Are the advantages of Typed Arrays in JavaScript is that they work the same or similar in C?

Question

I\'ve been playing around with Typed Arrays in JavaScript.

var buffer = new ArrayBuffer(16);
var int32View = new Int32Array(buffer);

I imagine

Answer 1

I'm not really contributor to any javascript engine, only had some readings on v8, so my answer might not be completely true:

Well values in arrays(only normal arrays with no holes/gaps, not sparse. Sparse arrays are treated as objects.) are all either pointers or a number with a fixed length(in v8 they are 32 bit, if a 31 bit integer then it's tagged with a 0 bit in the end, else it's a pointer).

So I don't think finding the memory location is any different than a typedArray, since the number of the bytes are the same all over the array. But the difference comes that if it's an a object, then you have to add one unboxing layer, which doesn't happen for normal typedArrays.

And ofcourse when accessing typedArrays, definitely doesn't have type checking's that a normal array have(though that might be remove in a higly optimized code, which is only generated for hot code).

For Writing, if it's the same type shouldn't be much slower. If it's a different type then the JS engine might generate polymorphic code for it, which is slower.

You can also try making some benchmarks on jsperf.com to confirm.

Answer 2

When it comes to performance, things can change fast. As AshleysBrain says, it comes down to whether the VM can deduce that a normal array can be implemented as a typed array quickly and accurately. That depends on the particular optimizations of the particular JavaScript VM, and it can change in any new browser version.

This Chrome developer comment provides some guidance that worked as of June 2012:

1. Normal arrays can be as fast as typed arrays if you do a lot of sequential access. Random access outside the bounds of the array causes the array to grow.
2. Typed arrays are fast for access, but slow to be allocated. If you create temporary arrays frequently, avoid typed arrays. (Fixing this is possible, but it's low priority.)
3. Micro-benchmarks such as JSPerf are not reliable for real-world performance.

If I might elaborate on the last point, I've seen this phenomenon with Java for years. When you test the speed of a small piece of code by running it over and over again in isolation, the VM optimizes the heck out of it. It makes optimizations which only make sense for that specific test. Your benchmark can get a hundredfold speed improvement compared to running the same code inside another program, or compared to running it immediately after running several different tests that optimize the same code differently.

Answer 3

Yes, you are mostly correct. With a standard JavaScript array, the JavaScript engine has to assume that the data in the array is all objects. It can still store this as a C-like array/vector, where the access to the memory is still like you described. The problem is that the data is not the value, but something referencing that value (the object).

So, performing a[i] = b[i] + 2 requires the engine to:

1. access the object in b at index i;
2. check what type the object is;
3. extract the value out of the object;
4. add 2 to the value;
5. create a new object with the newly computed value from 4;
6. assign the new object from step 5 into a at index i.

With a typed array, the engine can:

1. access the value in b at index i (including placing it in a CPU register);
2. increment the value by 2;
3. assign the new object from step 2 into a at index i.

NOTE: These are not the exact steps a JavaScript engine will perform, as that depends on the code being compiled (including surrounding code) and the engine in question.

This allows the resulting computations to be much more efficient. Also, the typed arrays have a memory layout guarantee (arrays of n-byte values) and can thus be used to directly interface with data (audio, video, etc.).

Answer 4

Typed Arrays were designed by the WebGL standards committee, for performance reasons. Typically Javascript arrays are generic and can hold objects, other arrays and so on - and the elements are not necessarily sequential in memory, like they would be in C. WebGL requires buffers to be sequential in memory, because that's how the underlying C API expects them. If Typed Arrays are not used, passing an ordinary array to a WebGL function requires a lot of work: each element must be inspected, the type checked, and if it's the right thing (e.g. a float) then copy it out to a separate sequential C-like buffer, then pass that sequential buffer to the C API. Ouch - lots of work! For performance-sensitive WebGL applications this could cause a big drop in the framerate.

On the other hand, like you suggest in the question, Typed Arrays use a sequential C-like buffer already in their behind-the-scenes storage. When you write to a typed array, you are indeed assigning to a C-like array behind the scenes. For the purposes of WebGL, this means the buffer can be used directly by the corresponding C API.

Note your memory address calculation isn't quite enough: the browser must also bounds-check the array, to prevent out-of-range accesses. This has to happen with any kind of Javascript array, but in many cases clever Javascript engines can omit the check when it can prove the index value is already within bounds (such as looping from 0 to the length of the array). It also has to check the array index is really a number and not a string or something else! But it is in essence like you describe, using C-like addressing.

BUT... that's not all! In some cases clever Javascript engines can also deduce the type of ordinary Javascript arrays. In an engine like V8, if you make an ordinary Javascript array and only store floats in it, V8 may optimistically decide it's an array of floats and optimise the code it generates for that. The performance can then be equivalent to typed arrays. So typed arrays aren't actually necessary to reach maximum performance: just use arrays predictably (with every element the same type) and some engines can optimise for that as well.

So why do typed arrays still need to exist?

• Optimisations like deducing the type of arrays is really complicated. If V8 deduces an ordinary array has only floats in it, then you store an object in an element, it has to de-optimise and regenerate code that makes the array generic again. It's quite an achievement that all this works transparently. Typed Arrays are much simpler: they're guaranteed to be one type, and you just can't store other things like objects in them.
• Optimisations are never guaranteed to happen; you may store only floats in an ordinary array, but the engine may decide for various reasons not to optimise it.
• The fact they're much simpler means other less-sophisticated javascript engines can easily implement them. They don't need all the advanced deoptimisation support.
• Even with really advanced engines, proving optimisations can be used is extremely difficult and can sometimes be impossible. A typed array significantly simplifies the level of proof the engine needs to be able to optimise around it. A value returned from a typed array is certainly of a certain type, and engines can optimise for the result being that type. A value returned from an ordinary array could in theory have any type, and the engine may not be able to prove it will always have the same type result, and therefore generates less efficient code. Therefore code around a typed array is more easily optimised.
• Typed arrays remove the opportunity to make a mistake. You just can't accidentally store an object and suddenly get far worse performance.

So, in short, ordinary arrays can in theory be equally fast as typed arrays. But typed arrays make it much easier to reach peak performance.