Fixed-length untyped arrays

# Isiah Meadows (6 months ago)

Usually, the native resizable array is good enough. But sometimes, you don't want to resize it, just allocate a slab of memory for objects. Typed arrays already implement this for integers, but I find it quite often in certain applications that I want to do this for arbitrary objects, too. So how about a FixedArray analogue to TypedArray that implements most the same stuff TypedArray does, just without the binary data integration or numeric coercion?

There's a few other benefits to having a native fixed-size untyped array:

  1. Engines can make major assumptions about allocation and use that to improve performance and minimize allocations - notably, they can choose to allocate the array's entries in the object itself, since it can't change for the lifetime of that object. This is the norm in some languages, like Rust.
  2. Engines can optimize things like .map, .filter, .concat, and similar far more easily. Those don't have to be generic, and since they can assume everything's contiguous, it's much easier to implement.
  3. Engines can make guarantees about allocation size, and that's the whole point of this proposal. These objects only require a minimum of type info, GC info, its length, and length number of machine pointers.

Of course, yes, normal arrays could provide most of these, but consider how V8 internally uses an InternalArray extensively to implement many JS built-ins in JS and its CodeStubAssembler DSL. It's also worth noting V8 didn't start inlining Array.prototype.map and friends for dense arrays until it figured out how to bail out of a JIT-compiled loop from within that loop with negligible overhead (it took an internal redesign), and no other engine I'm aware of performs this optimization, either - it's all because JS's resizable arrays can also be sparse.


Isiah Meadows contact at isiahmeadows.com, www.isiahmeadows.com

# Jordan Harband (6 months ago)

Is there any evidence that existing arrays prevent engines from making optimizations? I believe (and any implementors, please correct me if I'm wrong) that the most common approach is to guess the likely maximum size for the array, double it, and then allocate that much memory - and thus all the array methods can remain optimized until the memory size needs to double again.

# Isiah Meadows (6 months ago)

I believe (and any implementors, please correct me if I'm wrong) that the most common approach is to guess the likely maximum size for the array, double it, and then allocate that much memory - and thus all the array methods can remain optimized until the memory size needs to double again.

They usually don't guess, they just start with some fixed number and increase the length until they have enough, even with . With some arrays, like that from Array.prototype.map, I think, for at least V8, they allocate

Is there any evidence that existing arrays prevent engines from making optimizations?

Not always prevent, but they do make it far more complicated in many cases. For example: bugs.chromium.org/p/v8/issues/detail?id=2229

They do block optimization in memory-constrained scenarios, though, because the fast path requires too much memory. For example, Moddable's XS and JerryScript won't be able to elide the check without doubling their code size for those builtins.

Most of the blocked optimizations are that of memory size and allocation overhead. An implementation might choose to allocate the array of objects inline with the object itself, rather than as a separate pointer. You can only really do this with objects of fixed size. It's worth noting that if you start with room for 8 items and double on every allocation, 100 arrays of length 8 would end up allocating room for 800 entries, but 100 arrays of length 9 would end up with room for 1600 entries, not 900. And this is just the overhead of the object's entries, which itself has to be a pointer member on a parent object instance. So if there's a lot of these kinds of arrays, it adds up really fast.


Isiah Meadows contact at isiahmeadows.com, www.isiahmeadows.com