如何对C语言的FOR语句给出一个生成中间代码的语法制导定义？

select_or_iter_stmt:
	  /* ... 省略 ... */
	| FOR
		{ $<ttype>$ = build_stmt (FOR_STMT, NULL_TREE, NULL_TREE,
					  NULL_TREE, NULL_TREE);
		  add_stmt ($<ttype>$); }
	  '(' for_init_stmt
		{ stmt_count++;
		  RECHAIN_STMTS ($<ttype>2, FOR_INIT_STMT ($<ttype>2)); }
	  xexpr ';'
                { if ($6)
		    FOR_COND ($<ttype>2)
		      = c_common_truthvalue_conversion ($6); }
	  xexpr ')'
                { c_in_iteration_stmt++;
		  FOR_EXPR ($<ttype>2) = $9; }
	  c99_block_lineno_labeled_stmt
                { RECHAIN_STMTS ($<ttype>2, FOR_BODY ($<ttype>2));
		  c_in_iteration_stmt--;}
	| /* ... 省略 ... */
	;

for_init_stmt:
	  xexpr ';'
		{ add_stmt (build_stmt (EXPR_STMT, $1)); }
	| decl
		{ check_for_loop_decls (); }
	;

/* Parse a single real statement, not including any labels.  */
stmt:
	  /* ... 省略 ... */
	| c99_block_start select_or_iter_stmt c99_block_end
		{ if (flag_isoc99)
		    RECHAIN_STMTS ($1, COMPOUND_BODY ($1));
		  $$ = NULL_TREE; }
	| BREAK ';'
	        { stmt_count++;
		if (!(c_in_iteration_stmt || c_in_case_stmt))
		  {
		    error ("break statement not within loop or switch");
		    $$ = NULL_TREE;
		  }
		else
		  $$ = add_stmt (build_break_stmt ()); }
	| CONTINUE ';'
                { stmt_count++;
		if (!c_in_iteration_stmt)
		  {
		    error ("continue statement not within a loop");
		    $$ = NULL_TREE;
		  }
		else
		  $$ = add_stmt (build_continue_stmt ()); }
	| /* ... 省略 ... */
	;

static void forstmt(int lab, Swtch swp, int lev) {
	int once = 0;
	Tree e1 = NULL, e2 = NULL, e3 = NULL;
	Coordinate pt2, pt3;
	
	t = gettok();
	expect('(');
	definept(NULL);
	if (kind[t] == ID)
		e1 = texpr(expr0, ';', FUNC);
	else
		expect(';');
	walk(e1, 0, 0);
	pt2 = src;
	refinc *= 10.0;
	if (kind[t] == ID)
		e2 = texpr(conditional, ';', FUNC);
	else
		expect(';');
	pt3 = src;
	if (kind[t] == ID)
		e3 = texpr(expr0, ')', FUNC);
	else {
		static char stop[] = { IF, ID, '}', 0 };
		test(')', stop);
	}
	if (e2) {
		once = foldcond(e1, e2);
		if (!once)
			branch(lab + 3);
	}
	definelab(lab);
	statement(lab, swp, lev);
	definelab(lab + 1);
	definept(&pt3);
	if (e3)
		walk(e3, 0, 0);
	if (e2) {
		if (!once)
			definelab(lab + 3);
		definept(&pt2);
		walk(e2, lab, 0);
	} else {
		definept(&pt2);
		branch(lab);
	}
	if (findlabel(lab + 2)->ref)
		definelab(lab + 2);
}

for (init; cond; step)
  body
/* after */

  ...
  for_init
Label_for_cond:
  if (for_cond) goto Label_for_after
  for_body
Label_for_step:
  for_step
  goto Label_for_cond
Label_for_after:
  ...

struct loop_t {
  struct label_t *cond, *step, *after;
  struct loop_t *prev; // 构成栈用的单向链
};

struct loop_t *current_enclosing_loop = NULL; // 当前parse所处的循环

for_statement:
    FOR
        {
          /* 创建AST用。可选 */
          $$ = make_for_statement();
          /* 记录循环嵌套层次 */
          struct loop_t* l = make_loop();
          l->cond  = make_label();
          l->step  = make_label();
          l->after = make_label();
          l->prev  = current_enclosing_loop;
          current_enclosing_loop = l;
        }
    '(' for_init_stmt
        {
          $$->init = $4;
          /* 生成for_init的代码 */
          gen_expression($$->init);
          /* 把Label_for_cond绑定在for_init之后 */
          bind_label(current_enclosing_loop->cond);
        }
    for_cond_stmt
        {
          $$->cond = $6;
          /* 生成跳转到循环之后的条件跳转 */
          gen_conditional_jump($$->cond, current_enclosing_loop->after);
        }
    for_expr ')'
        {
          $$->step = $8;
        }
    statement
        {
          $$->body = $11;
          /* 生成for_body的代码 */
          gen_statement($$->body);
          /* 把Label_for_step绑定在for_body之后 */
          bind_label(current_enclosing_loop->step);
          /* 生成for_step的代码 */
          gen_expression($$->step);
          /* 生成跳转回循环条件的代码 */
          gen_jump(current_enclosing_loop->cond);
          /* 循环结束。把Label_for_after绑定在循环之后 */
          bind_label(current_enclosing_loop->after);
          /* 把之前用到尚未绑定的label的IR修正到绑定后的值。可选 */
          patch_labels();
          /* 把当前循环从循环层次记录栈中弹出 */
          current_enclosing_loop = current_enclosing_loop->prev;
        }
    ;

break_statement:
    BREAK
        {
          if (current_enclosing_loop == NULL) report_error();
          else {
            gen_jump(current_enclosing_loop->after);
          }
        }
    ;

continue_statement:
    CONTINUE
        {
          if (current_enclosing_loop == NULL) report_error();
          else {
            gen_jump(current_enclosing_loop->step);
          }
        }
    ;