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
