[Bug gdb/31590] New: (-3) >> 1 should be -2.

[Bug gdb/31590] New: (-3) >> 1 should be -2.

[tsqurt at outlook dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31590

            Bug ID: 31590
           Summary: (-3) >> 1 should be -2.
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: tsqurt at outlook dot com
  Target Milestone: ---

Affected versions (Ubuntu 20.04 amd64)
- GNU gdb (GDB) 14.2
- GNU gdb (GDB) 15.0.50.20240402-git

The result is correct (-2) for gdb 13.2 using 2's complement. But for gdb 14.2:
(gdb) p (-3) >> 1
$1 = -1

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[Bug c++/31590] (-3) >> 1 should be -2.

[tromey at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31590

Tom Tromey <tromey at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|gdb                         |c++
                 CC|                            |tromey at sourceware dot org

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[Bug c++/31590] (-3) >> 1 should be -2.

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31590

Hannes Domani <ssbssa at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
                 CC|                            |ssbssa at sourceware dot org
   Last reconfirmed|                            |2024-05-31

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[Bug c++/31590] (-3) >> 1 should be -2.

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31590

--- Comment #1 from Hannes Domani <ssbssa at sourceware dot org> ---
https://sourceware.org/pipermail/gdb-patches/2024-May/209509.html

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[Bug c++/31590] (-3) >> 1 should be -2.

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31590

--- Comment #2 from Sourceware Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Hannes Domani <ssbssa@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=d17731525424349d7e63b517acf9f45114979fbb

commit d17731525424349d7e63b517acf9f45114979fbb
Author: Hannes Domani <ssbssa@yahoo.de>
Date:   Tue Jun 11 20:32:27 2024 +0200

    Fix right shift of negative numbers

    PR31590 shows that right shift of negative numbers doesn't work
    correctly since GDB 14:

    (gdb) p (-3) >> 1
    $1 = -1

    GDB 13 and earlier returned the correct value -2.
    And there actually is one test that shows the failure:

    print -1 >> 1
    $84 = 0
    (gdb) FAIL: gdb.base/bitshift.exp: lang=asm: rsh neg lhs: print -1 >> 1

    The problem was introduced with the change to gmp functions in
    commit 303a881f87.
    It's wrong because gdb_mpz::operator>> uses mpz_tdif_q_2exp, which
    always rounds toward zero, and the gmp docu says this:

    For positive n both mpz_fdiv_q_2exp and mpz_tdiv_q_2exp are simple
    bitwise right shifts.
    For negative n, mpz_fdiv_q_2exp is effectively an arithmetic right shift
    treating n as two's complement the same as the bitwise logical functions
    do, whereas mpz_tdiv_q_2exp effectively treats n as sign and magnitude.

    So this changes mpz_tdiv_q_2exp to mpz_fdiv_q_2exp, since it
    does right shifts for both positive and negative numbers.

    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31590
    Approved-By: Tom Tromey <tom@tromey.com>

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[Bug c++/31590] (-3) >> 1 should be -2.

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31590

Hannes Domani <ssbssa at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |16.1
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Hannes Domani <ssbssa at sourceware dot org> ---
Fixed.

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