JSValue是什么

在JavaScript中，变量是弱类型的。但是C是一个强类型语言。但是QuicksJS作为一个使用C语言编写的虚拟机，少不了和JavaScript代码中的变量交互。如何来解决这个问题呢？ QuickJS中，使用JSValue来表示这种变量。因此，JSValue是一个JavaScript变量的代表。它既可以是基础类型也可以是一个对象。它用到了引用计数，因此，明确的对它进行复制（JS_DupValue()，添加引用计数）和释放（JS_FreeValue()，减少引用计数）很重要。
先看它的定义。

typedef union JSValueUnion {
    int32_t int32;
    double float64;
    void *ptr;
} JSValueUnion;

typedef struct JSValue {
    JSValueUnion u;
    int64_t tag;
} JSValue;

enum {
    /* all tags with a reference count are negative */
    JS_TAG_FIRST       = -11, /* first negative tag */
    JS_TAG_BIG_DECIMAL = -11,
    JS_TAG_BIG_INT     = -10,
    JS_TAG_BIG_FLOAT   = -9,
    JS_TAG_SYMBOL      = -8,
    JS_TAG_STRING      = -7,
    JS_TAG_MODULE      = -3, /* used internally */
    JS_TAG_FUNCTION_BYTECODE = -2, /* used internally */
    JS_TAG_OBJECT      = -1,

    JS_TAG_INT         = 0,
    JS_TAG_BOOL        = 1,
    JS_TAG_NULL        = 2,
    JS_TAG_UNDEFINED   = 3,
    JS_TAG_UNINITIALIZED = 4,
    JS_TAG_CATCH_OFFSET = 5,
    JS_TAG_EXCEPTION   = 6,
    JS_TAG_FLOAT64     = 7,
    /* any larger tag is FLOAT64 if JS_NAN_BOXING */
};

JSValue是一个结构体，它定义了tag和u两个字段。其中tag，表示它的值的类型，有上述代码中的19种类型。另外一个u是JSValueUnion，用于存储值或指针，分别包含：int32，float64和指针ptr。

JSValue的创建

数字类型的JSValue的创建

#define JS_MKVAL(tag, val) (JSValue){ (JSValueUnion){ .int32 = val }, tag }

/* value handling */
static js_force_inline JSValue JS_NewBool(JSContext *ctx, JS_BOOL val)
{
    return JS_MKVAL(JS_TAG_BOOL, (val != 0));
}

static js_force_inline JSValue JS_NewInt32(JSContext *ctx, int32_t val)
{
    return JS_MKVAL(JS_TAG_INT, val);
}

static js_force_inline JSValue JS_NewCatchOffset(JSContext *ctx, int32_t val)
{
    return JS_MKVAL(JS_TAG_CATCH_OFFSET, val);
}

static js_force_inline JSValue JS_NewInt64(JSContext *ctx, int64_t val)
{
    JSValue v;
    if (val == (int32_t)val) {
        v = JS_NewInt32(ctx, val);
    } else {
        v = __JS_NewFloat64(ctx, val);
    }
    return v;
}

static js_force_inline JSValue JS_NewUint32(JSContext *ctx, uint32_t val)
{
    JSValue v;
    if (val <= 0x7fffffff) {
        v = JS_NewInt32(ctx, val);
    } else {
        v = __JS_NewFloat64(ctx, val);
    }
    return v;
}

数字型的JSValue创建非常简单，使用宏定义，定义了一个结构体。如果是32位的整数，放到JSValueUnion的int32字段中。如果是浮点数字，就放到float64字段中。以JS_NewInt32()为例，上面的宏定义，预处理时，将会变成下面的代码

static js_force_inline JSValue JS_NewInt32(JSContext *ctx, int32_t val)
{   
    return (JSValue){ 
        (JSValueUnion){
         .int32 = val 
         }, 
         JS_TAG_INT 
    };
}

其中的(JSValueUnion)和(JSValue)其实就是c语言中的强制转换。希望读者在此不要困惑。

字符串类型的JSValue的创建

//由纯ascii字符构成的字符串，创建JSValue
static JSValue js_new_string8(JSContext *ctx, const uint8_t *buf, int len)
{
    JSString *str;

    // 如果是空字符串，直接返回JS_ATOM_empty_string
    if (len <= 0) {
        return JS_AtomToString(ctx, JS_ATOM_empty_string);
    }
    str = js_alloc_string(ctx, len, 0);
    if (!str)
        return JS_EXCEPTION;
    memcpy(str->u.str8, buf, len);
    str->u.str8[len] = '\0';
    return JS_MKPTR(JS_TAG_STRING, str);
}


typedef struct StringBuffer {
    JSContext *ctx;
    JSString *str;
    int len;
    int size;
    int is_wide_char;
    int error_status;
} StringBuffer;

