Why useless if statement is improving performance?

I'm writing a small library in rust as a 'collision detection' module. In trying to make a line-segment - line-segment intersection test as fast as possible, I've run into a strange discrepancy where seemingly useless code is making the function run faster.

My goal is optimising this method as much as possible, and knowing why the executable code changes performance as significantly as it does I would imagine would be helpful in achieving that goal. Whether it be a quirk in the rust compiler or something else, if anyone is knowledgeable to be able to understand this, here's what I have so far:

The two functions:

  fn line_line(a1x: f64, a1y: f64, a2x: f64, a2y: f64, b1x: f64, b1y: f64, b2x: f64, b2y: f64) -> bool {
     let dax = a2x - a1x;
     let day = a2y - a1y;
     let dbx = b2x - b1x;
     let dby = b2y - b1y;

     let dot = dax * dby - dbx * day;
     let dd = dot * dot;

     let nd1x = a1x - b1x;
     let nd1y = a1y - b1y;
     let t = (dax * nd1y - day * nd1x) * dot;
     let u = (dbx * nd1y - dby * nd1x) * dot;
     u >= 0.0 && u <= dd && t >= 0.0 && t <= dd
  }
  
  fn line_line_if(a1x: f64, a1y: f64, a2x: f64, a2y: f64, b1x: f64, b1y: f64, b2x: f64, b2y: f64) -> bool {
     let dax = a2x - a1x;
     let day = a2y - a1y;
     let dbx = b2x - b1x;
     let dby = b2y - b1y;

     let dot = dax * dby - dbx * day;
     if dot == 0.0 { return false } // useless branch if dot isn't 0 ?
     let dd = dot * dot;

     let nd1x = a1x - b1x;
     let nd1y = a1y - b1y;
     let t = (dax * nd1y - day * nd1x) * dot;
     let u = (dbx * nd1y - dby * nd1x) * dot;
     u >= 0.0 && u <= dd && t >= 0.0 && t <= dd
  }

Criterion-rs bench code:

  fn criterion_benchmark(c: &mut Criterion) {
     c.bench_function("line-line", |b| b.iter(|| line_line(
        black_box(0.0), black_box(1.0), black_box(2.0), black_box(0.0), 
        black_box(1.0), black_box(0.0), black_box(2.0), black_box(4.0))));
     c.bench_function("line-line-if", |b| b.iter(|| line_line_if(
        black_box(0.0), black_box(1.0), black_box(2.0), black_box(0.0), 
        black_box(1.0), black_box(0.0), black_box(2.0), black_box(4.0))));
  }

Note that with these inputs, the if statement will not evaluate to true.

assert_eq!(line_line_if(0.0, 1.0, 2.0, 0.0, 1.0, 0.0, 2.0, 4.0), true); // does not panic

Criterion-rs bench results:

line-line               time:   [5.6718 ns 5.7203 ns 5.7640 ns]
line-line-if            time:   [5.1215 ns 5.1791 ns 5.2312 ns]

Godbolt.org rust compilation results for these two pieces of code.

Unfortunately I cannot read asm as well as I'd like, but hopefully this may be helpful.
Thank you in advance if you're able to answer this.