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binutils-gdb/gdbsupport/enum-flags.h
Carlos Galvez 4a0b2cb721 Fix -Wenum-constexpr-conversion in enum-flags.h 
This fixes PR 31331:
https://sourceware.org/bugzilla/show_bug.cgi?id=31331

Currently, enum-flags.h is suppressing the warning
-Wenum-constexpr-conversion coming from recent versions of Clang.
This warning is intended to be made a compiler error
(non-downgradeable) in future versions of Clang:

https://github.com/llvm/llvm-project/issues/59036

The rationale is that casting a value of an integral type into an
enumeration is Undefined Behavior if the value does not fit in the
range of values of the enum:
https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1766

Undefined Behavior is not allowed in constant expressions, leading to
an ill-formed program.

In this case, in enum-flags.h, we are casting the value -1 to an enum
of a positive range only, which is UB as per the Standard and thus not
allowed in a constexpr context.

The purpose of doing this instead of using std::underlying_type is
because, for C-style enums, std::underlying_type typically returns
"unsigned int". However, when operating with it arithmetically, the
enum is promoted to *signed* int, which is what we want to avoid.

This patch solves this issue as follows:

* Use std::underlying_type and remove the custom enum_underlying_type.

* Ensure that operator~ is called always on an unsigned integer. We do
  this by casting the input enum into std::size_t, which can fit any
  unsigned integer. We have the guarantee that the cast is safe,
  because we have checked that the underlying type is unsigned. If the
  enum had negative values, the underlying type would be signed.

  This solves the issue with C-style enums, but also solves a hidden
  issue: enums with underlying type of std::uint8_t or std::uint16_t are
  *also* promoted to signed int. Now they are all explicitly casted
  to the largest unsigned int type and operator~ is safe to use.

* There is one more thing that needs fix. Currently, operator~ is
  implemented as follows:

  return (enum_type) ~underlying(e);

  After applying ~underlying(e), the result is a very large value,
  which we then cast to "enum_type". This cast is Undefined Behavior
  if the large value does not fit in the range of the enum. For
  C++ enums (scoped and/or with explicit underlying type), the range
  of the enum is the entire range of the underlying type, so the cast
  is safe. However, for C-style enums, the range is the smallest
  bit-field that can hold all the values of the enumeration. So the
  range is a lot smaller and casting a large value to the enum would
  invoke Undefined Behavior.

  To solve this problem, we create a new trait
  EnumHasFixedUnderlyingType, to ensure operator~ may only be called
  on C++-style enums. This behavior is roughly the same as what we
  had on trunk, but relying on different properties of the enums.

* Once this is implemented, the following tests fail to compile:

  CHECK_VALID (true,  int,  true ? EF () : EF2 ())

  This is because it expects the enums to be promoted to signed int,
  instead of unsigned int (which is the true underlying type).

  I propose to remove these tests altogether, because:

  - The comment nearby say they are not very important.
  - Comparing 2 enums of different type like that is strange, relies
    on integer promotions and thus hurts readability. As per comments
    in the related PR, we likely don't want this type of code in gdb
    code anyway, so there's no point in testing it.
  - Most importantly, this type of comparison will be ill-formed in
    C++26 for regular enums, so enum_flags does not need to emulate
    that.

Since this is the only place where the warning was suppressed, remove
also the corresponding macro in include/diagnostics.h.

The change has been tested by running the entire gdb test suite
(make check) and comparing the results (testsuite/gdb.sum) against
trunk. No noticeable differences have been observed.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31331
Tested-by: Keith Seitz <keiths@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>
2024-12-22 10:48:58 -07:00

