From: Richard Sandiford <richard.sandiford@arm.com>
To: gcc-patches@gcc.gnu.org
Subject: [10/23] Tweak the way that is_a is implemented
Date: Fri, 13 Nov 2020 08:16:25 +0000 [thread overview]
Message-ID: <mpta6vl8yee.fsf@arm.com> (raw)
In-Reply-To: <mpth7ptad81.fsf@arm.com> (Richard Sandiford's message of "Fri, 13 Nov 2020 08:10:54 +0000")
At the moment, class hierarchies that use is_a are expected
to define specialisations like:
template <>
template <>
inline bool
is_a_helper <cgraph_node *>::test (symtab_node *p)
{
return p->type == SYMTAB_FUNCTION;
}
But this doesn't scale well to larger hierarchies, because it only
defines ::test for an argument that is exactly “symtab_node *”
(and not for example “const symtab_node *” or something that
comes between cgraph_node and symtab_node in the hierarchy).
For example:
struct A { int x; };
struct B : A {};
struct C : B {};
template <>
template <>
inline bool
is_a_helper <C *>::test (A *a)
{
return a->x == 1;
}
bool f(B *b) { return is_a<C *> (b); }
gives:
warning: inline function ‘static bool is_a_helper<T>::test(U*) [with U = B; T = C*]’ used but never defined
and:
bool f(const A *a) { return is_a<const C *> (a); }
gives:
warning: inline function ‘static bool is_a_helper<T>::test(U*) [with U = const A; T = const C*]’ used but never defined
This patch instead allows is_a to be implemented by specialising
is_a_helper as a whole, for example:
template<>
struct is_a_helper<C *> : static_is_a_helper<C *>
{
static inline bool test (const A *a) { return a->x == 1; }
};
It also adds a general specialisation of is_a_helper for const
pointers. Together, this makes both of the above examples work.
gcc/
* is-a.h (reinterpret_is_a_helper): New class.
(static_is_a_helper): Likewise.
(is_a_helper): Inherit from reinterpret_is_a_helper.
(is_a_helper<const T *>): New specialization.
---
gcc/is-a.h | 81 ++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 63 insertions(+), 18 deletions(-)
diff --git a/gcc/is-a.h b/gcc/is-a.h
index e84c3e4880c..26f53a5ba4a 100644
--- a/gcc/is-a.h
+++ b/gcc/is-a.h
@@ -116,9 +116,30 @@ 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,
+Method 1
+--------
+
+If DERIVED is derived from BASE, and if BASE contains enough information
+to determine whether an object is actually an instance of DERIVED,
+then you can make the above routines work for DERIVED by defining
+a specialization of is_a_helper such as:
+
+ template<>
+ struct is_a_helper<DERIVED *> : static_is_a_helper<DERIVED *>
+ {
+ static inline bool test (const BASE *p) { return ...; }
+ };
+
+This test function should return true if P is an instanced of DERIVED.
+This on its own is enough; the comments below for method 2 do not apply.
+
+Method 2
+--------
+
+Alternatively, if two types are connected in ways other than C++
+inheritance, each connection between them must be made by defining a
+specialization of the template member function 'test' of the template
+class 'is_a_helper'. For example,
template <>
template <>
@@ -145,15 +166,52 @@ when needed may result in a crash. For example,
#ifndef GCC_IS_A_H
#define GCC_IS_A_H
+/* A base class that specializations of is_a_helper can use if casting
+ U * to T is simply a reinterpret_cast. */
+
+template <typename T>
+struct reinterpret_is_a_helper
+{
+ template <typename U>
+ static inline T cast (U *p) { return reinterpret_cast <T> (p); }
+};
+
+/* A base class that specializations of is_a_helper can use if casting
+ U * to T is simply a static_cast. This is more type-safe than
+ reinterpret_is_a_helper. */
+
+template <typename T>
+struct static_is_a_helper
+{
+ template <typename U>
+ static inline T cast (U *p) { return static_cast <T> (p); }
+};
+
/* A generic type conversion internal helper class. */
template <typename T>
-struct is_a_helper
+struct is_a_helper : reinterpret_is_a_helper<T>
{
template <typename U>
static inline bool test (U *p);
+};
+
+/* Reuse the definition of is_a_helper<T *> to implement
+ is_a_helper<const T *>. */
+
+template <typename T>
+struct is_a_helper<const T *>
+{
template <typename U>
- static inline T cast (U *p);
+ static inline const T *cast (const U *p)
+ {
+ return is_a_helper<T *>::cast (const_cast <U *> (p));
+ }
+ template <typename U>
+ static inline bool test (const U *p)
+ {
+ return is_a_helper<T *>::test (p);
+ }
};
/* Note that we deliberately do not define the 'test' member template. Not
