7.3 QuickJS核心代码流程 JS_Eval

JSRuntime *rt = JS_NewRuntime();
JSContext *ctx = JS_NewContext(rt);
js_std_add_helpers(ctx, 0, NULL);

const char *scripts = "console.log('hello quickjs')";
JS_Eval(ctx, scripts, strlen(scripts), "main", 0);

1 函数调用链

JS_Eval 方法就是执行 JS 脚本的入口方法。函数原型如下：

JSValue JS_Eval(JSContext *ctx, const char *input, size_t input_len, const char *filename, int eval_flags){
    return JS_EvalThis(ctx, ctx->global_obj, input, input_len, filename, eval_flags);
}

JS_EvalThis函数原型如下：

JSValue JS_EvalThis(JSContext *ctx, JSValueConst this_obj, const char *input, 
                    size_t input_len, const char *filename, int eval_flags){
           
    int eval_type = eval_flags & JS_EVAL_TYPE_MASK;
    JSValue ret;

    assert(eval_type == JS_EVAL_TYPE_GLOBAL ||
           eval_type == JS_EVAL_TYPE_MODULE);
    ret = JS_EvalInternal(ctx, this_obj, input, input_len, filename,
                          eval_flags, -1);
    return ret;
}

参数解释：

eval_flags

表示eval的类型，JS_EVAL_TYPE_MASK定义如下：

/* JS_Eval() flags */
#define JS_EVAL_TYPE_GLOBAL   (0 << 0) /* global code (default) */
#define JS_EVAL_TYPE_MODULE   (1 << 0) /* module code */
#define JS_EVAL_TYPE_DIRECT   (2 << 0) /* direct call (internal use) */
#define JS_EVAL_TYPE_INDIRECT (3 << 0) /* indirect call (internal use) */
#define JS_EVAL_TYPE_MASK     (3 << 0)

eval的类型有四种：

• JS_EVAL_TYPE_GLOBAL：default，表示global code
• JS_EVAL_TYPE_MODULE：表示module code
• JS_EVAL_TYPE_DIRECT：表示direct call (internal use)
• JS_EVAL_TYPE_INDIRECT：表示indirect call (internal use)

JS_EvalInternal函数原型如下：

/* the indirection is needed to make 'eval' optional */
static JSValue JS_EvalInternal(JSContext *ctx, JSValueConst this_obj,
                               const char *input, size_t input_len,
                               const char *filename, int flags, int scope_idx)
{
    if (unlikely(!ctx->eval_internal)) {
        return JS_ThrowTypeError(ctx, "eval is not supported");
    }
    return ctx->eval_internal(ctx, this_obj, input, input_len, filename,
                              flags, scope_idx);
}

在初始化上下文环境时已经通过JS_AddIntrinsicEval函数将eval_internal设定为了__JS_EvalInternal，具体函数定义如下：

/* 'input' must be zero terminated i.e. input[input_len] = '\0'. */
static JSValue __JS_EvalInternal(JSContext *ctx, JSValueConst this_obj,
                                 const char *input, size_t input_len,
                                 const char *filename, int flags, int scope_idx)
{
    JSParseState s1, *s = &s1;
    int err, js_mode, eval_type;
    JSValue fun_obj, ret_val;
    JSStackFrame *sf;
    JSVarRef **var_refs;
    JSFunctionBytecode *b;
    JSFunctionDef *fd;
    JSModuleDef *m;

	//解析1
    js_parse_init(ctx, s, input, input_len, filename);
    skip_shebang(s);

