https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102490
Bug ID: 102490 Summary: Erroneous optimization of default constexpr operator== of struct with bitfields Product: gcc Version: 10.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: luc.briand35 at gmail dot com Target Milestone: --- Hello, For the following code, gcc version 10 and up wrongly optimizes the default operator==(). This occurs for O1 and up. Removing the 'constexpr' qualifier fixes everything. The size of the bitfields doesn't matter. No warnings are appear with "-Wall -Wextra". Godbolt link: https://gcc.godbolt.org/z/j4fG3sKze struct A { unsigned char foo : 1; unsigned char bar : 1; constexpr bool operator==(const A&) const = default; }; int main() { A a{}, b{}; a.bar = 0b1; return a == b; } With the options "-std=c++2a -O1", the assembly generated is simply: main: mov eax, 1 ret In this similar example, we can see that the generated assembly for the equality operator ignores the 'bar' bitfield (Godbolt link: https://gcc.godbolt.org/z/3K75xx1on) : struct A { unsigned char foo : 3; unsigned char bar : 1; constexpr bool operator==(const A&) const = default; }; void change(A& a); int main() { A a{}, b{}; change(a); return a == b; } The assembly for gcc version 10.X and 11.X is a bit different, but have the same problem.