Using libfuncs

You know how felt division is “disabled” by default in Cairo 1? But what if we really know what we’re doing and are not afraid of shooting ourselves in the foot? It’s actually quite easy to bring it back:

impl Felt252Div of Div<felt252> {
    fn div(lhs: felt252, rhs: felt252) -> felt252 {
        felt252_div(lhs, rhs.try_into().expect('div by zero'))
    }
}

use debug::PrintTrait;

fn main() {
    let r = 2 / 3;
    r.print();
    // 0x555555555555560aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaac, obviously
}

The Div impl is using felt252_div, a Sierra libfunc. Since it’s already declared in corelib it’s straightforward to use - we don’t even have to import it anymore.

By default, we can use any of the audited libfuncs and if we tweak Scarb.toml we can use even the experimental ones, but only locally.

However the problem then is to 1) knowing what the libfuncs are doing and 2) how to declare and use them. For old time’s sake, let’s take an example of the first libfunc in the audited list: alloc_local. Here’s a Cairo program using it:

extern fn alloc_local<u128>() -> u128 nopanic;

fn main() {
    let l: u128 = alloc_local();
}

It compiles down to the following Sierra code:

type [1] = u128 [storable: true, drop: true, dup: true, zero_sized: false];
type [2] = Uninitialized<[1]> [storable: false, drop: true, dup: false, zero_sized: false];
type [0] = Struct<ut@Tuple> [storable: true, drop: true, dup: true, zero_sized: true];

libfunc [1] = alloc_local<[1]>;
libfunc [3] = drop<[1]>;
libfunc [0] = struct_construct<[0]>;
libfunc [4] = store_temp<[0]>;

[1]() -> ([0]); // 0
[3]([0]) -> (); // 1
[0]() -> ([1]); // 2
[4]([1]) -> ([2]); // 3
return([2]); // 4

[0]@0() -> ([0]);

But trying to cairo-run it fails:

Error: Failed setting up runner.

Caused by:
    #1: One of the arguments does not match the expected type of the libfunc or return statement.

Why? I don’t know 😅 I guess you’ll have to dive into the depths of the compiler to figure that out. If you do, please reach out.