/* It is valid to call string_buffer_end() and all string_buffer functions even
   if string_buffer_init() or another string_buffer function returns an error.
   If the error_status is set, string_buffer_end() returns JS_EXCEPTION.
 */
static int string_buffer_init2(JSContext *ctx, StringBuffer *s, int size,
                               int is_wide)
{
    s->ctx = ctx;
    s->size = size;
    s->len = 0;
    s->is_wide_char = is_wide;
    s->error_status = 0;
    s->str = js_alloc_string(ctx, size, is_wide);
    if (unlikely(!s->str)) {
        s->size = 0;
        return s->error_status = -1;
    }
#ifdef DUMP_LEAKS
    /* the StringBuffer may reallocate the JSString, only link it at the end */
    list_del(&s->str->link);
#endif
    return 0;
}

static inline int string_buffer_init(JSContext *ctx, StringBuffer *s, int size)
{
    return string_buffer_init2(ctx, s, size, 0);
}

static void string_buffer_free(StringBuffer *s)
{
    js_free(s->ctx, s->str);
    s->str = NULL;
}

static int string_buffer_set_error(StringBuffer *s)
{
    js_free(s->ctx, s->str);
    s->str = NULL;
    s->size = 0;
    s->len = 0;
    return s->error_status = -1;
}

static no_inline int string_buffer_widen(StringBuffer *s, int size)
{
    JSString *str;
    size_t slack;
    int i;

    if (s->error_status)
        return -1;

    str = js_realloc2(s->ctx, s->str, sizeof(JSString) + (size << 1), &slack);
    if (!str)
        return string_buffer_set_error(s);
    size += slack >> 1;
    for(i = s->len; i-- > 0;) {
        str->u.str16[i] = str->u.str8[i];
    }
    s->is_wide_char = 1;
    s->size = size;
    s->str = str;
    return 0;
}

static no_inline int string_buffer_realloc(StringBuffer *s, int new_len, int c)
{
    JSString *new_str;
    int new_size;
    size_t new_size_bytes, slack;

    if (s->error_status)
        return -1;

    if (new_len > JS_STRING_LEN_MAX) {
        JS_ThrowInternalError(s->ctx, "string too long");
        return string_buffer_set_error(s);
    }
    new_size = min_int(max_int(new_len, s->size * 3 / 2), JS_STRING_LEN_MAX);
    if (!s->is_wide_char && c >= 0x100) {
        return string_buffer_widen(s, new_size);
    }
    new_size_bytes = sizeof(JSString) + (new_size << s->is_wide_char) + 1 - s->is_wide_char;
    new_str = js_realloc2(s->ctx, s->str, new_size_bytes, &slack);
    if (!new_str)
        return string_buffer_set_error(s);
    new_size = min_int(new_size + (slack >> s->is_wide_char), JS_STRING_LEN_MAX);
    s->size = new_size;
    s->str = new_str;
    return 0;
}

static no_inline int string_buffer_putc_slow(StringBuffer *s, uint32_t c)
{
    if (unlikely(s->len >= s->size)) {
        if (string_buffer_realloc(s, s->len + 1, c))
            return -1;
    }
    if (s->is_wide_char) {
        s->str->u.str16[s->len++] = c;
    } else if (c < 0x100) {
        s->str->u.str8[s->len++] = c;
    } else {
        if (string_buffer_widen(s, s->size))
            return -1;
        s->str->u.str16[s->len++] = c;
    }
    return 0;
}

/* 0 <= c <= 0xff */
static int string_buffer_putc8(StringBuffer *s, uint32_t c)
{
    if (unlikely(s->len >= s->size)) {
        if (string_buffer_realloc(s, s->len + 1, c))
            return -1;
    }
    if (s->is_wide_char) {
        s->str->u.str16[s->len++] = c;
    } else {
        s->str->u.str8[s->len++] = c;
    }
    return 0;
}

/* 0 <= c <= 0xffff */
static int string_buffer_putc16(StringBuffer *s, uint32_t c)
{
    if (likely(s->len < s->size)) {
        if (s->is_wide_char) {
            s->str->u.str16[s->len++] = c;
            return 0;
        } else if (c < 0x100) {
            s->str->u.str8[s->len++] = c;
            return 0;
        }
    }
    return string_buffer_putc_slow(s, c);
}

