[PATCHv4 5/6] gdbsupport: add checked_static_cast

[Andrew Burgess <aburgess@redhat.com>]

Pedro,

This patch is based on the idea you put forward in your feedback.  In
terms of naming I went with gdb::checked_static_cast in the end as I
feel the new function is really replacing static_cast.  The use of
dynamic_cast is really an implementation detail.

Let me know what you think.

Thanks,
Andrew

---

This commit was inspired by these mailing list posts:

  https://sourceware.org/pipermail/gdb-patches/2022-June/190323.html
  https://sourceware.org/pipermail/gdb-patches/2022-April/188098.html

The idea is to add a new function gdb::checked_static_cast, which can,
in some cases, be used as a drop-in replacement for static_cast.  And
so, if I previously wrote this:

  BaseClass *base = get_base_class_pointer ();
  DerivedClass *derived = static_cast<DerivedClass *> (base);

I can now write:

  BaseClass *base = get_base_class_pointer ();
  DerivedClass *derived = gdb::checked_static_cast<DerivedClass *> (base);

The requirement is that BaseClass and DerivedClass must be
polymorphic.

The benefit of making this change is that, when GDB is built in
developer mode, a run-time check will be made to ensure that `base`
really is of type DerivedClass before the cast is performed.  If
`base` is not of type DerivedClass then GDB will assert.

In a non-developer build gdb::checked_static_cast is equivalent to a
static_cast, and there should be no performance difference.

This commit adds the support function, but does not make use of this
function, a use will be added in the next commit.

Co-Authored-By: Pedro Alves <pedro@palves.net>
Co-Authored-By: Tom Tromey <tom@tromey.com>
---
 gdbsupport/gdb-checked-static-cast.h | 68 ++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 gdbsupport/gdb-checked-static-cast.h

diff --git a/gdbsupport/gdb-checked-static-cast.h b/gdbsupport/gdb-checked-static-cast.h
new file mode 100644
index 00000000000..62378e458f8
--- /dev/null
+++ b/gdbsupport/gdb-checked-static-cast.h
@@ -0,0 +1,68 @@
+/* Copyright (C) 2022 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef COMMON_GDB_CHECKED_DYNAMIC_CAST_H
+#define COMMON_GDB_CHECKED_DYNAMIC_CAST_H
+
+#include "gdbsupport/traits.h"
+
+namespace gdb
+{
+
+/* This function can be used in place of static_cast when casting between
+   pointers of polymorphic types.  The benefit of using this call is that,
+   when compiling in developer mode, dynamic_cast will be used to validate
+   the cast.  This use of dynamic_cast is why this function will only
+   work for polymorphic types.
+
+   In non-developer (i.e. production) builds, the dynamic_cast is replaced
+   with a static_cast which is usually significantly faster.  */
+
+template<typename T, typename V>
+T
+checked_static_cast (V *v)
+{
+  /* We only support casting to pointer types.  */
+  static_assert (std::is_pointer<T>::value, "target must be a pointer type");
+
+  /* Check for polymorphic types explicitly in case we're in release mode.  */
+  static_assert (std::is_polymorphic<V>::value, "types must be polymorphic");
+
+  /* Figure out the type that T points to.  */
+  using T_no_P = typename std::remove_pointer<T>::type;
+
+  /* In developer mode this cast uses dynamic_cast to confirm at run-time
+     that the cast from V* to T is valid.  However, we can catch some
+     mistakes at compile time, this assert prevents anything other than
+     downcasts, or casts to same type.  */
+  static_assert (std::is_base_of<V, T_no_P>::value
+                 || std::is_base_of<T_no_P, V>::value,
+                 "types must be related");
+
+#ifdef DEVELOPMENT
+  T result = dynamic_cast<T> (v);
+  gdb_assert (result != nullptr);
+#else
+  T result = static_cast<T> (v);
+#endif
+
+  return result;
+}
+
+}
+
+#endif /* COMMON_GDB_CHECKED_DYNAMIC_CAST_H */
-- 
2.25.4