深入理解PHP原理之foreach.doc

foreach是PHP中很常用的一个用作数组循环的控制语句。因为它的方便和易用，自然也就在后端隐藏着很复杂的具体实现方式(对用户透明)今天，我们就来一起分析分析，foreach是如何实现数组（对象）的遍历的。本节内容涉及到较多编译原理（lex and yacc）的知识，所以如果您觉得看不太懂，可以先找相关的资料看看。 我们知道PHP是一个脚本语言，也就是说，用户编写的PHP代码最终都是会被PHP解释器解释执行，特别的，对于PHP来说，所有的用户编写的PHP代码，都会被翻译成PHP的虚拟机ZE的虚拟指令（OPCODES）来执行(参看：深入理解PHP原理之Opcodes). 不论细节的话，就是说，我们所编写的任何PHP脚本，都会最终被翻译成一条条的指令，从而根据指令，由相应的C编写的函数来执行。 那么foreach会被翻译成什么样子呢？1. foreach($arr as $key = $val)2. echo $key . = . $val . n;3. /SPAN在词法分析阶段，foreach会被识别为一个TOKEN：T_FOREACH，在语法分析阶段，会被规则：1. unticked_statement: /没有被绑定ticks的语句2. /有省略3. | T_FOREACH ( variable T_AS4. zend_do_foreach_begin(&$1, &$2, &$3, &$4, 1 TSRMLS_CC); 5. foreach_variable foreach_optional_arg ) zend_do_foreach_cont(&$1, &$2, &$4, &$6, &$7 TSRMLS_CC); 6. foreach_statement zend_do_foreach_end(&$1, &$4 TSRMLS_CC); 7. | T_FOREACH ( expr_without_variable T_AS8. zend_do_foreach_begin(&$1, &$2, &$3, &$4, 0 TSRMLS_CC); 9. variable foreach_optional_arg ) zend_check_writable_variable(&$6); zend_do_foreach_cont(&$1, &$2, &$4, &$6, &$7 TSRMLS_CC); 10. foreach_statement zend_do_foreach_end(&$1, &$4 TSRMLS_CC); 11. /有省略12. ;/SPAN仔细分析这段语法规则，我们可以发现，对于:foreach($arr as $key = $val)echo $key . = . $val .”n”;会被分析为：1. T_FOREACH ( variable T_AS zend_do_foreach_begin(foreach, (, $arr, as, 1 TSRMLS_CC); 2. foreach_variable foreach_optional_arg(T_DOUBLE_ARROW foreach_variable) ) zend_do_foreach_cont(foreach, (, as, $key, $val TSRMLS_CC); 3. foreach_satement zend_do_foreach_end(foreach, as);/SPAN然后，让我们来看看foreach_statement：它其实就是一个代码块，体现了我们的 echo $key . = . $val .”n”;T_ECHO expr; 显然，实现foreach的核心就是如下3个函数：zend_do_foreach_beginzend_do_foreach_contzend_do_foreach_end其中，zend_do_foreach_begin (代码太长，直接写伪码) 主要做了：1. 记录当前的opline行数（为以后跳转而记录）2. 对数组进行RESET（讲内部指针指向第一个元素）3. 获取临时变量 （$val）4. 设置获取变量的OPCODE FE_FETCH，结果存第3步的临时变量4. 记录获取变量的OPCODES的行数而对于 zend_do_foreach_cont来说：1. 根据foreach_variable的u.EA.type来判断是否引用2. 根据是否引用来调整zend_do_foreach_begin中生成的FE_FETCH方式3. 根据zend_do_foreach_begin中记录的取变量的OPCODES的行数，来初始化循环(主要处理在循环内部的循环:do_begin_loop)最后zend_do_foreach_end:1. 根据zend_do_foreach_begin中记录的行数信息，设置ZEND_JMP OPCODES2. 根据当前行数，设置循环体下一条opline, 用以跳出循环3. 结束循环（处理循环内循环:do_end_loop)4. 清理临时变量当然， 在zend_do_foreach_cont 和 zend_do_foreach_end之间 会在语法分析阶段被填充foreach_satement的语句代码。 这样，就实现了foreach的OPCODES line。比如对于我们开头的实例代码，最终生成的OPCODES是：1. filename: /home/huixinchen/foreach.php2. function name: (null)3. number of ops: 174. compiled vars: !0 = $arr, !1 = $key, !2 = $val5. line # op fetch ext return operands6. -7. 2 0 SEND_VAL 18. 1 SEND_VAL 1009. 2 DO_FCALL 2 range10. 3 ASSIGN !0, $011. 3 4 FE_RESET $2 !0, -1412. 5 FE_FETCH $3 $2, -1413. 6 ZEND_OP_DATA 514. 7 ASSIGN !2, $315. 8 ASSIGN !1, 516. 4 9 CONCAT 7 !1, -17. 