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 --git a/gdb/gmp-utils.h b/gdb/gmp-utils.h
index 51e06abc0504dfd2c5503dafb12bea8edd92f4b7..878ce1da43a80e60d23a11312a56d2d063685e33 100644 (file)
--- 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 61c7eca2747392f9cc65ab12d2050b8ad96349fe..17f6b78fed20e44bc9c26d180d323d45645550ce 100644 (file)
--- 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"
        }