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authorBen Cheng <bccheng@google.com>2014-03-25 22:37:19 -0700
committerBen Cheng <bccheng@google.com>2014-03-25 22:37:19 -0700
commit1bc5aee63eb72b341f506ad058502cd0361f0d10 (patch)
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Initial checkin of GCC 4.9.0 from trunk (r208799).
Change-Id: I48a3c08bb98542aa215912a75f03c0890e497dba
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+/* Dynamic testing for abstract is-a relationships.
+ Copyright (C) 2012-2014 Free Software Foundation, Inc.
+ Contributed by Lawrence Crowl.
+
+This file is part of GCC.
+
+GCC 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, or (at your option) any later
+version.
+
+GCC 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 GCC; see the file COPYING3. If not see
+<http://www.gnu.org/licenses/>. */
+
+
+/* This header generic type query and conversion functions.
+
+
+USING THE GENERIC TYPE FACILITY
+
+
+The user functions are:
+
+bool is_a <TYPE> (pointer)
+
+ Tests whether the pointer actually points to a more derived TYPE.
+
+ Suppose you have a symtab_node *ptr, AKA symtab_node *ptr. You can test
+ whether it points to a 'derived' cgraph_node as follows.
+
+ if (is_a <cgraph_node> (ptr))
+ ....
+
+
+TYPE *as_a <TYPE> (pointer)
+
+ Converts pointer to a TYPE*.
+
+ You can just assume that it is such a node.
+
+ do_something_with (as_a <cgraph_node> *ptr);
+
+TYPE *dyn_cast <TYPE> (pointer)
+
+ Converts pointer to TYPE* if and only if "is_a <TYPE> pointer". Otherwise,
+ returns NULL. This function is essentially a checked down cast.
+
+ This functions reduce compile time and increase type safety when treating a
+ generic item as a more specific item.
+
+ You can test and obtain a pointer to the 'derived' type in one indivisible
+ operation.
+
+ if (cgraph_node *cptr = dyn_cast <cgraph_node> (ptr))
+ ....
+
+ As an example, the code change is from
+
+ if (symtab_function_p (node))
+ {
+ struct cgraph_node *cnode = cgraph (node);
+ ....
+ }
+
+ to
+
+ if (cgraph_node *cnode = dyn_cast <cgraph_node> (node))
+ {
+ ....
+ }
+
+ The necessary conditional test defines a variable that holds a known good
+ pointer to the specific item and avoids subsequent conversion calls and
+ the assertion checks that may come with them.
+
+ When, the property test is embedded within a larger condition, the
+ variable declaration gets pulled out of the condition. (This approach
+ leaves some room for using the variable inappropriately.)
+
+ if (symtab_variable_p (node) && varpool (node)->finalized)
+ varpool_analyze_node (varpool (node));
+
+ becomes
+
+ varpool_node *vnode = dyn_cast <varpool_node> (node);
+ if (vnode && vnode->finalized)
+ varpool_analyze_node (vnode);
+
+ Note that we have converted two sets of assertions in the calls to varpool
+ into safe and efficient use of a variable.
+
+
+If you use these functions and get a 'inline function not defined' or a
+'missing symbol' error message for 'is_a_helper<....>::test', it means that
+the connection between the types has not been made. See below.
+
+
+EXTENDING THE GENERIC TYPE FACILITY
+
+Each connection between types must be made by defining a specialization of the
+template member function 'test' of the template class 'is_a_helper'. For
+example,
+
+ template <>
+ template <>
+ inline bool
+ is_a_helper <cgraph_node>::test (symtab_node *p)
+ {
+ return p->type == SYMTAB_FUNCTION;
+ }
+
+If a simple reinterpret_cast between the pointer types is incorrect, then you
+must also specialize the template member function 'cast'. Failure to do so
+when needed may result in a crash. For example,
+
+ template <>
+ template <>
+ inline bool
+ is_a_helper <cgraph_node>::cast (symtab_node *p)
+ {
+ return &p->x_function;
+ }
+
+*/
+
+#ifndef GCC_IS_A_H
+#define GCC_IS_A_H
+
+/* A generic type conversion internal helper class. */
+
+template <typename T>
+struct is_a_helper
+{
+ template <typename U>
+ static inline bool test (U *p);
+ template <typename U>
+ static inline T *cast (U *p);
+};
+
+/* Note that we deliberately do not define the 'test' member template. Not
+ doing so will result in a build-time error for type relationships that have
+ not been defined, rather than a run-time error. See the discussion above
+ for when to define this member. */
+
+/* This is the generic implementation for casting from one type to another.
+ Do not use this routine directly; it is an internal function. See the
+ discussion above for when to define this member. */
+
+template <typename T>
+template <typename U>
+inline T *
+is_a_helper <T>::cast (U *p)
+{
+ return reinterpret_cast <T *> (p);
+}
+
+
+/* The public interface. */
+
+/* A generic test for a type relationship. See the discussion above for when
+ to use this function. The question answered is "Is type T a derived type of
+ type U?". */
+
+template <typename T, typename U>
+inline bool
+is_a (U *p)
+{
+ return is_a_helper<T>::test (p);
+}
+
+/* A generic conversion from a base type U to a derived type T. See the
+ discussion above for when to use this function. */
+
+template <typename T, typename U>
+inline T *
+as_a (U *p)
+{
+ gcc_checking_assert (is_a <T> (p));
+ return is_a_helper <T>::cast (p);
+}
+
+/* A generic checked conversion from a base type U to a derived type T. See
+ the discussion above for when to use this function. */
+
+template <typename T, typename U>
+inline T *
+dyn_cast (U *p)
+{
+ if (is_a <T> (p))
+ return is_a_helper <T>::cast (p);
+ else
+ return static_cast <T *> (0);
+}
+
+#endif /* GCC_IS_A_H */