> > # 内存地址相关

[nacyzhouw]

内存地址相关

简介

https://blog.csdn.net/qq_23079443/article/details/81143877

• 32位系统最大可用内存2^32个字节，2^10*2^10*2^10*2^2，1024个字节等于1KB 1024个KB等于1MB，1024个MB等于1GB，2^30 = 1GB，2^40 = 1TB， 1111 1111 1111 ...1111，32位共32个*8个1（1Byte = 8bit）
• 在64位操作系统中: 程序员可以使用的内存只要有前面的48位就可以了。也就是0X7fffffffffffffff(0x7fffffffffff = 01111111111111111111111111111111111111111111111)以下的。
• javascript的帧栈：
  https://github.com/stacktracejs/stackframe/blob/0d7a328bf05f6867e62245e3c9574aab2c97fd3b/stackframe.js#L29

var stackFrame = new StackFrame({
    functionName: 'funName',
    args: ['args'],
    fileName: 'http://localhost:3000/file.js',
    lineNumber: 1,
    columnNumber: 3288, 
    isEval: true,
    isNative: false,
    source: 'ORIGINAL_STACK_LINE'
});
   var booleanProps = ['isConstructor', 'isEval', 'isNative', 'isToplevel'];
    var numericProps = ['columnNumber', 'lineNumber'];
    var stringProps = ['fileName', 'functionName', 'source'];
    var arrayProps = ['args'];

• ecma_init()的调用栈

jmem_heap_alloc_block_internal(const size_t size) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/jmem/jmem-heap.c:163)
jmem_heap_gc_and_alloc_block(const size_t size, _Bool ret_null_on_error) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/jmem/jmem-heap.c:327)
jmem_heap_alloc_block(const size_t size) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/jmem/jmem-heap.c:373)
ecma_alloc_extended_object(size_t size) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/ecma/base/ecma-alloc.c:113)
ecma_create_object(ecma_object_t * prototype_object_p, size_t ext_object_size, ecma_object_type_t type) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/ecma/base/ecma-helpers.c:81)
ecma_instantiate_builtin(ecma_builtin_id_t obj_builtin_id) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/ecma/builtin-objects/ecma-builtins.c:385)
ecma_instantiate_builtin(ecma_builtin_id_t obj_builtin_id) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/ecma/builtin-objects/ecma-builtins.c:360)
ecma_builtin_get(ecma_builtin_id_t builtin_id) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/ecma/builtin-objects/ecma-builtins.c:297)
ecma_init_global_lex_env() (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/ecma/operations/ecma-lex-env.c:42)
ecma_init() (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/ecma/base/ecma-init-finalize.c:38)
jerry_init(jerry_init_flag_t flags) (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/jerry-core/api/jerry.c:185)
main() (/home/cisen/桌面/develop/jerryscript/demo-jerryscript/main.c:13)

• jmem_heap_alloc_block_internal函数是内存分配的最底层调用，该函数直接从全局的jerry_global_heap堆中申请内存，jerry_global_heap是固定512KB。jerry_global_heap堆的结构如下：

/**
 *  Free region node
 * 自由区域节点
 */
typedef struct
{
  uint32_t next_offset; /**< Offset of next region in list 列表中下一个区域的偏移量 */
  uint32_t size; /**< Size of region 区域大小 */
} jmem_heap_free_t;

struct jmem_heap_t
{
  jmem_heap_free_t first; /**< first node in free region list 自由区域列表中的第一个节点 */
  uint8_t area[JMEM_HEAP_AREA_SIZE]; /**< heap area 头部区域 JMEM_HEAP_AREA_SIZE是512*1024 -8预留8字节*/
};

• jmen就是jerry-memory的意思
• jerry_context_t是一个独立的栈，不是存在上面jerry_global_heap堆里面的

内部

• 貌似定义的地址范围是0x7FFFFFFFFFFF，即可

总结

• 说有的数据变更基本都是变更global_heap， 所以一定要搞清楚，全局堆的操作方式，具体见：jmem_heap_alloc_block_internal函数
• parser_list的每个项目叫做page
• parser_context的literal_pool就是存储很多literal_p的地方，详见：

literal_p = (lexer_literal_t *) parser_list_append (context_p, &context_p->literal_pool);

parser_context.lit_object只是存储当前的literal_object。每个literal_object其实是存在global_heap里面的，详见上面的函数调用了：parser_malloc函数申请heap堆内存