10 CONCAT 8 7, !218. 11 CONCAT 9 8, %0A19. 12 ECHO 920. 5 13 JMP -521. 14 SWITCH_FREE $222. 7 15 RETURN 123. 16* ZEND_HANDLE_EXCEPTIO我们注意到FE_FETCH的op2的操作数是14，也就是JMP后一条opline，也就是说，在获取完最后一个数组元素以后，FE_FETCH失败的情况下，会跳到第14行opline，从而实现了循环的结束。而15行opline的op1的操作数是指向了FE_FETCH，也就是无条件跳转到第5行opline，从而实现了循环。 附录:1. void zend_do_foreach_begin(znode *foreach_token, znode *open_brackets_token, znode *array, znode *as_token, int variable TSRMLS_DC)2. 3. zend_op *opline;4. zend_bool is_variable;5. zend_bool push_container = 0;6. zend_op dummy_opline;7. 8. if (variable) 9. /是否是匿名数组10. if (zend_is_function_or_method_call(array) 11. /是否是函数返回值12. is_variable = 0;13. else 14. is_variable = 1;15. 16. /* 使用括号记录FE_RESET的opline行数 */17. open_brackets_token-u.opline_num = get_next_op_number(CG(active_op_array);18. zend_do_end_variable_parse(BP_VAR_W, 0 TSRMLS_CC); /获取数组/对象和zend_do_begin_variable_parse对应19. if (CG(active_op_array)-last 0 &20. CG(active_op_array)-opcodesCG(active_op_array)-last-1.opcode = ZEND_FETCH_OBJ_W) 21. /* Only lock the container if we are fetching from a real container and not $this */22. if (CG(active_op_array)-opcodesCG(active_op_array)-last-1.op1.op_type = IS_VAR) 23. CG(active_op_array)-opcodesCG(active_op_array)-last-1.extended_value |= ZEND_FETCH_ADD_LOCK;24. push_container = 1;25. 26. 27. else 28. is_variable = 0;29. open_brackets_token-u.opline_num = get_next_op_number(CG(active_op_array);30. 31. 32. foreach_token-u.opline_num = get_next_op_number(CG(active_op_array); /记录数组Reset Opline number33. 34. opline = get_next_op(CG(active_op_array) TSRMLS_CC); /生成Reset数组Opcode35. 36. opline-opcode = ZEND_FE_RESET;37. opline-result.op_type = IS_VAR;38. opline-result.u.var = get_temporary_variable(CG(active_op_array);39. opline-op1 = *array;40. SET_UNUSED(opline-op2);41. opline-extended_value = is_variable ? ZEND_FE_RESET_VARIABLE : 0;42. 43. dummy_opline.result = opline-result;44. if (push_container) 45. dummy_opline.op1 = CG(active_op_array)-opcodesCG(active_op_array)-last-2.op1;46. else 47. znode tmp;48. 49. tmp.op_type = IS_UNUSED;50. dummy_opline.op1 = tmp;51. 52. zend_stack_push(&CG(foreach_copy_stack), (void *) &dummy_opline, sizeof(zend_op); 53. 54. as_token-u.opline_num = get_next_op_number(CG(active_op_array); /记录循环起始点55. 56. opline = get_next_op(CG(active_op_array) TSRMLS_CC);57. opline-opcode = ZEND_FE_FETCH;58. opline-result.op_type = IS_VAR;59. opline-result.u.var = get_temporary_variable(CG(active_op_array);60. opline-op1 = dummy_opline.result; /被操作数组61. opline-extended_value = 0;62. SET_UNUSED(opline-op2);63. 