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| author | Hannes Domani <ssbssa@yahoo.de> | 2024-06-11 20:32:27 +0200 |
|---|---|---|
| committer | Hannes Domani <ssbssa@yahoo.de> | 2024-07-05 21:42:53 +0200 |
| commit | edd3f7be255e2e51ff7928cabb744af5d3d20018 (patch) | |
| tree | 9479b48a3fa522de5c632d675e37e8efb37b43d3 | |
| parent | 041fb9da7dc674bb335a0a26443a4b4339e995f9 (diff) | |
| download | gdb-edd3f7be255e2e51ff7928cabb744af5d3d20018.zip gdb-edd3f7be255e2e51ff7928cabb744af5d3d20018.tar.gz gdb-edd3f7be255e2e51ff7928cabb744af5d3d20018.tar.bz2 | |
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>
| -rw-r--r-- | gdb/gmp-utils.h | 4 | ||||
| -rw-r--r-- | 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 51e06ab..878ce1d 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 61c7eca..17f6b78 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" } |