global語句的作用是定義全局變量,例如如果想在函數内通路全局作用域内的變量則可以通過global聲明來定義。 下面從文法解釋開始分析。
1. 詞法解析
檢視 Zend/zend_language_scanner.l檔案,搜尋 global關鍵字。我們可以找到如下代碼:
<ST_IN_SCRIPTING>"global" {
return T_GLOBAL;
}
2. 文法解析
在詞法解析完後,獲得了token,此時通過這個token,我們去Zend/zend_language_parser.y檔案中查找。找到相關代碼如下:
| 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); }
;
上面代碼中的$3是指global_var(如果不清楚yacc的文法,可以查閱yacc入門類的文章。)
從上面的代碼可以知道,對于全局變量的聲明調用的是zend_do_fetch_global_variable函數,查找此函數的實作在Zend/zend_compile.c檔案。
void zend_do_fetch_global_variable(znode *varname, const znode *static_assignment, int fetch_type TSRMLS_DC)
{
...//省略
opline->opcode = ZEND_FETCH_W; /* the default mode must be Write, since fetch_simple_variable() is used to define function arguments */
opline->result.op_type = IS_VAR;
opline->result.u.EA.type = 0;
opline->result.u.var = get_temporary_variable(CG(active_op_array));
opline->op1 = *varname;
SET_UNUSED(opline->op2);
opline->op2.u.EA.type = fetch_type;
result = opline->result;
... // 省略
fetch_simple_variable(&lval, varname, 0 TSRMLS_CC); /* Relies on the fact that the default fetch is BP_VAR_W */
zend_do_assign_ref(NULL, &lval, &result TSRMLS_CC);
CG(active_op_array)->opcodes[CG(active_op_array)->last-1].result.u.EA.type |= EXT_TYPE_UNUSED;
}
/* }}} */
上面的代碼确認了opcode為ZEND_FETCH_W外,還執行了zend_do_assign_ref函數。zend_do_assign_ref函數的實作如下:
void zend_do_assign_ref(znode *result, const znode *lvar, const znode *rvar TSRMLS_DC) /* {{{ */
{
zend_op *opline;
... //省略
opline = get_next_op(CG(active_op_array) TSRMLS_CC);
opline->opcode = ZEND_ASSIGN_REF;
...//省略
if (result) {
opline->result.op_type = IS_VAR;
opline->result.u.EA.type = 0;
opline->result.u.var = get_temporary_variable(CG(active_op_array));
*result = opline->result;
} else {
/* SET_UNUSED(opline->result); */
opline->result.u.EA.type |= EXT_TYPE_UNUSED;
}
opline->op1 = *lvar;
opline->op2 = *rvar;
}
從上面的zend_do_fetch_global_variable函數和zend_do_assign_ref函數的實作可以看出, 使用global聲明一個全局變量後,其執行了兩步操作,ZEND_FETCH_W和ZEND_ASSIGN_REF。
3. 生成并執行中間代碼
我們看下ZEND_FETCH_W的最後執行。從代碼中我們可以知道:
- ZEND_FETCH_W = 83
- op->op1.op_type = 4
- op->op2.op_type = 0
而計算最後調用的方法在代碼中的展現為:
zend_opcode_handlers[opcode * 25 + zend_vm_decode[op->op1.op_type] * 5 + zend_vm_decode[op->op2.op_type]];
計算,最後調用ZEND_FETCH_W_SPEC_CV_HANDLER函數。即
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中調用如下代碼擷取符号表
target_symbol_table = zend_get_target_symbol_table(opline, EX(Ts), type, varname TSRMLS_CC);
在zend_get_target_symbol_table函數的實作如下:
static inline HashTable *zend_get_target_symbol_table(const zend_op *opline, const temp_variable *Ts, int type, const zval *variable TSRMLS_DC)
{
switch (opline->op2.u.EA.type) {
... // 省略
case ZEND_FETCH_GLOBAL:
case ZEND_FETCH_GLOBAL_LOCK:
return &EG(symbol_table);
break;
... // 省略
}
return NULL;
}