Where is Go's string == function sources?

Where is Go's string == function (the function which gets called when you write if a == "a" {}) sources?

You're looking for runtime·cmpstring.

In plain Go, here: https://github.com/golang/go/blob/master/src/internal/bytealg/compare_generic.go

But many common architectures have specific implementations in assembly, like: https://github.com/golang/go/blob/master/src/internal/bytealg/compare_amd64.s