Compiler Explorer

Source code

struct FooBar {
    foo: bool,
    bar: isize,
    b: [isize; 3],
    c: f64,
    d: &'static str,
}

pub const FOO: bool = true;
pub const BAR: isize = 1;
pub const B: [isize; 3] = [1,2,-3];
pub const C: f64 = 3.14;
pub const D: &str = "ding!";

const FOO_BAR: FooBar = FooBar {
    foo: FOO,
    bar: BAR,
    b: B,
    c: C,
    d: D
};

// The branch in this example only contains conditional on constants
#[unsafe(no_mangle)]
#[inline(never)]
fn work_with_constant(loop_count: u32) -> isize {
    let mut res = 0;
    // I think the testcase is too quick to have precise measurements,
    // we try to repeat the work 1000 times to smooth the imprecision
    for _ in 0..1000 {
        if FOO_BAR.foo && FOO_BAR.bar == BAR && FOO_BAR.b == B && FOO_BAR.c == C && FOO_BAR.d == D
        // This condition is always true
        {
            // Spin loop to be able to control the amount of
            // time spent in the branch
            for _ in 0..loop_count {
                // black_box to avoid loop optimizations
                res = std::hint::black_box(FOO_BAR.bar + res);
            }
        }
    }
    res
}

// The branch in this example only contains conditional on constants
#[unsafe(no_mangle)]
#[inline(never)]
fn work(foo_bar: &FooBar, loop_count: u32) -> isize {
    let mut res = 0;
    // I think the testcase is too quick to have precise measurements,
    // we try to repeat the work 1000 times to smooth the imprecision
    for _ in 0..1000 {
        if foo_bar.foo &&
            foo_bar.bar == BAR &&
            foo_bar.b == B &&
            foo_bar.c == C && 
            foo_bar.d == D
        // This condition is always true
        {
            // Spin loop to be able to control the amount of
            // time spent in the branch
            for _ in 0..loop_count {
                // black_box to avoid loop optimizations
                res = std::hint::black_box(foo_bar.bar + res);
            }
        }
    }
    res
}