https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125619
Bug ID: 125619
Summary: Missed optimization with comparing struct with bit
field
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: dr.xiaosa at gmail dot com
Target Milestone: ---
GCC generate bad code when doing field-wise comparison of struct with bit field
member on x86-64 target. Given:
===
struct test0 {
std::uint16_t a:7;
std::uint16_t b:2;
std::uint16_t c:5;
std::uint16_t d:2;
friend constexpr bool operator==(const test0& lhs, const test0& rhs) =
default;
};
bool comp0(test0 a, test0 b) noexcept {
return a == b;
}
===
GCC now produces:
===
"comp0(test0, test0)":
mov edx, edi
xor eax, eax
xor edx, esi
test dx, 511
je .L5
ret
.L5:
mov edx, esi
mov ecx, edi
movzx eax, dh
xor al, ch
cmp al, 1
setbe al
ret
===
Full test cases in C++:
https://godbolt.org/z/f18edz9KP
