Type-safe wrapper for enum flags

This patch fixes C++ build errors like this:

/home/pedro/gdb/mygit/cxx-convertion/src/gdb/linux-tdep.c:1126:35: error: invalid conversion from ‘int’ to ‘filterflags’ [-fpermissive]
       | COREFILTER_HUGETLB_PRIVATE);
                                   ^

This is a case of enums used as bit flags.  Unlike "regular" enums,
these values are supposed to be or'ed together.  However, in C++, the
type of "(ENUM1 | ENUM2)" is int, and you then can't assign an int to
an enum variable without a cast.  That means that this:

  enum foo_flags flags = 0;

  if (...)
    flags |= FOO_FLAG1;
  if (...)
    flags |= FOO_FLAG2;

... would have to be written as:

  enum foo_flags flags = (enum foo_flags) 0;

  if (...)
    flags = (enum foo_flags) (flags | FOO_FLAG1);
  if (...)
    flags = (enum foo_flags) (flags | FOO_FLAG2);

which is ... ugly.  Alternatively, we'd have to use an int for the
variable's type, which isn't ideal either.

This patch instead adds an "enum flags" class.  "enum flags" are
exactly the enums where the values are bits that are meant to be ORed
together.

This allows writing code like the below, while with raw enums this
would fail to compile without casts to enum type at the assignments to
'f':

  enum some_flag
  {
     flag_val1 = 1 << 1,
     flag_val2 = 1 << 2,
     flag_val3 = 1 << 3,
     flag_val4 = 1 << 4,
  };
  DEF_ENUM_FLAGS_TYPE(enum some_flag, some_flags)

  some_flags f = flag_val1 | flag_val2;
  f |= flag_val3;

It's also possible to assign literal zero to an enum flags variable
(meaning, no flags), dispensing either adding an awkward explicit "no
value" value to the enumeration or the cast to assignments from 0.
For example:

  some_flags f = 0;
  f |= flag_val3 | flag_val4;

Note that literal integers other than zero do fail to compile:

  some_flags f = 1; // error

C is still supported -- DEF_ENUM_FLAGS_TYPE is just a typedef in that
case.

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* btrace.h: Include common/enum-flags.h.
	(btrace_insn_flags): Define.
	(struct btrace_insn) <flags>: Change type.
	(btrace_function_flags): Define.
	(struct btrace_function) <flags>: Change type.
	(btrace_thread_flags): Define.
	(struct btrace_thread_info) <flags>: Change type.
	* c-exp.y (token_flags): Rename to ...
	(token_flag): ... this.
	(token_flags): Define.
	(struct token) <flags>: Change type.
	* common/enum-flags.h: New file.
	* compile/compile-c-types.c (convert_qualified): Change type of
	'quals' local.
	* compile/compile-internal.h: Include "common/enum-flags.h".
	(gcc_qualifiers_flags): Define.
	* completer.c (enum reg_completer_targets): Rename to ...
	(enum reg_completer_target): ... this.
	(reg_completer_targets): Define.
	(reg_or_group_completer_1): Change type of 'targets' parameter.
	* disasm.c (do_mixed_source_and_assembly_deprecated): Change type
	of 'psl_flags' local.
	(do_mixed_source_and_assembly): Change type of 'psl_flags' local.
	* infrun.c: Include "common/enum-flags.h".
	(enum step_over_what): Rename to ...
	(enum step_over_what_flag): ... this.
	(step_over_what): Change type.
	(start_step_over): Change type of 'step_what' local.
	(thread_still_needs_step_over): Now returns a step_over_what.
	Adjust.
	(keep_going_pass_signal): Change type of 'step_what' local.
	* linux-tdep.c: Include "common/enum-flags.h".
	(enum filterflags): Rename to ...
	(enum filter_flag): ... this.
	(filter_flags): Define.
	(dump_mapping_p): Change type of 'filterflags' parameter.
	(linux_find_memory_regions_full): Change type of 'filterflags'
	local.
	(linux_find_memory_regions_full): Pass the address of an unsigned
	int to sscanf instead of the address of an enum.
	* record-btrace.c (btrace_print_lines): Change type of local
	'psl_flags'.
	(btrace_call_history): Replace 'flags' parameter
	with 'int_flags' parameter.  Adjust.
	(record_btrace_call_history, record_btrace_call_history_range)
	(record_btrace_call_history_from): Rename 'flags' parameter to
	'int_flags'.  Use record_print_flags.
	* record.h: Include "common/enum-flags.h".
	(record_print_flags): Define.
	* source.c: Include "common/enum-flags.h".
	(print_source_lines_base, print_source_lines): Change type of
	flags parameter.
	* symtab.h: Include "common/enum-flags.h".
	(enum print_source_lines_flags): Rename to ...
	(enum print_source_lines_flag): ... this.
	(print_source_lines_flags): Define.
	(print_source_lines): Change prototype.

