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From: "doko at debian dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/98731] New: s390x: Large classes of std::bitset and
 std::vector<bool> hash the same
Date: Mon, 18 Jan 2021 14:41:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98731

            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 =3D {false, true, false, true};
      std::vector<bool> d =3D {true, false, true, false};

      std::bitset<9> e("000000000"), f("010101010");
      std::vector<bool> g =3D {true, true, true, true, true, true, true, tr=
ue,
true};
      std::vector<bool> h =3D {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=E2=80=938-bit values all hash to 7857072875483051545;
9=E2=80=9316-bit values all hash to 4158372090644325695; and though bug.cc
doesn=E2=80=99t demonstrate it, 17-bit values hash to 14756137038141193723.=