refactor(irgen): 规范采用visitor生成

src/irgen/IRGenStmt.cpp

@@ -6,18 +6,34 @@
 #include "ir/IR.h"
 #include "utils/Log.h"
 
-bool IRGenImpl::GenStmt(SysYParser::StmtContext& stmt) {
-  if (stmt.returnStmt()) {
-    GenReturnStmt(*stmt.returnStmt());
-    return true;
+// 语句生成当前只实现了最小子集。
+// 目前支持：
+// - return <exp>;
+//
+// 还未支持：
+// - 赋值语句
+// - if / while 等控制流
+// - 空语句、块语句嵌套分发之外的更多语句形态
+
+std::any IRGenImpl::visitStmt(SysYParser::StmtContext* ctx) {
+  if (!ctx) {
+    throw std::runtime_error(FormatError("irgen", "缺少语句"));
+  }
+  if (ctx->returnStmt()) {
+    return ctx->returnStmt()->accept(this);
   }
   throw std::runtime_error(FormatError("irgen", "暂不支持的语句类型"));
 }
 
-void IRGenImpl::GenReturnStmt(SysYParser::ReturnStmtContext& ret) {
-  if (!ret.exp()) {
+
+std::any IRGenImpl::visitReturnStmt(SysYParser::ReturnStmtContext* ctx) {
+  if (!ctx) {
+    throw std::runtime_error(FormatError("irgen", "缺少 return 语句"));
+  }
+  if (!ctx->exp()) {
     throw std::runtime_error(FormatError("irgen", "return 缺少表达式"));
   }
-  ir::Value* v = GenExpr(*ret.exp());
+  ir::Value* v = EvalExpr(*ctx->exp());
   builder_.CreateRet(v);
+  return BlockFlow::Terminated;
 }