@@ -161,19 +219,6 @@ struct is_a_helper
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
--
2.17.1
next prev parent reply other threads:[~2020-11-13 8:16 UTC|newest]
Thread overview: 88+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-13 8:10 [00/23] Make fwprop use an on-the-side RTL SSA representation Richard Sandiford
2020-11-13 8:11 ` [01/23] vec: Silence clang warning Richard Sandiford
2020-11-25 19:58 ` Jeff Law
2020-11-13 8:12 ` [02/23] rtlanal: Remove noop_move_p REG_EQUAL condition Richard Sandiford
2020-11-25 20:00 ` Jeff Law
2020-11-13 8:12 ` [03/23] reginfo: Add a global_reg_set Richard Sandiford
2020-11-25 20:01 ` Jeff Law
2020-11-13 8:13 ` [04/23] Move iterator_range to a new iterator-utils.h file Richard Sandiford
2020-11-25 20:02 ` Jeff Law
2020-11-13 8:13 ` [05/23] Add more iterator utilities Richard Sandiford
2020-11-25 20:12 ` Jeff Law
2020-11-13 8:14 ` [06/23] Add an RAII class for managing obstacks Richard Sandiford
2020-11-25 20:15 ` Jeff Law
2020-11-13 8:14 ` [07/23] Add a class that multiplexes two pointer types Richard Sandiford
2020-11-25 20:23 ` Jeff Law
2020-11-26 16:15 ` Richard Sandiford
2020-11-30 1:28 ` Jeff Law
2020-11-25 23:33 ` Martin Sebor
2020-11-26 17:06 ` Richard Sandiford
2020-11-27 18:12 ` Richard Sandiford
2020-11-28 0:17 ` Martin Sebor
2020-12-17 0:17 ` Richard Sandiford
2020-12-17 14:21 ` Tom Tromey
2020-12-17 15:38 ` Richard Sandiford
2020-12-17 15:44 ` Nathan Sidwell
2021-01-04 15:32 ` Jeff Law
2020-11-13 8:15 ` [08/23] Add an alternative splay tree implementation Richard Sandiford
2020-12-02 20:36 ` Jeff Law
2020-12-17 0:29 ` Richard Sandiford
2021-01-04 15:27 ` Jeff Law
2021-01-01 8:25 ` Andreas Schwab
2021-01-04 14:53 ` Richard Sandiford
2021-01-04 15:02 ` Andreas Schwab
2021-01-04 15:42 ` Richard Sandiford
2021-01-05 12:13 ` Richard Biener
2020-11-13 8:15 ` [09/23] Add a cut-down version of std::span (array_slice) Richard Sandiford
2020-11-30 19:56 ` Jeff Law
2022-08-03 15:13 ` Martin Jambor
2022-08-03 15:31 ` Richard Sandiford
2022-08-10 16:03 ` Martin Jambor
2022-08-11 6:58 ` Richard Biener
2022-08-16 7:59 ` Richard Sandiford
2020-11-13 8:16 ` Richard Sandiford [this message]
2020-12-02 5:15 ` [10/23] Tweak the way that is_a is implemented Jeff Law
2020-11-13 8:16 ` [11/23] Split update_cfg_for_uncondjump out of combine Richard Sandiford
2020-11-30 6:14 ` Jeff Law
2020-11-13 8:17 ` [12/23] Export print-rtl.c:print_insn_with_notes Richard Sandiford
2020-11-25 20:24 ` Jeff Law
2020-11-13 8:18 ` [13/23] recog: Split out a register_asm_p function Richard Sandiford
2020-11-25 20:24 ` Jeff Law
2020-11-13 8:18 ` [14/23] simplify-rtx: Put simplify routines into a class Richard Sandiford
2020-11-30 19:54 ` Jeff Law
2020-11-13 8:19 ` [15/23] recog: Add a validate_change_xveclen function Richard Sandiford
2020-11-30 20:03 ` Jeff Law
2020-11-13 8:19 ` [16/23] recog: Add a way of temporarily undoing changes Richard Sandiford
2020-11-25 20:27 ` Jeff Law
2020-12-17 0:22 ` Richard Sandiford
2020-11-13 8:20 ` [17/23] recog: Add a class for propagating into insns Richard Sandiford
2020-12-03 22:32 ` Jeff Law
2020-11-13 8:20 ` [18/23] recog: Add an RAII class for undoing insn changes Richard Sandiford
2020-11-25 20:27 ` Jeff Law
2020-11-13 8:20 ` [19/23] rtlanal: Add some new helper classes Richard Sandiford
2020-12-13 17:30 ` Jeff Law
2020-12-14 16:37 ` Richard Sandiford
2020-12-14 20:02 ` Jeff Law
2020-11-13 8:21 ` [20/23] rtlanal: Add simple_regno_set Richard Sandiford
2020-11-25 20:31 ` Jeff Law
2020-12-17 0:47 ` Richard Sandiford
2021-01-04 15:28 ` Jeff Law
2020-11-13 8:22 ` [21/23] doc: Add documentation for rtl-ssa Richard Sandiford
2020-11-30 6:26 ` Jeff Law
2020-11-13 8:23 ` [PATCH 22/23] Add rtl-ssa Richard Sandiford
2020-12-16 3:31 ` Jeff Law
2020-12-17 0:33 ` Richard Sandiford
2020-12-19 20:01 ` Jeff Law
2020-11-13 8:24 ` [PATCH 23/23] fwprop: Rewrite to use RTL SSA Richard Sandiford
2020-12-16 3:52 ` Jeff Law
2020-12-17 0:34 ` Richard Sandiford
2020-11-25 19:58 ` [00/23] Make fwprop use an on-the-side RTL SSA representation Jeff Law
2020-11-26 16:03 ` Richard Sandiford
2020-11-27 15:56 ` Michael Matz
2020-11-27 16:31 ` Richard Sandiford
2020-11-30 21:13 ` Jeff Law
2020-12-01 0:03 ` Michael Matz
2020-12-01 10:15 ` Richard Sandiford
2020-12-02 0:25 ` Jeff Law
2020-11-30 6:45 ` Jeff Law
2020-11-30 14:12 ` Richard Sandiford
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