【PHP内核学习】global关键字的解析过程分析

本文github地址：

https://github.com/wusuopubupt/phpLib/blob/master/global%E5%85%B3%E9%94%AE%E5%AD%97%E7%9A%84%E8%A7%A3%E6%9E%90%E8%BF%87%E7%A8%8B%E5%88%86%E6%9E%90

|=-----------------------------------------------------------------------=|
|=--------------------=[ global关键字的解析过程分析 ]=-------------------=|
|=-----------------------------------------------------------------------=|
|=--------------------------=[  by d4shman  ]=---------------------------=|
|=-----------------------------------------------------------------------=|
|=-------------------------=[  May 8, 2014  ]=---------------------------=|
|=-----------------------------------------------------------------------=|

[目录]
0x01 词法分析 
0X02 语法分析
0X03 解释执行
0X04 参考文献

0x01 词法分析

d4shman@gentoo# vi /php-dev/php-5.4.8/Zend/zend_language_scanner.l
找到global：

    <ST_IN_SCRIPTING>"global" {
        return T_GLOBAl;
    }
发现返回一个token T_GLOBAL

0X02 语法分析

通过token T_GLOBAL来到zend_language_parser.y找到：
    |   T_GLOBAL global_var_list ‘;‘

    global_var_list:
            global_var_list ‘,‘ global_var  { zend_do_fetch_global_variable(&$3, NULL, ZEND_FETCH_GLOBAL_LOCK TSRMLS_CC); }
        |   global_var                      { zend_do_fetch_global_variable(&$1, NULL, ZEND_FETCH_GLOBAL_LOCK TSRMLS_CC); }
    ;
上面的$3指的是global_val，可以看到，对于全局变量，语法分析器调用的是Zend引擎
的zend_do_fetch_globa_variable函数。此函数的声明在Zend/zend_compile.c

0X03 解释执行

    在Zend/zend_compile.c中找到zend_do_fetch_global_variable函数定义：

    void zend_do_fetch_global_variable(znode *varname, const znode *static_assignment, int fetch_type TSRMLS_DC) 
    {
        zend_op *opline;
        znode lval;
        znode result;

		/*如果变量类型是常量且不是字符串，则将其转化成字符串类型*/
        if (varname->op_type == IS_CONST) {  
            if (Z_TYPE(varname->u.constant) != IS_STRING) {
                convert_to_string(&varname->u.constant);
            }
        }
    
        opline = get_next_op(CG(active_op_array) TSRMLS_CC); /* CG: compile_global */
        opline->opcode = ZEND_FETCH_W;      /* 默认的模式必须是Write */
        opline->result_type = IS_VAR;
        opline->result.var = get_temporary_variable(CG(active_op_array));
        SET_NODE(opline->op1, varname);
        if (opline->op1_type == IS_CONST) {
            CALCULATE_LITERAL_HASH(opline->op1.constant);
        }
        SET_UNUSED(opline->op2);
        opline->extended_value = fetch_type;
        GET_NODE(&result, opline->result);
    
        if (varname->op_type == IS_CONST) {
            zval_copy_ctor(&varname->u.constant);
        }
		/* Relies on the fact that the default fetch is BP_VAR_W */
        fetch_simple_variable(&lval, varname, 0 TSRMLS_CC); 
    
        zend_do_assign_ref(NULL, &lval, &result TSRMLS_CC);
        CG(active_op_array)->opcodes[CG(active_op_array)->last-1].result_type |= EXT_TYPE_UNUSED;
    }

    上面的代码确认了opcode为ZEND_FETCH_W外，还执行了zend_do_assign_ref函数。zend_do_assign_ref函数中
有这么一个关键语句：

    opline->opcode = ZEND_ASSIGN_REF;

    由此可知，语法分析过程中，实际执行了2个opcode: ZEND_FETCH_W和ZEND_ASSIGN_REF，在zend_vm_opcodes.h
中发现，它们对应的opcode分别是83和39。而计算最后调用的方法是(定义在zend_execute.c:)：

    zend_opcode_handlers[opcode * 25 + zend_vm_decode[op->op1.op_type] * 5 + zend_vm_decode[op->op2.op_type]];

    计算后（///////////我没搞清楚是怎么计算出的//////////）,得到调用的函数是：

    static int ZEND_FASTCALL  ZEND_FETCH_W_SPEC_CV_HANDLER(ZEND_OPCODE_HANDLER_ARGS)
    {
        return zend_fetch_var_address_helper_SPEC_CV(BP_VAR_W, ZEND_OPCODE_HANDLER_ARGS_PASSTHRU);
    }	

    在zend_fetch_var_address_helper_SPEC_CV中调用如下代码获取符号表：

    target_symbol_table = zend_get_target_symbol_table(opline, EX(Ts), type, varname TSRMLS_CC);

	zend_get_target_symbol_table函数的实现如下（在）：

    static inline HashTable *zend_get_target_symbol_table(int fetch_type TSRMLS_DC)
    {
        switch (fetch_type) {
            case ZEND_FETCH_LOCAL:  
                if (!EG(active_symbol_table)) {
                    zend_rebuild_symbol_table(TSRMLS_C);
                }
                return EG(active_symbol_table);
                break;
            case ZEND_FETCH_GLOBAL:
            case ZEND_FETCH_GLOBAL_LOCK:
                return &EG(symbol_table); /*返回global 变量符号表的地址*/
                break;
            case ZEND_FETCH_STATIC:
                if (!EG(active_op_array)->static_variables) {
                    ALLOC_HASHTABLE(EG(active_op_array)->static_variables);
                    zend_hash_init(EG(active_op_array)->static_variables, 2, NULL, ZVAL_PTR_DTOR, 0);
                }
                return EG(active_op_array)->static_variables;
                break;
            EMPTY_SWITCH_DEFAULT_CASE()
        }
        return NULL;
    }

	通过代码可以看到，当传递过来的fetch_type是ZEND_FETCH_GLOBAL(_LOCK)时，函数使用EG(excutor_global)宏
返回了global变量的符号表地址。

    以上就是global变量解析执行的整个过程。

0X04 参考文献

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