1 files changed, 224 insertions, 0 deletions

diff --git a/gdb/common/enum-flags.h b/gdb/common/enum-flags.h
new file mode 100644
index 0000000..d7046e0
--- /dev/null
+++ b/gdb/common/enum-flags.h
@@ -0,0 +1,224 @@
+/* Copyright (C) 2015 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_ENUM_FLAGS_H
+#define COMMON_ENUM_FLAGS_H
+
+/* Type-safe wrapper for enum flags.  enum flags are enums where the
+   values are bits that are meant to be ORed together.
+
+   This allows writing code like the below, while with raw enums this
+   would fail to compile without casts to enum type at the assignments
+   to 'f':
+
+    enum some_flag
+    {
+       flag_val1 = 1 << 1,
+       flag_val2 = 1 << 2,
+       flag_val3 = 1 << 3,
+       flag_val4 = 1 << 4,
+    };
+    DEF_ENUM_FLAGS_TYPE(enum some_flag, some_flags)
+
+    some_flags f = flag_val1 | flag_val2;
+    f |= flag_val3;
+
+   It's also possible to assign literal zero to an enum flags variable
+   (meaning, no flags), dispensing adding an awkward explicit "no
+   value" value to the enumeration.  For example:
+
+    some_flags f = 0;
+    f |= flag_val3 | flag_val4;
+
+   Note that literal integers other than zero fail to compile:
+
+    some_flags f = 1; // error
+*/
+
+#ifdef __cplusplus
+
+/* Traits type used to prevent the global operator overloads from
+   instantiating for non-flag enums.  */
+template<typename T> struct enum_flags_type {};
+
+/* Use this to mark an enum as flags enum.  It defines FLAGS as
+   enum_flags wrapper class for ENUM, and enables the global operator
+   overloads for ENUM.  */
+#define DEF_ENUM_FLAGS_TYPE(enum_type, flags_type)	\
+  typedef enum_flags<enum_type> flags_type;		\
+  template<>						\
+  struct enum_flags_type<enum_type>			\
+  {							\
+    typedef enum_flags<enum_type> type;			\
+  }
+
+/* Until we can rely on std::underlying type being universally
+   available (C++11), roll our own for enums.  */
+template<int size, bool sign> class integer_for_size { typedef void type; };
+template<> struct integer_for_size<1, 0> { typedef uint8_t type; };
+template<> struct integer_for_size<2, 0> { typedef uint16_t type; };
+template<> struct integer_for_size<4, 0> { typedef uint32_t type; };
+template<> struct integer_for_size<8, 0> { typedef uint64_t type; };
+template<> struct integer_for_size<1, 1> { typedef int8_t type; };
+template<> struct integer_for_size<2, 1> { typedef int16_t type; };
+template<> struct integer_for_size<4, 1> { typedef int32_t type; };
+template<> struct integer_for_size<8, 1> { typedef int64_t type; };
+
+template<typename T>
+struct enum_underlying_type
+{
+  typedef typename
+    integer_for_size<sizeof (T), static_cast<bool>(T (-1) < T (0))>::type
+    type;
+};
+
+template <typename E>
+class enum_flags
+{
+public:
+  typedef E enum_type;
+  typedef typename enum_underlying_type<enum_type>::type underlying_type;
+
+private:
+  /* Private type used to support initializing flag types with zero:
+
+       foo_flags f = 0;
