PHP7 Kernel Analysis 7 Zend Engine Execution Process

The content of this article is about the Zend engine execution process of PHP7 Kernel Analysis 7. Now I share it with you. Friends in need can refer to it

1.EG(executor_globals/zend_executor_globals )

The most important structure in the entire life cycle of PHP is a global variable, which is allocated before main execution (not under ZTS) until PHP exits. It records all the information of the current request

2.EX(execute_data/zend_execute_data)

at The core structure in the execution process. Each function call, include/require, eval, etc. will generate a new structure, which represents the current scope, code execution position, allocation of local variables, etc.,

#define EX(element)             ((execute_data)->element)

struct _zend_execute_data {
    const zend_op       *opline;  //指向当前执行的opcode，初始时指向zend_op_array起始位置
    zend_execute_data   *call;             /* current call                   */
    zval                *return_value;  //返回值指针
    zend_function       *func;          //当前执行的函数（非函数调用时为空）
    zval                 This;          //这个值并不仅仅是面向对象的this，还有另外两个值也通过这个记录：call_info + num_args，分别存在zval.u1.reserved、zval.u2.num_args
    zend_class_entry    *called_scope;  //当前call的类
    zend_execute_data   *prev_execute_data; //函数调用时指向调用位置作用空间
    zend_array          *symbol_table; //全局变量符号表
#if ZEND_EX_USE_RUN_TIME_CACHE
    void               **run_time_cache;   /* cache op_array->run_time_cache */
#endif
#if ZEND_EX_USE_LITERALS
    zval                *literals;  //字面量数组，与func.op_array->literals相同
#endif
};

3.Zend’s execution process

In Zend VM zend_execute_data.opline, zend_execute_data.prev_execute_data of zend_execute_data implement call/ret

step1: 为当前作用域分配一块内存，充当运行栈，zend_execute_data结构、所有局部变量、中间变量等等都在此内存上分配
step2: 初始化全局变量符号表，然后将全局执行位置指针EG(current_execute_data)指向step1新分配的zend_execute_data，然后将zend_execute_data.opline指向op_array的起始位置
step3: 从EX(opline)开始调用各opcode的C处理handler(即_zend_op.handler)，每执行完一条opcode将EX(opline)++继续执行下一条，直到执行完全部opcode，函数/类成员方法调用、if的执行过程：
step3.1: if语句将根据条件的成立与否决定EX(opline) + offset所加的偏移量，实现跳转
step3.2: 如果是函数调用，则首先从EG(function_table)中根据function_name取出此function对应的编译完成的zend_op_array，然后像step1一样新分配一个zend_execute_data结构，
将EG(current_execute_data)赋值给新结构的prev_execute_data，再将EG(current_execute_data)指向新的zend_execute_data，最后从新的zend_execute_data.opline开始执行，切换
到函数内部，函数执行完以后将EG(current_execute_data)重新指向EX(prev_execute_data)，释放分配的运行栈，销毁局部变量，继续从原来函数调用的位置执行
step4: 全部opcode执行完成后将step1分配的内存释放，这个过程会将所有的局部变量"销毁"，执行阶段结束

#4 .Runtime cache

During execution, PHP often needs to look up constants, functions, classes, member methods, member attributes, etc. in different hash tables based on their names, so PHP provides a cache The mechanism is used to cache the results found based on the name, so that when the same opcode is executed again, the last cached value can be directly reused without repeated searches, thereby improving execution efficiency. The runtime caching mechanism will only take effect when the same opcode is executed multiple times. Please note that the same opcode here does not refer to the same opcode value, but the same data in the memory.

In fact, the runtime cache is It is stored based on the CONST operand in the opcode it belongs to. That is to say, only operands containing the IS_CONST type may use this mechanism, and other types will not be used. This is because only the CONST operand is fixed, and other types are not used. Type values ​​such as CV and VAR are not fixed. Since their values ​​are not fixed, the cached values ​​are not fixed, so opcode operations of types other than CONST will not be cached.

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