终于调用的顺序清楚了，yyparse()函数调用了expand_expr_stmt()函数，expand_expr_stmt()函数调用了expand_expr()函数。
下面的语法规则对应printf("Hello,World!\n");。
stmt:
compstmt {}
| expr ‘;‘
{ emit_line_note (input_filename, lineno);
/* Do default conversion if safe and possibly important,
in case within ({...}). */
if ((TREE_CODE (TREE_TYPE ($1)) == ARRAY_TYPE
&& lvalue_p ($1))
|| TREE_CODE (TREE_TYPE ($1)) == FUNCTION_TYPE)
$1 = default_conversion ($1);
expand_expr_stmt ($1);
clear_momentary (); }
/* Generate RTL to evaluate the expression EXP
and remember it in case this is the VALUE in a ({... VALUE; }) constr. */void
expand_expr_stmt (exp)
tree exp;
{
/* If -W, warn about statements with no side effects,
except inside a ({...}) where they may be useful. */
if (expr_stmts_for_value == 0 && exp != error_mark_node)
{
if (! TREE_VOLATILE (exp) && (extra_warnings || warn_unused))
warning_with_file_and_line (emit_filename, emit_lineno,
"statement with no effect");
else if (warn_unused)
warn_if_unused_value (exp);
}
last_expr_type = TREE_TYPE (exp);
if (! flag_syntax_only)
last_expr_value = expand_expr (exp, expr_stmts_for_value ? 0 : const0_rtx,
VOIDmode, 0);
emit_queue ();
}