/* 0 <= c <= 0x10ffff */
static int string_buffer_putc(StringBuffer *s, uint32_t c)
{
    if (unlikely(c >= 0x10000)) {
        /* surrogate pair */
        c -= 0x10000;
        if (string_buffer_putc16(s, (c >> 10) + 0xd800))
            return -1;
        c = (c & 0x3ff) + 0xdc00;
    }
    return string_buffer_putc16(s, c);
}


static int string_buffer_write8(StringBuffer *s, const uint8_t *p, int len)
{
    int i;

    if (s->len + len > s->size) {
        if (string_buffer_realloc(s, s->len + len, 0))
            return -1;
    }
    if (s->is_wide_char) {
        for (i = 0; i < len; i++) {
            s->str->u.str16[s->len + i] = p[i];
        }
        s->len += len;
    } else {
        memcpy(&s->str->u.str8[s->len], p, len);
        s->len += len;
    }
    return 0;
}

static int string_buffer_write16(StringBuffer *s, const uint16_t *p, int len)
{
    int c = 0, i;

    for (i = 0; i < len; i++) {
        c |= p[i];
    }
    if (s->len + len > s->size) {
        if (string_buffer_realloc(s, s->len + len, c))
            return -1;
    } else if (!s->is_wide_char && c >= 0x100) {
        if (string_buffer_widen(s, s->size))
            return -1;
    }
    if (s->is_wide_char) {
        memcpy(&s->str->u.str16[s->len], p, len << 1);
        s->len += len;
    } else {
        for (i = 0; i < len; i++) {
            s->str->u.str8[s->len + i] = p[i];
        }
        s->len += len;
    }
    return 0;
}

static int string_buffer_concat(StringBuffer *s, const JSString *p,
                                uint32_t from, uint32_t to)
{
    if (to <= from)
        return 0;
    if (p->is_wide_char)
        return string_buffer_write16(s, p->u.str16 + from, to - from);
    else
        return string_buffer_write8(s, p->u.str8 + from, to - from);
}

static JSValue string_buffer_end(StringBuffer *s)
{
    JSString *str;
    str = s->str;
    if (s->error_status)
        return JS_EXCEPTION;
    if (s->len == 0) {
        js_free(s->ctx, str);
        s->str = NULL;
        return JS_AtomToString(s->ctx, JS_ATOM_empty_string);
    }
    if (s->len < s->size) {
        /* smaller size so js_realloc should not fail, but OK if it does */
        /* XXX: should add some slack to avoid unnecessary calls */
        /* XXX: might need to use malloc+free to ensure smaller size */
        str = js_realloc_rt(s->ctx->rt, str, sizeof(JSString) +
                            (s->len << s->is_wide_char) + 1 - s->is_wide_char);
        if (str == NULL)
            str = s->str;
        s->str = str;
    }
    if (!s->is_wide_char)
        str->u.str8[s->len] = 0;
#ifdef DUMP_LEAKS
    list_add_tail(&str->link, &s->ctx->rt->string_list);
#endif
    str->is_wide_char = s->is_wide_char;
    str->len = s->len;
    s->str = NULL;
    return JS_MKPTR(JS_TAG_STRING, str);
}

/* create a string from a UTF-8 buffer */
//根据utf-8编码的字符串创建JSValue（JS_TAG_String）
JSValue JS_NewStringLen(JSContext *ctx, const char *buf, size_t buf_len)
{
    const uint8_t *p, *p_end, *p_start, *p_next;
    uint32_t c;
    StringBuffer b_s, *b = &b_s;
    size_t len1;
    
    p_start = (const uint8_t *)buf;
    p_end = p_start + buf_len;
    p = p_start;
    //遍历字符串，直到字符串中非ascii字符部分结束
    while (p < p_end && *p < 128)
        p++;
    len1 = p - p_start;
    if (len1 > JS_STRING_LEN_MAX)
        return JS_ThrowInternalError(ctx, "string too long");
    
    // 判断字符串是否位纯ascii字符    
    if (p == p_end) {
        /* ASCII string */
        //如果是纯ascii，调用js_new_string8来创建JSValue
        return js_new_string8(ctx, (const uint8_t *)buf, buf_len);
    } else {
        if (string_buffer_init(ctx, b, buf_len))
            goto fail;
        string_buffer_write8(b, p_start, len1);
        while (p < p_end) {
            if (*p < 128) {
                string_buffer_putc8(b, *p++);
            } else {
                /* parse utf-8 sequence, return 0xFFFFFFFF for error */
                c = unicode_from_utf8(p, p_end - p, &p_next);
                if (c < 0x10000) {
                    p = p_next;
                } else if (c <= 0x10FFFF) {
                    p = p_next;
                    /* surrogate pair */
                    c -= 0x10000;
                    string_buffer_putc16(b, (c >> 10) + 0xd800);
                    c = (c & 0x3ff) + 0xdc00;
                } else {
                    /* invalid char */
                    c = 0xfffd;
                    /* skip the invalid chars */
                    /* XXX: seems incorrect. Why not just use c = *p++; ? */
                    while (p < p_end && (*p >= 0x80 && *p < 0xc0))
                        p++;
                    if (p < p_end) {
                        p++;
                        while (p < p_end && (*p >= 0x80 && *p < 0xc0))
                            p++;
                    }
                }
                string_buffer_putc16(b, c);
            }
        }
    }
    return string_buffer_end(b);

