排序优化提供基础架构

src/mir/AsmPrinter.cpp

@@ -260,6 +260,15 @@ namespace mir
         std::string g_cached_adrp_symbol;
         bool g_adrp_cache_valid = false;
 
+        // 帧基址缓存——x13 持有 x29 + g_frame_base_offset，避免重复计算地址
+        int g_frame_base_offset = 0;
+        bool g_frame_base_valid = false;
+
+        void InvalidateFrameBase()
+        {
+            g_frame_base_valid = false;
+        }
+
         void InvalidateAdrpCache()
         {
             g_adrp_cache_valid = false;
@@ -270,6 +279,7 @@ namespace mir
             if (amount > 12285)
             {
                 InvalidateAdrpCache();
+                InvalidateFrameBase();
                 os << "    movz x13, #" << (amount & 0xFFFF) << "\n";
                 if ((amount >> 16) != 0)
                     os << "    movk x13, #" << ((amount >> 16) & 0xFFFF) << ", lsl #16\n";
@@ -295,9 +305,17 @@ namespace mir
         void EmitAddressFromBase(PhysReg target_xreg, PhysReg base_reg, int offset,
                                  std::ostream &os)
         {
-            if (offset > 12285)
+            // 使用 x13 时，ADRP 和帧基址缓存同时失效
+            if (target_xreg == PrinterScratchXReg())
             {
                 InvalidateAdrpCache();
+                InvalidateFrameBase();
+            }
+
+            if (offset > 12285)
+            {
+                if (target_xreg != PrinterScratchXReg())
+                    InvalidateAdrpCache();
                 os << "    movz x13, #" << (offset & 0xFFFF) << "\n";
                 if ((offset >> 16) != 0)
                     os << "    movk x13, #" << ((offset >> 16) & 0xFFFF) << ", lsl #16\n";
@@ -308,7 +326,8 @@ namespace mir
             if (offset < -12285)
             {
                 int abs_off = -offset;
-                InvalidateAdrpCache();
+                if (target_xreg != PrinterScratchXReg())
+                    InvalidateAdrpCache();
                 os << "    movz x13, #" << (abs_off & 0xFFFF) << "\n";
                 if ((abs_off >> 16) != 0)
                     os << "    movk x13, #" << ((abs_off >> 16) & 0xFFFF) << ", lsl #16\n";
@@ -341,6 +360,7 @@ namespace mir
             const char *narrow_op = (opcode == Opcode::LoadStack) ? "ldur" : "stur";
             const char *wide_op = (opcode == Opcode::LoadStack) ? "ldr" : "str";
 
+            // x29 可达的窄范围直接用 ldur/stur
             if (offset >= -256 && offset <= 255)
             {
                 os << "    " << narrow_op << " ";
@@ -350,7 +370,41 @@ namespace mir
             }
 
             const PhysReg scratch_xreg = PrinterScratchXReg();
+            bool is_32bit = IsWReg(reg.GetReg()) || IsSReg(reg.GetReg());
+
+            // 尝试帧基址缓存——x13 已持有之前的地址
+            if (g_frame_base_valid)
+            {
+                int diff = offset - g_frame_base_offset;
+
+                // ldur/stur（范围 ±256）
+                if (diff >= -256 && diff <= 255)
+                {
+                    os << "    " << narrow_op << " ";
+                    PrintOperand(reg, os);
+                    os << ", [" << PhysRegName(scratch_xreg) << ", #" << diff << "]\n";
+                    return;
+                }
+
+                // ldr/str 无符号立即数（正偏移）
+                if (diff >= 0)
+                {
+                    int max_imm = is_32bit ? 16380 : 32760;
+                    int align = is_32bit ? 4 : 8;
+                    if (diff <= max_imm && diff % align == 0)
+                    {
+                        os << "    " << wide_op << " ";
+                        PrintOperand(reg, os);
+                        os << ", [" << PhysRegName(scratch_xreg) << ", #" << diff << "]\n";
+                        return;
+                    }
+                }
+            }
+
+            // 缓存未命中——完整计算地址到 x13
             EmitAddressFromBase(scratch_xreg, PhysReg::X29, offset, os);
+            g_frame_base_offset = offset;
+            g_frame_base_valid = true;
 
             os << "    " << wide_op << " ";
             PrintOperand(reg, os);
@@ -386,6 +440,7 @@ namespace mir
                 os << "    adrp " << PhysRegName(scratch_xreg) << ", " << asm_symbol << "\n";
                 g_cached_adrp_symbol = asm_symbol;
                 g_adrp_cache_valid = true;
+                InvalidateFrameBase();
             }
 
             os << "    " << (opcode == Opcode::LoadGlobal ? "ldr " : "str ");
@@ -944,6 +999,7 @@ namespace mir
     void PrintAsm(const MachineFunction &function, std::ostream &os)
     {
         g_adrp_cache_valid = false;
+        g_frame_base_valid = false;
         const std::string asm_name = NormalizeAsmSymbol(function.GetName());
 
         os << "    .text\n";
@@ -959,8 +1015,9 @@ namespace mir
             }
 
             const auto &block = *block_ptr;
-            // 每个基本块重置 ADRP 缓存——跨块时 x13 可能已被 call/clobber 破坏
+            // 每个基本块重置缓存——跨块时 x13 可能已被 call/clobber 破坏
             g_adrp_cache_valid = false;
+            g_frame_base_valid = false;
             PrintBlockLabelRef(function, block.GetLabelId(), os);
             os << ":\n";