+
+     but not other integers:
+
+       foo_flags f = 1;
+
+     The way this works is that we define an implicit constructor that
+     takes a pointer to this private type.  Since nothing can
+     instantiate an object of this type, the only possible pointer to
+     pass to the constructor is the NULL pointer, or, zero.  */
+  struct zero_type;
+
+  underlying_type
+  underlying_value () const
+  {
+    return m_enum_value;
+  }
+
+public:
+  /* Allow default construction, just like raw enums.  */
+  enum_flags ()
+  {}
+
+  enum_flags (const enum_flags &other)
+    : m_enum_value (other.m_enum_value)
+  {}
+
+  enum_flags &operator= (const enum_flags &other)
+  {
+    m_enum_value = other.m_enum_value;
+    return *this;
+  }
+
+  /* If you get an error saying these two overloads are ambiguous,
+     then you tried to mix values of different enum types.  */
+  enum_flags (enum_type e)
+    : m_enum_value (e)
+  {}
+  enum_flags (struct enum_flags::zero_type *zero)
+    : m_enum_value ((enum_type) 0)
+  {}
+
+  enum_flags &operator&= (enum_type e)
+  {
+    m_enum_value = (enum_type) (underlying_value () & e);
+    return *this;
+  }
+  enum_flags &operator|= (enum_type e)
+  {
+    m_enum_value = (enum_type) (underlying_value () | e);
+    return *this;
+  }
+  enum_flags &operator^= (enum_type e)
+  {
+    m_enum_value = (enum_type) (underlying_value () ^ e);
+    return *this;
+  }
+
+  operator enum_type () const
+  {
+    return m_enum_value;
+  }
+
+  enum_flags operator& (enum_type e) const
+  {
+    return (enum_type) (underlying_value () & e);
+  }
+  enum_flags operator| (enum_type e) const
+  {
+    return (enum_type) (underlying_value () | e);
+  }
+  enum_flags operator^ (enum_type e) const
+  {
+    return (enum_type) (underlying_value () ^ e);
+  }
+  enum_flags operator~ () const
+  {
+    return (enum_type) ~underlying_value ();
+  }
+
+private:
+  /* Stored as enum_type because GDB knows to print the bit flags
+     neatly if the enum values look like bit flags.  */
+  enum_type m_enum_value;
+};
+
+/* Global operator overloads.  */
+
+template <typename enum_type>
+typename enum_flags_type<enum_type>::type
+operator& (enum_type e1, enum_type e2)
+{
+  return enum_flags<enum_type> (e1) & e2;
+}
+
+template <typename enum_type>
+typename enum_flags_type<enum_type>::type
+operator| (enum_type e1, enum_type e2)
+{
+  return enum_flags<enum_type> (e1) | e2;
+}
+
+template <typename enum_type>
+typename enum_flags_type<enum_type>::type
+operator^ (enum_type e1, enum_type e2)
+{
+  return enum_flags<enum_type> (e1) ^ e2;
+}
+
+template <typename enum_type>
+typename enum_flags_type<enum_type>::type
+operator~ (enum_type e)
+{
+  return ~enum_flags<enum_type> (e);
+}
+
+#else /* __cplusplus */
+
+/* In C, the flags type is just a typedef for the enum type.  */
+
+#define DEF_ENUM_FLAGS_TYPE(enum_type, flags_type) \
+  typedef enum_type flags_type
+
+#endif /* __cplusplus */
+
+#endif /* COMMON_ENUM_FLAGS_H */