	//解析2
    eval_type = flags & JS_EVAL_TYPE_MASK;
    m = NULL;
    if (eval_type == JS_EVAL_TYPE_DIRECT) {
        JSObject *p;
        sf = ctx->rt->current_stack_frame;
        assert(sf != NULL);
        assert(JS_VALUE_GET_TAG(sf->cur_func) == JS_TAG_OBJECT);
        p = JS_VALUE_GET_OBJ(sf->cur_func);
        assert(js_class_has_bytecode(p->class_id));
        b = p->u.func.function_bytecode;
        var_refs = p->u.func.var_refs;
        js_mode = b->js_mode;
    } else {
        sf = NULL;
        b = NULL;
        var_refs = NULL;
        js_mode = 0;
        if (flags & JS_EVAL_FLAG_STRICT)
            js_mode |= JS_MODE_STRICT;
        if (flags & JS_EVAL_FLAG_STRIP)
            js_mode |= JS_MODE_STRIP;
        if (eval_type == JS_EVAL_TYPE_MODULE) {
            JSAtom module_name = JS_NewAtom(ctx, filename);
            if (module_name == JS_ATOM_NULL)
                return JS_EXCEPTION;
            m = js_new_module_def(ctx, module_name);
            if (!m)
                return JS_EXCEPTION;
            js_mode |= JS_MODE_STRICT;
        }
    }

	//解析3，补充1
    fd = js_new_function_def(ctx, NULL, TRUE, FALSE, filename, 1);
    if (!fd)
        goto fail1;
    s->cur_func = fd;
    fd->eval_type = eval_type;
    fd->has_this_binding = (eval_type != JS_EVAL_TYPE_DIRECT);
    fd->backtrace_barrier = ((flags & JS_EVAL_FLAG_BACKTRACE_BARRIER) != 0);
    if (eval_type == JS_EVAL_TYPE_DIRECT) {
        fd->new_target_allowed = b->new_target_allowed;
        fd->super_call_allowed = b->super_call_allowed;
        fd->super_allowed = b->super_allowed;
        fd->arguments_allowed = b->arguments_allowed;
    } else {
        fd->new_target_allowed = FALSE;
        fd->super_call_allowed = FALSE;
        fd->super_allowed = FALSE;
        fd->arguments_allowed = TRUE;
    }
    fd->js_mode = js_mode;
    fd->func_name = JS_DupAtom(ctx, JS_ATOM__eval_);
    if (b) {
        if (add_closure_variables(ctx, fd, b, scope_idx))
            goto fail;
    }
    fd->module = m;
    s->is_module = (m != NULL);
    s->allow_html_comments = !s->is_module;

    push_scope(s); /* body scope */
    fd->body_scope = fd->scope_level;
    
    err = js_parse_program(s);
    if (err) {
    fail:
        free_token(s, &s->token);
        js_free_function_def(ctx, fd);
        goto fail1;
    }

    /* create the function object and all the enclosed functions */
    fun_obj = js_create_function(ctx, fd);
    if (JS_IsException(fun_obj))
        goto fail1;
    /* Could add a flag to avoid resolution if necessary */
    if (m) {
        m->func_obj = fun_obj;
        if (js_resolve_module(ctx, m) < 0)
            goto fail1;
        fun_obj = JS_DupValue(ctx, JS_MKPTR(JS_TAG_MODULE, m));
    }
    if (flags & JS_EVAL_FLAG_COMPILE_ONLY) {
        ret_val = fun_obj;
    } else {
        ret_val = JS_EvalFunctionInternal(ctx, fun_obj, this_obj, var_refs, sf);
    }
    return ret_val;
 fail1:
    /* XXX: should free all the unresolved dependencies */
    if (m)
        js_free_module_def(ctx, m);
    return JS_EXCEPTION;
}

实际起作用的是 __JS_EvalInternal 函数，内部会先声明整个脚本解析涉及的内容，比如 JSParseState s、函数对象和返回值 fun_obj、栈帧 sf、变量指针 var_refs、函数字节码 b、函数 fd、模块 m 等。通过函数 js_parse_init 来设置初始化 JSParseState。有上下文 ctx、文件名 filename、根据输入 input 字符串长度设置缓存大小和初始化 token 等。

2 解析

解析1

js_parse_init 函数执行完后会过滤 Shebang，Shebang 一般是会在类 Unix 脚本的第一行，规则是开头两个字符是#!，作用是告诉系统希望用什么解释器执行脚本，比如#!/bin/bash 表示希望用 bash 来执行脚本。在 QuickJS 里显然 Shebang 起不了什么用，因此通过 skip_shebang 函数过滤掉。

