Godbolt is such an amazing tool, and amazing that it's free.
For a random example from a few days ago, I wanted to understand how Rust compiles various approaches to doing pairwise addition between a f64 vector and a f32 vector: https://godbolt.org/z/9envsT. Profiling can tell me which is fastest, but godbolt is really helpful for understanding why.
(Fun fact I learned recently, after years of using it: Godbolt is named after its creator, Matt Godbolt [0]).