https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105903
Bug ID: 105903
Summary: Missed optimization for __synth3way
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Consider this example:
#include <compare>
#include <vector>
inline constexpr auto synth3way = std::__detail::__synth3way;
struct Iterator {
std::vector<int>::iterator it;
constexpr bool operator<(Iterator const& rhs) const {
return it < rhs.it;
}
constexpr bool operator>(Iterator const& rhs) const {
return it > rhs.it;
}
};
bool less(Iterator const& a, Iterator const& b) {
return a < b;
}
bool less3way(Iterator const& a, Iterator const& b) {
return synth3way(a, b) < 0;
}
Here, synth3way(a, b) < 0 produces identical code to a < b (compiling with gcc
12.1 --std=c++20 -O3), which is great. The compiler recognizes that it doesn't
have to do the second synthesized comparison.
However, if we instead compared (sorry):
bool greater(Iterator const& a, Iterator const& b) {
return a > b;
}
bool greater3way(Iterator const& a, Iterator const& b) {
return synth3way(a, b) > 0;
}
This is now much worse:
greater(Iterator const&, Iterator const&):
mov rax, QWORD PTR [rdi]
cmp QWORD PTR [rsi], rax
setb al
ret
greater3way(Iterator const&, Iterator const&):
mov rdx, QWORD PTR [rdi]
mov rcx, QWORD PTR [rsi]
xor eax, eax
cmp rdx, rcx
jb .L3
cmp rcx, rdx
setb al
.L3:
ret
Interestingly, if we write this out:
bool greater3way(Iterator const& a, Iterator const& b) {
auto const cmp = [&]{
if (a < b) return std::weak_ordering::less;
if (b < a) return std::weak_ordering::greater;
return std::weak_ordering::equivalent;
}();
return cmp > 0;
}
bool greater3way_fold(Iterator const& a, Iterator const& b) {
return [&]{
if (a < b) return false;
if (b < a) return true;
return false;
}();
}
The greater3way_fold implementation generates the same code as >, which
definitely suggests to me that the greater3way version is simply a missed
optimization.
On compiler explorer: https://godbolt.org/z/1xvfsMrnf
Because synth3way is only used in contexts that require a weak ordering anyway,
it would be a valid optimization to replace synth3way(a, b) > 0 with b < a,
synth3way(a, b) <= 0 with !(b < a) and synth3way(a, b) >= 0 with !(a < b).
These replacements aren't generally true (because partial orders), but the
precondition on this type is that we have a weak order, so we should be able to
do better.
