https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108635
Bug ID: 108635
Summary: Redundant calls to C++ spaceship operator<=> with
attribute pure or const
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jzwinck at gmail dot com
Target Milestone: ---
This code seems to have a missed optimization in GCC 10/11/12:
#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
