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TIL: Why ARM Has a JavaScript Instruction
(notnotp.com)
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this post was submitted on 14 Jan 2026
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Programming
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There are tons of languages that let you represent integers directly as integers rather than having everything be a float so that you do not have to worry about this problem at all.
So by insanity you mean having just one number type?
Yes. Types are good. Numeric operations have specific hardware behavior that depends on whether you're using floating-point or not. Having exclusively floating-point semantics is wildly wrong for a programming language.
The whole reason why an entire instruction was added to ARM to facilitate conversion to integers is because people need integer semantics from their numbers and so the language has to support this efficiently. Thus, in practice there already two number types, it's just that they have been merged together in this incredibly messy way so that you have the worst of both worlds.
Yeah. I think the smallest number of number types you can reasonably have is two - f64 and arbitrary precision integers types. One of the few good decisions Python made.
Well, I think I’m happy to never have to choose a number type in JS. I also think that insanity is how C and Intel handle NaN conversions.
What does it mean to access the element at index π of an array?
What does it mean to access the 0th element of an array?
It is the 0-th element after the start of the array. 0-based indexing is very common in both mathematics and computer science.
Try interacting with anything that uses u64 and you'll be a lot less happy!
Anyway JavaScript does have BigInt so technically you are choosing.
It's not actually quite as bad as the article says. While it's UB for C, and it can return garbage. The actual x86 conversion instruction will never return garbage. Unfortunately the value it returns is 0x8000... whereas JS apparently wants 0. And it sets a floating point exception flag, so you still need extra instructions to handle it. Probably not many though.
Also in practice on a modern JS engine it won't actually need to do this operation very often anyway.
I’m sorry you had to experience this, but in all my years of development I hadn’t.
0x8000 is garbage. Insane.