The compiler can see the loop, so it can optimize it. Hosting constant expressions out of loops is a long-standing optimization in all compilers. Here, the expression "s" is unchanged* inside the loop, so the compiler can hoist it. If the expression were "s[0]" it could do the same. But can it do it for strlen(s)? It can, if it knows that strlen() has no side effects. So gcc extends C's syntax to add a "pure" attribute to tell the compiler exactly that.

* Pedants will point out all subtleties about C pointer aliasing rules apply here. But for the purpose of explanation, assume that nothing assigning throuh s means "s is constant".