[Bug tree-optimization/45115] pure functions returning structs are not optimized.

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This affects C++20 three-way comparisons, which return trivial structs wrapping
an integer.

From PR 108635:

    #include <compare>

    struct S
    {
        std::weak_ordering operator<=>(const S&) const __attribute__((const));
    };

    int compare3way(S& a, S& b)
    {
        return (a < b) ? -1 : (a > b) ? 1 : 0;
    }

I expect operator<=> to be called once, but it is called twice.  This can be a
major missed optimization if operator<=> is expensive.  It happens regardless
of:

 1. Using attribute((const)) or attribute((pure)).
 2. Making operator<=> a free function or a member.
 3. Comparing (a > b) or (a < b) in the second ternary expression.  This is
especially strange, because it's really calling the same pure function twice,
and that's optimized correctly when the function being called is operator<
instead of operator<=>.

Clang optimizes it as expected.

Demo: https://godbolt.org/z/jP51E6xaz