refactor(irgen): IR改成alloca和store形式

src/irgen/IRGenDecl.cpp

@@ -5,11 +5,18 @@
 #include "irgen/IRGen.h"
 
 #include <memory>
+#include <stdexcept>
 
 #include "ast/AstNodes.h"
 #include "ir/IR.h"
 
 void IRGenImpl::GenBlock(const ast::Block& block) {
+  // 先为所有局部变量创建栈槽，使 alloca 聚集在入口块前部。
+  for (const auto& decl : block.varDecls) {
+    auto* slot = builder_.CreateAllocaI32(ir::DefaultContext().NextTemp());
+    locals_[decl->name] = slot;
+  }
+
   for (const auto& decl : block.varDecls) {
     GenVarDecl(*decl);
   }
@@ -19,11 +26,16 @@ void IRGenImpl::GenBlock(const ast::Block& block) {
 }
 
 void IRGenImpl::GenVarDecl(const ast::VarDecl& decl) {
+  auto it = locals_.find(decl.name);
+  if (it == locals_.end()) {
+    throw std::runtime_error("变量栈槽未创建: " + decl.name);
+  }
+
   ir::Value* init = nullptr;
   if (decl.init) {
     init = GenExpr(*decl.init);
   } else {
     init = ir::DefaultContext().GetConstInt(0);
   }
-  locals_[decl.name] = init;
+  builder_.CreateStore(init, it->second);
 }