I think this would be better served by a very large sum, e.g.

```rust
#[bench]
fn sum_many_f64(bh: &mut BenchHarness) {
    let nums = [-1e30, 1e60, 1e30, 1.0, -1e60];
    // with these numbers, true sum is 100, naive summation gives 0.
    let v = vec::from_fn(5*100, |i| nums[i % 5]);

    bh.iter(|| { v.sum(); });
}
```