 fail:
    string_buffer_free(b);
    return JS_EXCEPTION;
}

static JSValue JS_ConcatString3(JSContext *ctx, const char *str1,
                                JSValue str2, const char *str3)
{
    StringBuffer b_s, *b = &b_s;
    int len1, len3;
    JSString *p;

    if (unlikely(JS_VALUE_GET_TAG(str2) != JS_TAG_STRING)) {
        str2 = JS_ToStringFree(ctx, str2);
        if (JS_IsException(str2))
            goto fail;
    }
    p = JS_VALUE_GET_STRING(str2);
    len1 = strlen(str1);
    len3 = strlen(str3);

    if (string_buffer_init2(ctx, b, len1 + p->len + len3, p->is_wide_char))
        goto fail;

    string_buffer_write8(b, (const uint8_t *)str1, len1);
    string_buffer_concat(b, p, 0, p->len);
    string_buffer_write8(b, (const uint8_t *)str3, len3);

    JS_FreeValue(ctx, str2);
    return string_buffer_end(b);

 fail:
    JS_FreeValue(ctx, str2);
    return JS_EXCEPTION;
}

JSValue JS_NewString(JSContext *ctx, const char *str)
{
    return JS_NewStringLen(ctx, str, strlen(str));
}

JSValue JS_NewAtomString(JSContext *ctx, const char *str)
{
    JSAtom atom = JS_NewAtom(ctx, str);
    if (atom == JS_ATOM_NULL)
        return JS_EXCEPTION;
    JSValue val = JS_AtomToString(ctx, atom);
    JS_FreeAtom(ctx, atom);
    return val;
}

JSAtom转JSValue

#define ATOM_GET_STR_BUF_SIZE 64

#define JS_MKPTR(tag, p) (JSValue){ (JSValueUnion){ .ptr = p }, tag }

#define JS_VALUE_GET_TAG(v) ((int32_t)(v).tag)

#define JS_VALUE_HAS_REF_COUNT(v) ((unsigned)JS_VALUE_GET_TAG(v) >= (unsigned)JS_TAG_FIRST)

#define JS_VALUE_GET_PTR(v) ((v).u.ptr)

//添加JSValue引用计数
static inline JSValue JS_DupValue(JSContext *ctx, JSValueConst v)
{
    if (JS_VALUE_HAS_REF_COUNT(v)) {
        JSRefCountHeader *p = (JSRefCountHeader *)JS_VALUE_GET_PTR(v);
        p->ref_count++;
    }
    return (JSValue)v;
}

static inline BOOL __JS_AtomIsTaggedInt(JSAtom v)
{
    return (v & JS_ATOM_TAG_INT) != 0;
}

static inline JSAtom __JS_AtomFromUInt32(uint32_t v)
{
    return v | JS_ATOM_TAG_INT;
}

static inline uint32_t __JS_AtomToUInt32(JSAtom atom)
{
    return atom & ~JS_ATOM_TAG_INT;
}

static const char *JS_AtomGetStr(JSContext *ctx, char *buf, int buf_size, JSAtom atom)
{
    return JS_AtomGetStrRT(ctx->rt, buf, buf_size, atom);
}

static JSValue __JS_AtomToValue(JSContext *ctx, JSAtom atom, BOOL force_string)
{
    char buf[ATOM_GET_STR_BUF_SIZE];

