There may need to be multiple calls to the allocator. Allocating all of the needed JavaScript objects will be slower.In JavaScript, there is indirection from the car variable to the car object, from the car object to the wheel array, from the wheel array to the wheel objects, and potentially from the wheel objects to the rotation angles. In C, accessing a wheel rotation angle only requires accessing the stack pointer at some fixed offset. These differences have a performance impact. Under Google V8, this would not be so, the rotation angles, if they are floating-point, would each be stored in objects of their own. In C, the rotation angles are contained within the wheel objects, in the same contiguous piece of memory.The JS car object only contains pointers. Modern JS engines will allocate a car object, a string object, an array object to store the wheels, and individual objects for each wheel. The C object, including the wheel objects and the rotation angles, is one contiguous piece of memory.
In JavaScript, it is implicitly allocated on the heap. In C, the car object is explicitly allocated on the stack.In practice, there are several differences as to how this will be implemented, however: In either language, one can write = 0.32, for example. In JavaScript, this could be done as follows: // Car object literalīoth of these accomplish the same goal. In C, you could do the following: struct CarĬhar makeModel // String for the car's make and modelĭouble direction // Wheel direction angleĭouble rotation // Current angle of rotation In JavaScript, this cannot be done manually.Īs a simple example, suppose you wanted to define and instantiate an object representing a car and its 4 wheels, with each wheel having angles of direction and rotation. There is one big difference, however: in C and C++, the programmer can intentionally nest (inline) objects inside of each other. Most objects in C/C++ are also heap-allocated, after all.
You might think that this isn’t such a big problem. This means that accessing object properties necessarily means going through one or more levels of indirection. Concretely speaking, all JavaScript objects are allocated on the heap and manipulated through pointers. What do I mean by indirection? Why is it a problem? In JavaScript (and most other dynamic languages), objects, arrays, functions and strings are manipulated through references values.
POINTER INDIRECTION CODE
When you ask someone why JavaScript code is slower than C, they will mention dynamic typing, run-time checks, number types and other sources of overhead, but they will seldom mention indirection. Surprisingly, this performance issue is one that is seldom talked about. This problem affects JavaScript, but also Python, Lua, Scheme and even Java to some extent. In this post I’m going to talk about a performance disadvantage inherent to most (if not all) current implementations of dynamic languages and, more generally, languages that make use of garbage collection.
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