Comment by spooneybarger | Hacker News Reader

Comment by spooneybarger | Hacker News Reader

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I can't really comment. I don't know how they are doing it and why they made that choice. I assume that Julia was already boxing integers in some fashion at which point this 100% makes sense to me.

Also, given the performance profile that you are seeking to allow programmers to achieve, I think boxing all integers makes sense, then you can give folks protection from integer overflow and underflow and otherwise make how the computer does math more closely approximate what we expect from doing math.

I think that following my above reasoning that only having floating point math can also make sense. That is... in most programming languages 3 and 3.0 are different. Such that 3/ 2 = 1 and 3.0/2.0 = 1.5.

We allow for 3/2=1 because, integer math is faster than floating point math and we want to allow folks to go as fast as possible when doing things like addition, subtraction and multiplication and accept that we can't represent all the numbers that come from division and give an approximation.

Julia doesn't box integers, that would destroy speed. It also doesn't protect from overflow/underflow. It specializes on Vector{Int} so that's it's just a standard vector of integers. Then Vector{Union{Int,Missing}} is stored in memory as a vector of integers with a bitarray declaring missings. Indexing operations are expanded to ifelse a check for the existence and then return the value if it exists or a missing. Then branches are added by the compiler to handle the small unions. This keeps pure integer use without overhead, and gives a small penalty when using missing (which doesn't turn out to be all that bad due to how pipelining is usually done), but it's safe and any type can have missing values.

Thank you. I look forward into learning more about how Julia handles this.

I would like to add, given that this is my favorite talk, that the decisions a language ends up making in things like this are a good reflection of its values. Bryan Cantrill has an incredibly good talk on this that I recommend to anyone reading this:

https://vimeo.com/230142234

If I understand correctly, instead of the usual kind of boxing, they use a separate type tag (one byte) and support union types with up to 255 other "special" enum values. The type tags and data are stored separately, so instead of an array of boxed values, they internally have two arrays, one for type tags and the other for data.

Apparently this can work well with SIMD instructions.

That's pretty cool. There's a lot that a couple of us on the Pony core team like about Julia from our limited knowledge. I'll have to look into that more.

> We allow for 3/2=1 because, integer math is faster than floating point math

Is it? I didn't measured it myself, but from reading Abrash's The Black Book I had got an idea that floating point division is faster on x86 with FPU.

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