[binutils-gdb] Fix right shift of negative numbers

[binutils-gdb] Fix right shift of negative numbers

[Hannes Domani]

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"
 	}