[Bug c++/115399] New: std::tr2::dynamic_bitset shift behaves differently from std::bitset

[Bug c++/115399] New: std::tr2::dynamic_bitset shift behaves differently from std::bitset

[adamant.pwn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115399

            Bug ID: 115399
           Summary: std::tr2::dynamic_bitset shift behaves differently
                    from std::bitset
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: adamant.pwn at gmail dot com
  Target Milestone: ---
            Target: x86-64 Linux
             Build: 20240522

Posting a bug on behalf of someone who didn't manage to create an account at
GCC bugzilla due to automatic registration being disabled...

The bug is due to the line
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/tr2/dynamic_bitset.tcc#L63
being commented out in dynamic_bitset implementation (unlike std::bitset, where
it is present). Uncommenting it should fix the problem.

A simple example program to highlight the discrepancy:

#include <iostream>
#include <bitset>
#include <tr2/dynamic_bitset>

int main() {
    std::tr2::dynamic_bitset<> test(91);
    test[50] = true;
    std::cout << test << "\n";
    std::cout << (test << 78) << "\n";
    std::cout << "\n";

    std::bitset<91> test2;
    test2[50] = true;
    std::cout << test2 << "\n";
    std::cout << (test2 << 78) << "\n";
}

Output:

0000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000

0000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

[Bug libstdc++/115399] std::tr2::dynamic_bitset shift behaves differently from std::bitset

[adamant.pwn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115399

--- Comment #1 from Oleksandr Kulkov <adamant.pwn at gmail dot com> ---
The bug also affects right shift for a similar reason
(https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/tr2/dynamic_bitset.tcc#L91):

#include <iostream>
#include <bitset>
#include <tr2/dynamic_bitset>
#include <boost/dynamic_bitset.hpp>

int main() {
    std::tr2::dynamic_bitset<> test(128);
    for (int i = 64; i < 128; i++) test[i] = 1;
    std::cout << test << "\n";
    std::cout << (test >> 65) << "\n\n";

    boost::dynamic_bitset<> test2(128);
    for (int i = 64; i < 128; i++) test2[i] = 1;
    std::cout << test2 << "\n";
    std::cout << (test2 >> 65) << "\n\n";

    std::bitset<128> test3;
    for (int i = 64; i < 128; i++) test3[i] = 1;
    std::cout << test3 << "\n";
    std::cout << (test3 >> 65) << "\n\n";
}

Output:

11111111111111111111111111111111111111111111111111111111111111110000000000000000000000000000000000000000000000000000000000000000
11111111111111111111111111111111111111111111111111111111111111110111111111111111111111111111111111111111111111111111111111111111

11111111111111111111111111111111111111111111111111111111111111110000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000000000111111111111111111111111111111111111111111111111111111111111111

11111111111111111111111111111111111111111111111111111111111111110000000000000000000000000000000000000000000000000000000000000000
00000000000000000000000000000000000000000000000000000000000000000111111111111111111111111111111111111111111111111111111111111111

See also https://github.com/emsr/tr2/issues/1.

[Bug libstdc++/115399] std::tr2::dynamic_bitset shift behaves differently from std::bitset

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115399

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=114498

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note I think std::tr2 should be deprecated in a similar way as std::tr1 is
being considered.

[Bug libstdc++/115399] std::tr2::dynamic_bitset shift behaves differently from std::bitset

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115399

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-06-10
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The tr2 extensions never even got proposed for standardization and are not in
any ISO document at all. They're not well tested and nobody uses them.

[Bug libstdc++/115399] std::tr2::dynamic_bitset shift behaves differently from std::bitset

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115399

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug libstdc++/115399] std::tr2::dynamic_bitset shift behaves differently from std::bitset

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115399

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:bd3a312728fbf8c35a09239b9180269f938f872e

commit r15-1213-gbd3a312728fbf8c35a09239b9180269f938f872e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jun 10 14:08:16 2024 +0100

    libstdc++: Fix std::tr2::dynamic_bitset shift operations [PR115399]

    The shift operations for dynamic_bitset fail to zero out words where the
    non-zero bits were shifted to a completely different word.

    For a right shift we don't need to sanitize the unused bits in the high
    word, because we know they were already clear and a right shift doesn't
    change that.

    libstdc++-v3/ChangeLog:

            PR libstdc++/115399
            * include/tr2/dynamic_bitset (operator>>=): Remove redundant
            call to _M_do_sanitize.
            * include/tr2/dynamic_bitset.tcc (_M_do_left_shift): Zero out
            low bits in words that should no longer be populated.
            (_M_do_right_shift): Likewise for high bits.
            * testsuite/tr2/dynamic_bitset/pr115399.cc: New test.

[Bug libstdc++/115399] std::tr2::dynamic_bitset shift behaves differently from std::bitset

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115399

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |15.0

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk. I'll probably backport it to the release branches at some
point.