From: Hannes Domani <ssbssa@yahoo.de>
Date: Tue, 11 Jun 2024 18:32:59 +0000 (+0200)
Subject: Fix too-large or negative right shift of negative numbers
X-Git-Tag: binutils-2_43~444
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=50f4e9c3c3e5115b346f361de1374837c990f14b;p=thirdparty%2Fbinutils-gdb.git

Fix too-large or negative right shift of negative numbers

As seen in these test failures:

print -1 >> -1
warning: right shift count is negative
$N = 0
(gdb) FAIL: gdb.base/bitshift.exp: lang=c: neg lhs/rhs: print -1 >> -1
print -4 >> -2
warning: right shift count is negative
$N = 0
(gdb) FAIL: gdb.base/bitshift.exp: lang=c: neg lhs/rhs: print -4 >> -2

Fixed by restoring the logic from before the switch to gmp.

Approved-By: Tom Tromey <tom@tromey.com>
---

diff --git a/gdb/testsuite/gdb.base/bitshift.exp b/gdb/testsuite/gdb.base/bitshift.exp
index 17f6b78fed2..d6e252102d9 100644
--- a/gdb/testsuite/gdb.base/bitshift.exp
+++ b/gdb/testsuite/gdb.base/bitshift.exp
@@ -348,6 +348,7 @@ proc test_shifts {} {
 	    test_shift $lang "print -3 >> 1" " = -2"
 	    test_shift $lang "print -8 >> 1" " = -4"
 	    test_shift $lang "print [make_int64 $lang -8] >> 1" " = -4"
+	    test_rshift_tl $lang "print -8 >> 100" " = -1"
 	}
 
 	# Make sure an unsigned 64-bit value with high bit set isn't
diff --git a/gdb/valarith.c b/gdb/valarith.c
index 7034fa6096b..6160b0e4438 100644
--- a/gdb/valarith.c
+++ b/gdb/valarith.c
@@ -1303,7 +1303,21 @@ scalar_binop (struct value *arg1, struct value *arg2, enum exp_opcode op)
 	  {
 	    unsigned long nbits;
 	    if (!check_valid_shift_count (op, result_type, type2, v2, nbits))
-	      v = 0;
+	      {
+		/* Pretend the too-large shift was decomposed in a
+		   number of smaller shifts.  An arithmetic signed
+		   right shift of a negative number always yields -1
+		   with such semantics.  This is the right thing to
+		   do for Go, and we might as well do it for
+		   languages where it is undefined.  Also, pretend a
+		   shift by a negative number was a shift by the
+		   negative number cast to unsigned, which is the
+		   same as shifting by a too-large number.  */
+		if (v1 < 0 && !result_type->is_unsigned ())
+		  v = -1;
+		else
+		  v = 0;
+	      }
 	    else
 	      v = v1 >> nbits;
 	  }