64. opline = get_next_op(CG(active_op_array) TSRMLS_CC);65. opline-opcode = ZEND_OP_DATA; /当使用key的时候附属操作数,当foreach中不包含key时忽略66. SET_UNUSED(opline-op1);67. SET_UNUSED(opline-op2);68. SET_UNUSED(opline-result);69. /SPAN1. void zend_do_foreach_cont(znode *foreach_token, const znode *open_brackets_token, const znode *as_token, znode *value, znode *key TSRMLS_DC)2. 3. zend_op *opline;4. znode dummy, value_node;5. zend_bool assign_by_ref=0;6. 7. opline = &CG(active_op_array)-opcodesas_token-u.opline_num; /获取FE_FETCH Opline8. if (key-op_type != IS_UNUSED) 9. znode *tmp;/交换key和val10. 11. tmp = key;12. key = value;13. value = tmp;14. 15. opline-extended_value |= ZEND_FE_FETCH_WITH_KEY; /表明需要同时获取key和val16. 17. 18. if (key-op_type != IS_UNUSED) & (key-u.EA.type & ZEND_PARSED_REFERENCE_VARIABLE) 19. /key不能以引用方式获取20. zend_error(E_COMPILE_ERROR, Key element cannot be a reference);21. 22. 23. if (value-u.EA.type & ZEND_PARSED_REFERENCE_VARIABLE) 24. /以引用方式获取值25. assign_by_ref = 1;26. if (!(opline-1)-extended_value) 27. /根据FE_FETCH的上一条Opline也就是获取数组的扩展值来判断数组是否是匿名数组28. zend_error(E_COMPILE_ERROR, Cannot create references to elements of a temporary array expression);29. 30. 31. opline-extended_value |= ZEND_FE_FETCH_BYREF; /指明按引用取32. CG(active_op_array)-opcodesforeach_token-u.opline_num.extended_value |= ZEND_FE_RESET_REFERENCE; /重置原数组33. else 34. zend_op *foreach_copy;35. zend_op *fetch = &CG(active_op_array)-opcodesforeach_token-u.opline_num;36. zend_op *end = &CG(active_op_array)-opcodesopen_brackets_token-u.opline_num;37. 38. /* Change write context into read context */39. fetch-extended_value = 0; /* reset ZEND_FE_RESET_VARIABLE */40. while (fetch != end) 41. -fetch;42. if (fetch-opcode = ZEND_FETCH_DIM_W & fetch-op2.op_type = IS_UNUSED) 43. zend_error(E_COMPILE_ERROR, Cannot use for reading);44. 45. fetch-opcode -= 3; /* FETCH_W - FETCH_R */46. 47. 48. /* prevent double SWITCH_FREE */49. zend_stack_top(&CG(foreach_copy_stack), (void *) &foreach_copy);50. foreach_copy-op1.op_type = IS_UNUSED;51. 52. 53. value_node = opline-result; 54. 55. if (assign_by_ref) 56. zend_do_end_variable_parse(value, BP_VAR_W, 0 TSRMLS_CC); /获取值（引用）57. zend_do_assign_ref(NULL, value, &value_node TSRMLS_CC);/指明value node的type是IS_VAR58. else 59. zend_do_assign(&dummy, value, &value_node TSRMLS_CC); /获取copy值60. zend_do_free(&dummy TSRMLS_CC);61. 62. 63. if (key-op_type != IS_UNUSED) 64. znode key_node;65. 66. opline = &CG(active_op_array)-opcodesas_token-u.opline_num+1;67. opline-result.op_type = IS_TMP_VAR;68. opline-result.u.EA.type = 0;69. opline-result.u.opline_num = get_temporary_variable(CG(active_op_array);70. key_node = opline-result;71. 72. zend_do_assign(&dummy, key, &key_node TSRMLS_CC);73. zend_do_free(&dummy TSRMLS_CC);74. 75. 76. do_begin_loop(TSRMLS_C);77. INC_BPC(CG(active_op_array);78. /SPAN1. vo