Introduce unop_sizeof_operation

This adds class unop_sizeof_operation, which implements UNOP_SIZEOF.

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

	* expop.h (class unop_sizeof_operation): New.
	* ax-gdb.c (unop_sizeof_operation::do_generate_ax): New method.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2021-03-08  Tom Tromey  <tom@tromey.com>
+
+	* expop.h (class unop_sizeof_operation): New.
+	* ax-gdb.c (unop_sizeof_operation::do_generate_ax): New method.
+
 2021-03-08  Tom Tromey  <tom@tromey.com>
 
 	* expop.h (class unop_addr_operation): New.

gdb/ax-gdb.c

@@ -2491,6 +2491,29 @@ comma_operation::do_generate_ax (struct expression *exp,
   /* It's the consumer's responsibility to trace the right operand.  */
 }
 
+void
+unop_sizeof_operation::do_generate_ax (struct expression *exp,
+				       struct agent_expr *ax,
+				       struct axs_value *value,
+				       struct type *cast_type)
+{
+  /* We don't care about the value of the operand expression; we only
+     care about its type.  However, in the current arrangement, the
+     only way to find an expression's type is to generate code for it.
+     So we generate code for the operand, and then throw it away,
+     replacing it with code that simply pushes its size.  */
+  int start = ax->len;
+
+  std::get<0> (m_storage)->generate_ax (exp, ax, value);
+
+  /* Throw away the code we just generated.  */
+  ax->len = start;
+
+  ax_const_l (ax, TYPE_LENGTH (value->type));
+  value->kind = axs_rvalue;
+  value->type = builtin_type (ax->gdbarch)->builtin_int;
+}
+
 }
 
 /* This handles the middle-to-right-side of code generation for binary

gdb/expop.h

@@ -1582,6 +1582,35 @@ protected:
   }
 };
 
+/* Implement 'sizeof'.  */
+class unop_sizeof_operation
+  : public maybe_constant_operation<operation_up>
+{
+public:
+
+  using maybe_constant_operation::maybe_constant_operation;
+
+  value *evaluate (struct type *expect_type,
+		   struct expression *exp,
+		   enum noside noside) override
+  {
+    if (noside == EVAL_SKIP)
+      return eval_skip_value (exp);
+    return std::get<0> (m_storage)->evaluate_for_sizeof (exp, noside);
+  }
+
+  enum exp_opcode opcode () const override
+  { return UNOP_SIZEOF; }
+
+protected:
+
+  void do_generate_ax (struct expression *exp,
+		       struct agent_expr *ax,
+		       struct axs_value *value,
+		       struct type *cast_type)
+    override;
+};
+
 } /* namespace expr */
 
 #endif /* EXPOP_H */