[gdb/exp] Fix UB in scalar_binop
When building gdb with -fsanitize=undefined, I run into:
...
$ gdb -q -batch -ex "p -(-0x7fffffffffffffff - 1)"
src/gdb/valarith.c:1385:10: runtime error: signed integer overflow: \
0 - -9223372036854775808 cannot be represented in type 'long int'
$1 = -9223372036854775808
...
Fix this by performing the substraction in scalar_binop using unsigned types.
Tested on x86_64-linux.
diff --git a/gdb/testsuite/gdb.base/arithmet.exp b/gdb/testsuite/gdb.base/arithmet.exp
index b6009a3..4905c2e 100644
--- a/gdb/testsuite/gdb.base/arithmet.exp
+++ b/gdb/testsuite/gdb.base/arithmet.exp
@@ -98,3 +98,5 @@
gdb_test "print x/(y*w)" "0"
gdb_test "print x-(y/w)" "9"
gdb_test "print (x+y)*w" "42"
+
+gdb_test "p /x -(-0x7fffffffffffffff - 1)" " = 0x8000000000000000"
diff --git a/gdb/valarith.c b/gdb/valarith.c
index 6210267..526cc02 100644
--- a/gdb/valarith.c
+++ b/gdb/valarith.c
@@ -1382,7 +1382,10 @@ scalar_binop (struct value *arg1, struct value *arg2, enum exp_opcode op)
break;
case BINOP_SUB:
- v = v1 - v2;
+ /* Avoid runtime error: signed integer overflow: \
+ 0 - -9223372036854775808 cannot be represented in type
+ 'long int'. */
+ v = (ULONGEST)v1 - (ULONGEST)v2;
break;
case BINOP_MUL:
