LYR-SEM0221

a large struct passed by value

warning · spec 03-types · lint large-value-copy, group perf, allow unless said

A parameter's type is a struct of more than 64 bytes, counted without padding. Not said unless the lint is set to warn.

Explanation

A struct is a value: each call copies it whole into the parameter. Past 64 bytes the copy costs more than a reference would. The size the lint counts is a floor — no padding, a reference as one word — so a struct it names is at least as large as it says. The lint is allow by default.

Make the type a class, or pass the part of it the function needs.

Wrong

struct Frame {
    a: int,
    b: int,
    c: int,
    d: int,
    e: int,
    f: int,
    g: int,
    h: int,
    i: int,
}

@Warn("large-value-copy")
fn total(frame: Frame): int {
    return frame.a + frame.i;
}

fn main(): void {
    let _ = total(Frame { a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8, i = 9 });
}

Right

struct Frame {
    a: int,
    b: int,
    c: int,
    d: int,
    e: int,
    f: int,
    g: int,
    h: int,
}

@Warn("large-value-copy")
fn total(frame: Frame): int {
    return frame.a + frame.h;
}

fn main(): void {
    let _ = total(Frame { a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8 });
}