对于expand_call函数来说最主要的参数就是exp这个tree树，

打印出来之后我们终于看到了printf和Hello,world!

expand_call

下面是在 函数开始处加入debug_tree ()函数的结果

expand_expr_stmt
 <call_expr 840f0
    type <integer_type 824d0 int permanent SI
        size <integer_cst 8254c literal permanent 4
        align 32 size_unit 8 sep_unit 32 symtab 0
        sep <integer_cst 8251c literal permanent -2147483648 precision 32 min <integer_cst 8251c -2147483648>
        max <integer_cst 82534 literal permanent 2147483647
        pointer_to_this <pointer_type 88b4c> chain <integer_type 825bc* char>
    volatile
    arg 0 <addr_expr 84090
        type <pointer_type 98f48 type <function_type 956e8>
            permanent unsigned SI size <integer_cst 8254c 4>
            align 32 size_unit 8 sep_unit 32 symtab 0
       
        arg 0 <function_decl 95740 printf type <function_type 956e8>
            external public permanent used QI file /usr/include/stdio.h line 214
            align 1 size_unit 1 offset 0 chain <function_decl 954d8 ungetc>
    arg 1 <tree_list 83270
        value <nop_expr 840d8 type <pointer_type 9117c>
            literal
            arg 0 <nop_expr 840c0 type <pointer_type 88a44>
                literal
                arg 0 <addr_expr 840a8 type <pointer_type 94f58>
                    literal
                    arg 0 <string_cst 84014 type <array_type 94ef4>
                        static literal "Hello, world!
"(nil)

在c-parse.tab.c文件里面的stmt的定义出有对expand_expr_stmt()的调用，调用关系终于明朗了！

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 (); }

gcc源代码分析，在expand_call ()函数 和expand_expr_stmt ()函数的开始处加入debug_tree ()函数