Every access to a module-level list constant re-materializes the list

[niclas-ahden]

Reading a module-level list constant seems to cause a heap alloc+free for every read. A loop that reads one element per iteration from a 3-element top-level list is ~40x slower than the identical loop with the list passed as an argument. Scalar top-level constants are fine.

app [main!] {
    pf: platform "https://github.com/lukewilliamboswell/roc-platform-template-zig/releases/download/0.9/8GdFEvQYS3TeAZxKvTzCLVdQiomweGtXcdZkXNDEeABq.tar.zst",
}
import pf.Stdout

bases : List(U64)
bases = [1, 2, 3]

loop : U64, U64, U64 -> U64
loop = |i, n, acc|
    if i >= n {
        acc
    } else {
        loop(i + 1, n, acc + (bases.get(i % 3) ?? 0))
    }

main! = |args| {
    Stdout.line!("sum: ${loop(0, 1000000 + args.len(), 0).to_str()}")
    Ok({})
}

roc main.roc takes 3.9s. Moving bases into main! and passing it as an argument to loop takes 0.09s.

Tested on NixOS, x86, Roc 0d86ed8b3f4e39a941a15677c64f3ac1ca17e8bb.