https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
Bug ID: 98731 Summary: s390x: Large classes of std::bitset and std::vector<bool> hash the same Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: doko at debian dot org Target Milestone: --- [forwarded from https://bugs.debian.org/977638] same behavior with GCC 8, 9 and 10. On s390x, std::hash returns identical values for large classes of std::bitset and std::vector<bool>: $ cat bug.cc #include <bitset> #include <functional> #include <iostream> #include <vector> int main() { std::bitset<2> a("00"), b("01"); std::vector<bool> c = {false, true, false, true}; std::vector<bool> d = {true, false, true, false}; std::bitset<9> e("000000000"), f("010101010"); std::vector<bool> g = {true, true, true, true, true, true, true, true, true}; std::vector<bool> h = {false, false, false, true, true, false, false, false, false}; std::hash<std::bitset<2>> h1; std::hash<std::bitset<9>> h2; std::hash<std::vector<bool>> h3; std::cout << h1(a) << '\n' << h1(b) << '\n' << h3(c) << '\n' << h3(d) << "\n\n" << h2(e) << '\n' << h2(f) << '\n' << h3(g) << '\n' << h3(h) << '\n'; } $ g++ -o bug bug.cc $ ./bug 7857072875483051545 7857072875483051545 7857072875483051545 7857072875483051545 4158372090644325695 4158372090644325695 4158372090644325695 4158372090644325695 It appears that the hash value is completely dependent on the size of the object in bytes. 1–8-bit values all hash to 7857072875483051545; 9–16-bit values all hash to 4158372090644325695; and though bug.cc doesn’t demonstrate it, 17-bit values hash to 14756137038141193723.