解析2

关于flag的四个宏定义：

#define JS_EVAL_FLAG_STRICT   (1 << 3)  			//10XX，表示force 'strict' mode
#define JS_EVAL_FLAG_STRIP    (1 << 4)  			//100XX，表示force 'strip' mode
#define JS_EVAL_FLAG_COMPILE_ONLY (1 << 5)       	//1000XX，表示编译但不运行，结果是an object with a JS_TAG_FUNCTION_BYTECODE or JS_TAG_MODULE tag.可以通过JS_EvalFunction()执行。
/*  in the Error() backtraces */
#define JS_EVAL_FLAG_BACKTRACE_BARRIER (1 << 6)  	//10000xx，表示在Error()回溯过程中don't include the stack frames before this eval

解析3

这里js_new_function_def函数创建了一个顶层的函数定义节点，其第二个参数（父函数）设置为 NULL，后面再解析出来的函数都会成为其子函数。js_new_function_def 会返回一个初始化的 JSFunctionDef。（补充2）

2 补充说明

2.1 补充1

js_new_function_def函数定义：

static JSFunctionDef *js_new_function_def(JSContext *ctx,
                                          JSFunctionDef *parent,
                                          BOOL is_eval,
                                          BOOL is_func_expr,
                                          const char *filename, int line_num)
{
    JSFunctionDef *fd;

    fd = js_mallocz(ctx, sizeof(*fd));
    if (!fd)
        return NULL;

    fd->ctx = ctx;
    init_list_head(&fd->child_list);

    /* insert in parent list */
    fd->parent = parent;
    fd->parent_cpool_idx = -1;
    if (parent) {
        list_add_tail(&fd->link, &parent->child_list);
        fd->js_mode = parent->js_mode;
        fd->parent_scope_level = parent->scope_level;
    }

    fd->is_eval = is_eval;
    fd->is_func_expr = is_func_expr;
    js_dbuf_init(ctx, &fd->byte_code);
    fd->last_opcode_pos = -1;
    fd->func_name = JS_ATOM_NULL;
    fd->var_object_idx = -1;
    fd->arg_var_object_idx = -1;
    fd->arguments_var_idx = -1;
    fd->arguments_arg_idx = -1;
    fd->func_var_idx = -1;
    fd->eval_ret_idx = -1;
    fd->this_var_idx = -1;
    fd->new_target_var_idx = -1;
    fd->this_active_func_var_idx = -1;
    fd->home_object_var_idx = -1;

    /* XXX: should distinguish arg, var and var object and body scopes */
    fd->scopes = fd->def_scope_array;
    fd->scope_size = countof(fd->def_scope_array);
    fd->scope_count = 1;
    fd->scopes[0].first = -1;
    fd->scopes[0].parent = -1;
    fd->scope_level = 0;  /* 0: var/arg scope */
    fd->scope_first = -1;
    fd->body_scope = -1;

    fd->filename = JS_NewAtom(ctx, filename);
    fd->line_num = line_num;

    js_dbuf_init(ctx, &fd->pc2line);
    //fd->pc2line_last_line_num = line_num;
    //fd->pc2line_last_pc = 0;
    fd->last_opcode_line_num = line_num;

    return fd;
}

2.2 补充2

结构体 JSFunctionDef 定义

typedef struct JSFunctionDef {

    JSContext *ctx;
    struct JSFunctionDef *parent;
	//当前函数在parent常量池中的索引，or -1 if none
    int parent_cpool_idx; 
    
	//scope level in parent at point of definition
    int parent_scope_level; 
	//子函数列表，为JSFunctionDef.link的列表
    struct list_head child_list; 
    struct list_head link;

	//如果为true，那么表示当前函数代码是在eval中调用的
    BOOL is_eval; 
    int eval_type; /* only valid if is_eval = TRUE */
    BOOL is_global_var; /* TRUE if variables are not defined locally:
                           eval global, eval module or non strict eval */
    BOOL is_func_expr; /* TRUE if function expression */
    //表示当前函数是否有home object，每个函数都有一个slot用来存函数的home object，
    BOOL has_home_object; /* TRUE if the home object is available */

