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From: Hannes Domani <ssbssa@sourceware.org> To: gdb-cvs@sourceware.org Subject: [binutils-gdb] Fix right shift of negative numbers Date: Tue, 11 Jun 2024 18:37:16 +0000 (GMT) [thread overview] Message-ID: <20240611183716.EA8C0385DDC0@sourceware.org> (raw) 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> Diff: --- gdb/gmp-utils.h | 4 ++-- gdb/testsuite/gdb.base/bitshift.exp | 2 ++ 2 files changed, 4 insertions(+), 2 deletions(-) diff --git a/gdb/gmp-utils.h b/gdb/gmp-utils.h index 51e06abc050..878ce1da43a 100644 --- a/gdb/gmp-utils.h +++ b/gdb/gmp-utils.h @@ -280,13 +280,13 @@ struct gdb_mpz gdb_mpz operator>> (unsigned long nbits) const { gdb_mpz result; - mpz_tdiv_q_2exp (result.m_val, m_val, nbits); + mpz_fdiv_q_2exp (result.m_val, m_val, nbits); return result; } gdb_mpz &operator>>= (unsigned long nbits) { - mpz_tdiv_q_2exp (m_val, m_val, nbits); + mpz_fdiv_q_2exp (m_val, m_val, nbits); return *this; } diff --git a/gdb/testsuite/gdb.base/bitshift.exp b/gdb/testsuite/gdb.base/bitshift.exp index 61c7eca2747..17f6b78fed2 100644 --- a/gdb/testsuite/gdb.base/bitshift.exp +++ b/gdb/testsuite/gdb.base/bitshift.exp @@ -344,6 +344,8 @@ proc test_shifts {} { with_test_prefix "rsh neg lhs" { test_shift $lang "print -1 >> 0" " = -1" test_shift $lang "print -1 >> 1" " = -1" + test_shift $lang "print -2 >> 1" " = -1" + test_shift $lang "print -3 >> 1" " = -2" test_shift $lang "print -8 >> 1" " = -4" test_shift $lang "print [make_int64 $lang -8] >> 1" " = -4" }
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