fast.js | Faster user-land reimplementations | Runtime Evironment library

(V8 developer here.)

Does anyone know which part of the code in V8 makes this behavior?

It's part of the "scope resolution" system, and most of the code is in src/ast/scopes.cc. (It's fairly complicated, because there are so many things that it has to deal with.)

Is it true that a number variable is allocated in heap when it's used in a closure that never runs and a variable and it's in a register when it's not used in a closure?

Yes.

It doesn't matter whether the variable contains a number or anything else.

It doesn't matter how often the closure will run, including whether it will run at all. At the time where the decision needs to be made where to allocate variables, there is no information available yet about what will or won't execute. (Also, closures that are statically guaranteed to never execute are so rare in real code that it most likely wouldn't be worth optimizing for them even if it was possible.)

Variables used by closures are "context-allocated", which makes accessing them a little bit slower than "stack-allocated" variables. If/when the function in question gets optimized, stack-allocated variables may or may not end up being kept in a register some or all of the time, depending on what other register-allocation decisions the optimizing compiler makes.

In most real-world situations, you won't notice the difference (so this is usually not something worth worrying about), but a microbenchmark like a loop that only does count++ makes it observable.