	//当前函数存在prototype时返回true，class都有prototype
    BOOL has_prototype; 
    BOOL has_simple_parameter_list;

	//当前函数是否有参数表达式，如果有的话创建参数作用域
    BOOL has_parameter_expressions; 
    BOOL has_use_strict; /* to reject directive in special cases */
    BOOL has_eval_call; /* true if the function contains a call to eval() */
    BOOL has_arguments_binding; /* true if the 'arguments' binding is
                                   available in the function */
    BOOL has_this_binding; /* true if the 'this' and new.target binding are
                              available in the function */
    BOOL new_target_allowed; /* true if the 'new.target' does not
                                throw a syntax error */
    BOOL super_call_allowed; /* true if super() is allowed */
    BOOL super_allowed; /* true if super. or super[] is allowed */
    BOOL arguments_allowed; /* true if the 'arguments' identifier is allowed */
    BOOL is_derived_class_constructor;
    BOOL in_function_body;
    BOOL backtrace_barrier;
    JSFunctionKindEnum func_kind : 8;
    JSParseFunctionEnum func_type : 8;
    uint8_t js_mode; /* bitmap of JS_MODE_x */
    JSAtom func_name; /* JS_ATOM_NULL if no name */

    JSVarDef *vars;
    int var_size; /* allocated size for vars[] */
    int var_count;
    JSVarDef *args;
    int arg_size; /* allocated size for args[] */
    int arg_count; /* number of arguments */
    int defined_arg_count;
    int var_object_idx; /* -1 if none */
    int arg_var_object_idx; /* -1 if none (var object for the argument scope) */
    int arguments_var_idx; /* -1 if none */
    int arguments_arg_idx; /* argument variable definition in argument scope, 
                              -1 if none */
    int func_var_idx; /* variable containing the current function (-1
                         if none, only used if is_func_expr is true) */
    int eval_ret_idx; /* variable containing the return value of the eval, -1 if none */
    int this_var_idx; /* variable containg the 'this' value, -1 if none */
    int new_target_var_idx; /* variable containg the 'new.target' value, -1 if none */
    int this_active_func_var_idx; /* variable containg the 'this.active_func' value, -1 if none */
    int home_object_var_idx;
    BOOL need_home_object;
    
    int scope_level;    /* index into fd->scopes if the current lexical scope */
    int scope_first;    /* index into vd->vars of first lexically scoped variable */
    int scope_size;     /* allocated size of fd->scopes array */
    int scope_count;    /* number of entries used in the fd->scopes array */
    JSVarScope *scopes;
    JSVarScope def_scope_array[4];
    int body_scope; /* scope of the body of the function or eval */

    int global_var_count;
    int global_var_size;
    JSGlobalVar *global_vars;

    DynBuf byte_code;
    int last_opcode_pos; /* -1 if no last opcode */
    int last_opcode_line_num;
    BOOL use_short_opcodes; /* true if short opcodes are used in byte_code */
    
    LabelSlot *label_slots;
    int label_size; /* allocated size for label_slots[] */
    int label_count;
    BlockEnv *top_break; /* break/continue label stack */

    /* constant pool (strings, functions, numbers) */
    JSValue *cpool;
    int cpool_count;
    int cpool_size;

    /* list of variables in the closure */
    int closure_var_count;
    int closure_var_size;
    JSClosureVar *closure_var;

    JumpSlot *jump_slots;
    int jump_size;
    int jump_count;

    LineNumberSlot *line_number_slots;
    int line_number_size;
    int line_number_count;
    int line_number_last;
    int line_number_last_pc;

    /* pc2line table */
    JSAtom filename;
    int line_num;
    DynBuf pc2line;

    char *source;  /* raw source, utf-8 encoded */
    int source_len;

    JSModuleDef *module; /* != NULL when parsing a module */
} JSFunctionDef;