您好，好巧我最近也在研究JerryScript，有个问题请教下。
jerry_global_heap 这个代码中要求8字节对齐，知道是什么原因吗？
JERRY_ASSERT ((uintptr_t) JERRY_HEAP_CONTEXT (area) % JMEM_ALIGNMENT == 0);

因为的它的存储结构是jmem_heap_free_t，刚好是8字节吧，(建议关注：#535 这个帖子，以后解读基本以那个为主)

Originally posted by @cisen in #479 (comment)

分析了下觉得和jmem_heap_free_t 8字节没关系。
与JerryScript中ecma的结构有关。
具体分析如下： jerry_global_heap是我们分配的堆的起始地址，以创建object为例子，jerry_global_heap的8字节对齐，决定了后面分配的ecma_object_point8字节对齐。（ecma_object_point 必须8字节对齐，因为ecma_object_point 转成ecma_data时，通过ecma_pointer_to_ecma_value直接（将指针地址转为值）强转得到的，后面3bit必须为000, 官网中已说明ecma_value后面的3bit需要用于标志type）

[cisen]

JERRY_ASSERT ((uintptr_t) JERRY_HEAP_CONTEXT (area) % JMEM_ALIGNMENT == 0);

上面这句应该只是要求jerry_global_heap.area为8字节对齐把？没有要求jerry_global_heap为8字节对齐把？分析area关键是这两个宏：

#define JMEM_HEAP_GET_OFFSET_FROM_ADDR(p) ((uint32_t) ((uint8_t *) (p) - JERRY_HEAP_CONTEXT (area)))
#define JMEM_HEAP_GET_ADDR_FROM_OFFSET(u) ((jmem_heap_free_t *) (JERRY_HEAP_CONTEXT (area) + (u)))

看这两个宏的读写操作赋值结果area的类型都是jmem_heap_free_t，没读写别的东西，所以只能是8字节

[nacyzhouw]

JERRY_ASSERT ((uintptr_t) JERRY_HEAP_CONTEXT (area) % JMEM_ALIGNMENT == 0);

上面这句应该只是要求jerry_global_heap.area为8字节对齐把？没有要求jerry_global_heap为8字节对齐把？分析area关键是这两个宏：

#define JMEM_HEAP_GET_OFFSET_FROM_ADDR(p) ((uint32_t) ((uint8_t *) (p) - JERRY_HEAP_CONTEXT (area)))
#define JMEM_HEAP_GET_ADDR_FROM_OFFSET(u) ((jmem_heap_free_t *) (JERRY_HEAP_CONTEXT (area) + (u)))

看这两个宏的读写操作赋值结果area的类型都是jmem_heap_free_t，没读写别的东西，所以只能是8字节

为了保证jerry_global_heap.area可以是8字节对齐，必须是 jerry_global_heap是8字节对齐，对吧

另外下面的宏 (jmem_heap_free_t *)的作用是将jerry_global_heap.area指向的类型转为jmem_heap_free_t，它的地址是不变，并不是对jerry_global_heap.area地址对齐有什么约束
#define JMEM_HEAP_GET_ADDR_FROM_OFFSET(u) ((jmem_heap_free_t *) (JERRY_HEAP_CONTEXT (area) + (u)))

[cisen]

我觉得你理解错我的意思了，我的意思是对area的操作基本都是通过JMEM_HEAP_GET_OFFSET_FROM_ADDR和JMEM_HEAP_GET_ADDR_FROM_OFFSET这两个宏进行的。那么我们可以看看jerryscript通过调用这两个宏往area塞入了什么数据。全局搜索这两个宏发现，只往area塞入了jmem_heap_free_t类型的数据，没有别的类型数据。所以area里面只有jmem_heap_free_t为结构单元的数据