How PHP does garbage collection

Posted by benyamin on Wed, 13 Nov 2019 20:14:37 +0100

Comparison of variable implementation and garbage collection with php 5 and php7

Variable implementation

PHP variables are of weak type, which can represent integer, floating-point number, string and other types. PHP variables are represented by the structure zval

PHP 5.* zval and zend_value structures

struct _zval_struct { // structural morphology
    zvalue_value value;
    zend_uint refcount__gc;
    zend_uchar type;
    zend_uchar is_ref__gc;
}

typedef union _zvalue_value { // Consortium
    long lval;
    double dval;
    struct {
        char *val;
        int len;
    } str; // Character string
    HashTable *ht; // array
    zend_object_value obj; // object
    zend_ast *ast;
} zvalue_value;

PHP 7.0 zval and zend_value structures

struct _zval_struct {
    union {
        zend_long         lval;             /* long value */
        double            dval;             /* double value */
        zend_refcounted  *counted;
        zend_string      *str;
        zend_array       *arr;
        zend_object      *obj;
        zend_resource    *res;
        zend_reference   *ref;
        zend_ast_ref     *ast;
        zval             *zv;
        void             *ptr;
        zend_class_entry *ce;
        zend_function    *func;
        struct {
            uint32_t w1;
            uint32_t w2;
        } ww;
    } value;
    union {
        struct {
            ZEND_ENDIAN_LOHI_4(
                zend_uchar    type,         /* active type */
                zend_uchar    type_flags,
                zend_uchar    const_flags,
                zend_uchar    reserved)     /* call info for EX(This) */
        } v;
        uint32_t type_info;
    } u1;
    union {
        uint32_t     var_flags;
        uint32_t     next;                 /* hash collision chain */
        uint32_t     cache_slot;           /* literal cache slot */
        uint32_t     lineno;               /* line number (for ast nodes) */
        uint32_t     num_args;             /* arguments number for EX(This) */
        uint32_t     fe_pos;               /* foreach position */
        uint32_t     fe_iter_idx;          /* foreach iterator index */
    } u2;
};