If you are wondering how it works, here is Matt Godbolt himself explaining it. Surprisingly simple and cheap. https://www.youtube.com/watch?v=bSkpMdDe4g4&feature=youtu.be...
By the way, what I think could make this tool more useful for the average user is if the assembly were decompiled into another language (like C).
It's somewhere in the ideas list... https://github.com/compiler-explorer/compiler-explorer/issue...
Something similar is Andreas Fertig's awesome https://cppinsights.io/