Is Julia not evaluating the Function?

Question

I'm playing around with the Collatz conjecture (see below), and I have this function:

function txpo(max_n)
    for n ∈ 1:max_n
        x = n
        while x ≥ n  &&  x != 1
            x = iseven(x) ? x÷2 : 3x+1
        end
    end
end

On purpose I don't return anything or check anything, to make it as fast as possible. And boy, is it fast... - actually too fast, I think.

using BenchmarkTools
@btime txpo(100_000_000_000_000_000_000_000_000_000_000_000_000)

2.337 ns (0 allocations: 0 bytes)

Running through 10^38 while loops takes only 2ns? Julia is fast, but I would be surprised if it was that fast. It seems like the code is not evaluated. Or what am I missing here?

Collatz Conjecture Take a whole number and build a series by using the following rule: if the number is even divide by 2; if the number is odd multiply by 3 and add 1. Conjecture: every series will end up in the loop 4-2-1-4.

user_4354477 · Answer

Indeed, the whole function body is optimized out:

julia> function txpo(max_n)
           for n ∈ 1:max_n
               x = n
               while x ≥ n  &&  x != 1
                   x = iseven(x) ? x÷2 : 3x+1
               end
           end
       end
txpo (generic function with 1 method)

julia> @code_llvm txpo(100000000000)
;  @ REPL[1]:1 within `txpo`
define void @julia_txpo_179(i64 signext %0) #0 {
top:
;  @ REPL[1]:5 within `txpo`
  ret void
}

If you use 10^38, this only changes the function signature to accept 128-bit integers.

The LLVM IR code is basically the same as this Julia code:

julia_txpo_179(_0::Int64) = nothing

This makes perfect sense because why execute a function that doesn't return anything that depends on its computations and has no side-effects?

Finally, the native code that your CPU executes is literally retq, a.k.a. "return to caller":

julia> @code_native txpo(100000000000)
    .section    __TEXT,__text,regular,pure_instructions
; ┌ @ REPL[1]:5 within `txpo`
    retq
    nopw    %cs:(%rax,%rax)
; └

julia>

Is Julia not evaluating the Function?

1 Answers