Compile-time calculation: Difference between revisions

<br/>

<br/>

However, with the introduction of [https://github.com/ocaml-flambda/flambda-backend Flambda] (an alternative intermediate language, inliner, and optimiser), OCaml compilers which equip this backend have a limited ability to reduce pure and annotated function calls to constants:

<lang ocaml>let fact10 =

let rec factorial n =

if n = 1 then n else n * factorial (n-1)

in

(factorial[@unrolled 10]) 10

(* The unrolled annotation is what allows flambda to keep reducing the call

* Beware that the number of unrollings must be greater than or equal to the

* number of iterations (recursive calls) for this to compile down to a constant. *)</lang>

The assembler output (cleaned up and demangled a little) shows exactly what's expected, stored as data

Example:

.quad <span style="color:red">7257601</span>

Example.entry:

movl $1, %eax

ret

And converting the tagged int to regular int we get

# 7257601 lsr 1 ;;

- : int = 3628800

<code>example.ml</code> is compiled with <code>ocaml.4.12.0+flambda</code> (2021); the unrolling annotation is older than that, compilers as old as <code>4.03.0+flambda</code> (2016) support it.