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From: "cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
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Subject: [Bug c++/31590] (-3) >> 1 should be -2.
Date: Fri, 05 Jul 2024 19:45:22 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31590

--- Comment #4 from Sourceware Commits <cvs-commit at gcc dot gnu.org> ---
The gdb-15-branch branch has been updated by Hannes Domani
<ssbssa@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=3Dbinutils-gdb.git;h=3Dedd3f7be255e=
2e51ff7928cabb744af5d3d20018

commit edd3f7be255e2e51ff7928cabb744af5d3d20018
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 =3D -1

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

    print -1 >> 1
    $84 =3D 0
    (gdb) FAIL: gdb.base/bitshift.exp: lang=3Dasm: 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=3D31590
    Approved-By: Tom Tromey <tom@tromey.com>

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