    //判断是否是整形，因为JSAtom存储时，发现存储的字符串是整形数字时，直接就转为u32去存储了
    if (__JS_AtomIsTaggedInt(atom)) {
        snprintf(buf, sizeof(buf), "%u", __JS_AtomToUInt32(atom));
        return JS_NewString(ctx, buf);
    } else {
        //从字符串池中取出来对应字符串。
        JSRuntime *rt = ctx->rt;
        JSAtomStruct *p;
        assert(atom < rt->atom_size);
        p = rt->atom_array[atom];
        if (p->atom_type == JS_ATOM_TYPE_STRING) {
            goto ret_string;
        } else if (force_string) {
            if (p->len == 0 && p->is_wide_char != 0) {
                /* no description string */
                p = rt->atom_array[JS_ATOM_empty_string];
            }
        ret_string:
        // #define JS_MKPTR(tag, p) (JSValue){ (JSValueUnion){ .ptr = p }, tag }
            // 创建JSValue
            return JS_DupValue(ctx, JS_MKPTR(JS_TAG_STRING, p));
        } else {
            return JS_DupValue(ctx, JS_MKPTR(JS_TAG_SYMBOL, p));
        }
    }
}

JSValue JS_AtomToValue(JSContext *ctx, JSAtom atom)
{
    return __JS_AtomToValue(ctx, atom, FALSE);
}

JSValue JS_AtomToString(JSContext *ctx, JSAtom atom)
{
    return __JS_AtomToValue(ctx, atom, TRUE);
}

JSValue的复制

#define JS_VALUE_GET_TAG(v) ((int32_t)(v).tag)

#define JS_VALUE_HAS_REF_COUNT(v) ((unsigned)JS_VALUE_GET_TAG(v) >= (unsigned)JS_TAG_FIRST)

#define JS_VALUE_GET_PTR(v) ((v).u.ptr)

//添加JSValue引用计数
static inline JSValue JS_DupValue(JSContext *ctx, JSValueConst v)
{
    if (JS_VALUE_HAS_REF_COUNT(v)) {
        JSRefCountHeader *p = (JSRefCountHeader *)JS_VALUE_GET_PTR(v);
        p->ref_count++;
    }
    return (JSValue)v;
}

JSValue的释放

void __JS_FreeValue(JSContext *ctx, JSValue v);
void __JS_FreeValue(JSContext *ctx, JSValue v)
{
    __JS_FreeValueRT(ctx->rt, v);
}
/* called with the ref_count of 'v' reaches zero. */
void __JS_FreeValueRT(JSRuntime *rt, JSValue v)
{
    uint32_t tag = JS_VALUE_GET_TAG(v);

#ifdef DUMP_FREE
    {
        printf("Freeing ");
        if (tag == JS_TAG_OBJECT) {
            JS_DumpObject(rt, JS_VALUE_GET_OBJ(v));
        } else {
            JS_DumpValueShort(rt, v);
            printf("\n");
        }
    }
#endif

    switch(tag) {
    case JS_TAG_STRING:
        {
            JSString *p = JS_VALUE_GET_STRING(v);
            if (p->atom_type) {
                JS_FreeAtomStruct(rt, p);
            } else {
#ifdef DUMP_LEAKS
                list_del(&p->link);
#endif
                js_free_rt(rt, p);
            }
        }
        break;
    case JS_TAG_OBJECT:
    case JS_TAG_FUNCTION_BYTECODE:
        {
            JSGCObjectHeader *p = JS_VALUE_GET_PTR(v);
            if (rt->gc_phase != JS_GC_PHASE_REMOVE_CYCLES) {
                list_del(&p->link);
                list_add(&p->link, &rt->gc_zero_ref_count_list);
                if (rt->gc_phase == JS_GC_PHASE_NONE) {
                    free_zero_refcount(rt);
                }
            }
        }
        break;
    case JS_TAG_MODULE:
        abort(); /* never freed here */
        break;
#ifdef CONFIG_BIGNUM
    case JS_TAG_BIG_INT:
    case JS_TAG_BIG_FLOAT:
        {
            JSBigFloat *bf = JS_VALUE_GET_PTR(v);
            bf_delete(&bf->num);
            js_free_rt(rt, bf);
        }
        break;
    case JS_TAG_BIG_DECIMAL:
        {
            JSBigDecimal *bf = JS_VALUE_GET_PTR(v);
            bfdec_delete(&bf->num);
            js_free_rt(rt, bf);
        }
        break;
#endif
    case JS_TAG_SYMBOL:
        {
            JSAtomStruct *p = JS_VALUE_GET_PTR(v);
            JS_FreeAtomStruct(rt, p);
        }
        break;
    default:
        printf("__JS_FreeValue: unknown tag=%d\n", tag);
        abort();
    }
}

static inline void JS_FreeValue(JSContext *ctx, JSValue v)
{
    if (JS_VALUE_HAS_REF_COUNT(v)) {
        JSRefCountHeader *p = (JSRefCountHeader *)JS_VALUE_GET_PTR(v);
        if (--p->ref_count <= 0) {
            __JS_FreeValue(ctx, v);
        }
    }
}