Introduce sub_operation

This adds class sub_operation, which implements BINOP_SUB.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* expop.h (class sub_operation): New.
	* eval.c (eval_op_sub): No longer static.  Remove "op" parameter.
	(evaluate_subexp_standard): Update.

gdb/expop.h

@@ -92,6 +92,10 @@ extern struct value *eval_op_add (struct type *expect_type,
 				  struct expression *exp,
 				  enum noside noside,
 				  struct value *arg1, struct value *arg2);
+extern struct value *eval_op_sub (struct type *expect_type,
+				  struct expression *exp,
+				  enum noside noside,
+				  struct value *arg1, struct value *arg2);
 
 namespace expr
 {
@@ -1001,6 +1005,42 @@ protected:
   }
 };
 
+class sub_operation
+  : public maybe_constant_operation<operation_up, operation_up>
+{
+public:
+
+  using maybe_constant_operation::maybe_constant_operation;
+
+  value *evaluate (struct type *expect_type,
+		   struct expression *exp,
+		   enum noside noside) override
+  {
+    value *lhs
+      = std::get<0> (m_storage)->evaluate_with_coercion (exp, noside);
+    value *rhs
+      = std::get<1> (m_storage)->evaluate_with_coercion (exp, noside);
+    return eval_op_sub (expect_type, exp, noside, lhs, rhs);
+  }
+
+  enum exp_opcode opcode () const override
+  { return BINOP_SUB; }
+
+protected:
+
+  void do_generate_ax (struct expression *exp,
+		       struct agent_expr *ax,
+		       struct axs_value *value,
+		       struct type *cast_type)
+    override
+  {
+    gen_expr_binop (exp, BINOP_SUB,
+		    std::get<0> (this->m_storage).get (),
+		    std::get<1> (this->m_storage).get (),
+		    ax, value);
+  }
+};
+
 } /* namespace expr */
 
 #endif /* EXPOP_H */