Also, if you’re building binaries for redistribution, requiring Haswell or newer is kind of an aggressive choice.

I have a box that old that can run image diffusion models (I upgraded the GPU during covid.)

My point being that compiler authors do worry about “obsolete” targets because they’re widely used for compatibility reasons.

Oh, yeah. Looking at the code the comparison function is bad in a way that will hit issues with cmovs. The "else if" there is a nearly 100% predictable branch which follows a branch that's probably completely unpredictable. One optimization you get with -O3 is that branches not marked likely or unlikely can be turned into dataflow. The proper ordering for that code is to pull the equality check forward into an if statement and mark with "unlikely" if you're using -O3, and then return the ordering of the floating point comparison.

Since we're using Rust there's some decent syntactic sugar for this function you can use instead:

    let cmp = |other: &Neighbor| -> Ordering {
        other.dist.partial_cmp(&neighbor.dist).unwrap()
            .then_with(|| other.id.cmp(&neighbor.id))
    };

What factors would be compelling to upgrade for?

Just curious, since perf alone doesn’t seem to be the factor.

https://browser.geekbench.com/processors/intel-core-i7-2600k

https://browser.geekbench.com/processors/intel-core-i9-14900...

For what it's worth, you may be pleasantly surprised by the performance if you upgrade. I went from an Ivy Bridge processor to a Zen 3 processor, and I found that there were a lot of real world scenarios which got noticably faster. For example, AI turns in a late game Civ 6 map went from 30s to 15s. I can't promise you'll see good results, but it's worth considering.