Fix fixed-point binary operation type handling

Testing showed that gdb was not correctly handling some fixed-point
binary operations correctly.

Addition and subtraction worked by casting the result to the type of
left hand operand.  So, "fixed+int" had a different type -- and
different value -- from "int+fixed".

Furthermore, for multiplication and division, it does not make sense
to first cast both sides to the fixed-point type.  For example, this
can prevent "f * 1" from yielding "f", if 1 is not in the domain of
"f".  Instead, this patch changes gdb to use the value.  (This is
somewhat different from Ada semantics, as those can yield a "universal
fixed point".)

This includes a new test case.  It is only run in "minimal" mode, as
the old-style fixed point works differently, and is obsolete, so I
have no plans to change it.

gdb/ChangeLog
2021-01-06  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_evaluate_subexp) <BINOP_ADD, BINOP_SUB>:
	Do not cast result.
	* valarith.c (fixed_point_binop): Handle multiplication
	and division specially.
	* valops.c (value_to_gdb_mpq): New function.
	(value_cast_to_fixed_point): Use it.

gdb/testsuite/ChangeLog
2021-01-06  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/fixed_points/pck.ads (Delta4): New constant.
	(FP4_Type): New type.
	(FP4_Var): New variable.
	* gdb.ada/fixed_points/fixed_points.adb: Update.
	* gdb.ada/fixed_points.exp: Add tests for binary operators.

1 files changed, 30 insertions, 16 deletions

diff --git a/gdb/valarith.c b/gdb/valarith.c
index f3cc500eb18..81d48aae82a 100644
--- a/gdb/valarith.c
+++ b/gdb/valarith.c
@@ -903,27 +903,41 @@ fixed_point_binop (struct value *arg1, struct value *arg2, enum exp_opcode op)
   struct gdbarch *gdbarch = get_type_arch (type1);
   struct value *val;
 
+  gdb_mpq v1, v2, res;
+
   gdb_assert (is_fixed_point_type (type1) || is_fixed_point_type (type2));
-  if (!is_fixed_point_type (type1))
+  if (op == BINOP_MUL || op == BINOP_DIV)
     {
-      arg1 = value_cast (type2, arg1);
-      type1 = type2;
+      v1 = value_to_gdb_mpq (arg1);
+      v2 = value_to_gdb_mpq (arg2);
+
+      /* The code below uses TYPE1 for the result type, so make sure
+	 it is set properly.  */
+      if (!is_fixed_point_type (type1))
+	type1 = type2;
     }
-  if (!is_fixed_point_type (type2))
+  else
     {
-      arg2 = value_cast (type1, arg2);
-      type2 = type1;
-    }
+      if (!is_fixed_point_type (type1))
+	{
+	  arg1 = value_cast (type2, arg1);
+	  type1 = type2;
+	}
+      if (!is_fixed_point_type (type2))
+	{
+	  arg2 = value_cast (type1, arg2);
+	  type2 = type1;
+	}
 
-  gdb_mpq v1, v2, res;
-  v1.read_fixed_point (gdb::make_array_view (value_contents (arg1),
-					     TYPE_LENGTH (type1)),
-		       type_byte_order (type1), type1->is_unsigned (),
-		       type1->fixed_point_scaling_factor ());
-  v2.read_fixed_point (gdb::make_array_view (value_contents (arg2),
-					     TYPE_LENGTH (type2)),
-		       type_byte_order (type2), type2->is_unsigned (),
-		       type2->fixed_point_scaling_factor ());
+      v1.read_fixed_point (gdb::make_array_view (value_contents (arg1),
+						 TYPE_LENGTH (type1)),
+			   type_byte_order (type1), type1->is_unsigned (),
+			   type1->fixed_point_scaling_factor ());
+      v2.read_fixed_point (gdb::make_array_view (value_contents (arg2),
+						 TYPE_LENGTH (type2)),
+			   type_byte_order (type2), type2->is_unsigned (),
+			   type2->fixed_point_scaling_factor ());
+    }
 
   auto fixed_point_to_value = [type1] (const gdb_mpq &fp)
     {