javascript引擎v8内联缓存

Question

正如我们所知，v8 启用内联缓存以提高性能，

谁能解释一下 v8 到底做了什么以及它如何提高性能？

如果有示例就更好了。

提前致谢。

Answer 1

直接取自Chrome V8 page

V8 compiles JavaScript source code directly into machine code when it is first executed. There are no intermediate byte codes, no interpreter. Property access is handled by inline cache code that may be patched with other machine instructions as V8 executes....

和

...V8 optimizes property access by predicting that this [the object's] class will also be used for all future objects accessed in the same section of code and uses the information in the class to patch the inline cache code to use the hidden class. If V8 has predicted correctly the property's value is assigned (or fetched) in a single operation. If the prediction is incorrect, V8 patches the code to remove the optimisation.

例如，从 Point 对象访问属性 x 的 JavaScript 代码是:

point.x

在V8中，访问x生成的机器码是:

# ebx = the point object
cmp [ebx,<hidden class offset>],<cached hidden class>
jne <inline cache miss>
mov eax,[ebx, <cached x offset>]