feat(mir): 增加 Lab3 AArch64 MVP 后端与 --emit-asm 支持

src/mir/FrameLowering.cpp

@@ -1,4 +1,44 @@
-// 栈帧构建与序言尾声插入：
-// - 计算栈大小与对齐需求，分配栈槽
-// - 插入 prologue/epilogue，保存/恢复 callee-saved 等
+#include "mir/MIR.h"
 
+#include <stdexcept>
+#include <vector>
+
+namespace mir {
+namespace {
+
+int AlignTo(int value, int align) {
+  return ((value + align - 1) / align) * align;
+}
+
+}  // namespace
+
+void RunFrameLowering(MachineFunction& function) {
+  int cursor = 0;
+  for (const auto& slot : function.frame_slots()) {
+    cursor += slot.size;
+    if (-cursor < -256) {
+      throw std::runtime_error(
+          "Lab3 MVP 后端暂不支持超过 64 个 i32 栈槽的函数");
+    }
+  }
+
+  cursor = 0;
+  for (const auto& slot : function.frame_slots()) {
+    cursor += slot.size;
+    function.frame_slot(slot.index).offset = -cursor;
+  }
+  function.set_frame_size(AlignTo(cursor, 16));
+
+  auto& insts = function.entry().instructions();
+  std::vector<MachineInstr> lowered;
+  lowered.emplace_back(Opcode::Prologue);
+  for (const auto& inst : insts) {
+    if (inst.opcode() == Opcode::Ret) {
+      lowered.emplace_back(Opcode::Epilogue);
+    }
+    lowered.push_back(inst);
+  }
+  insts = std::move(lowered);
+}
+
+}  // namespace mir