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/* Copyright (C) 2015-2024 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 GDBSUPPORT_ENUM_FLAGS_H
#define GDBSUPPORT_ENUM_FLAGS_H
#include "traits.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
*/
/* Use this to mark an enum as flags enum. It defines FLAGS_TYPE as
enum_flags wrapper class for ENUM, and enables the global operator
overloads for ENUM. */
#define DEF_ENUM_FLAGS_TYPE(enum_type, flags_type) \
using flags_type = enum_flags<enum_type>; \
void is_enum_flags_enum_type (enum_type *)
/* To enable the global enum_flags operators for enum, declare an
"is_enum_flags_enum_type" overload that has exactly one parameter,
of type a pointer to that enum class. E.g.,:
void is_enum_flags_enum_type (enum some_flag *);
The function does not need to be defined, only declared.
DEF_ENUM_FLAGS_TYPE declares this.
A function declaration is preferred over a traits type, because the
former allows calling the DEF_ENUM_FLAGS_TYPE macro inside a
namespace to define the corresponding enum flags type in that
namespace. The compiler finds the corresponding
is_enum_flags_enum_type function via ADL. */
namespace enum_flags_detail
{
/* 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;
/* gdb::Requires trait helpers. */
template <typename enum_type>
using EnumIsUnsigned
= std::is_unsigned<typename std::underlying_type<enum_type>::type>;
/* Helper to detect whether an enum has a fixed underlying type. This can be
achieved by using a scoped enum (in which case the type is "int") or
an explicit underlying type. C-style enums that are unscoped or do not
have an explicit underlying type have an implementation-defined underlying
type.
https://timsong-cpp.github.io/cppwp/n4659/dcl.enum#5
We need this trait in order to ensure that operator~ below does NOT
operate on old-style enums. This is because we apply operator~ on
the value and then cast the result to the enum_type. This is however
Undefined Behavior if the result does not fit in the range of possible
values for the enum. For enums with fixed underlying type, the entire
range of the integer is available. However, for old-style enums, the range
is only the smallest bit-field that can hold all the values of the
enumeration, typically much smaller than the underlying integer:
https://timsong-cpp.github.io/cppwp/n4659/expr.static.cast#10
https://timsong-cpp.github.io/cppwp/n4659/dcl.enum#8
To implement this, we leverage the fact that, since C++17, enums with
fixed underlying type can be list-initialized from an integer:
https://timsong-cpp.github.io/cppwp/n4659/dcl.init.list#3.7
Old-style enums cannot be initialized like that, leading to ill-formed
code.
We then use this together with SFINAE to create the desired trait.
*/
template <typename enum_type, typename = void>
struct EnumHasFixedUnderlyingType : std::false_type
{
static_assert(std::is_enum<enum_type>::value);
};
/* Specialization that is active only if enum_type can be
list-initialized from an integer (0). Only enums with fixed
underlying type satisfy this property in C++17. */
template <typename enum_type>
struct EnumHasFixedUnderlyingType<enum_type, std::void_t<decltype(enum_type{0})>> : std::true_type
{
static_assert(std::is_enum<enum_type>::value);
};
template <typename enum_type>
using EnumIsSafeForBitwiseComplement = std::conjunction<
EnumIsUnsigned<enum_type>,
EnumHasFixedUnderlyingType<enum_type>
>;
template <typename enum_type>
using EnumIsUnsafeForBitwiseComplement = std::negation<EnumIsSafeForBitwiseComplement<enum_type>>;
}
template <typename E>
class enum_flags
{
public:
using enum_type = E;
using underlying_type = typename std::underlying_type<enum_type>::type;
/* For to_string. Maps one enumerator of E to a string. */
struct string_mapping
{
E flag;
const char *str;
};
/* Convenience for to_string implementations, to build a
string_mapping array. */
#define MAP_ENUM_FLAG(ENUM_FLAG) { ENUM_FLAG, #ENUM_FLAG }
public:
/* Allow default construction. */
constexpr enum_flags ()
: m_enum_value ((enum_type) 0)
{}
/* The default move/copy ctor/assignment do the right thing. */
/* If you get an error saying these two overloads are ambiguous,
then you tried to mix values of different enum types. */
constexpr enum_flags (enum_type e)
: m_enum_value (e)
{}
constexpr enum_flags (enum_flags_detail::zero_type *zero)
: m_enum_value ((enum_type) 0)
{}
enum_flags &operator&= (enum_flags e) &
{
m_enum_value = (enum_type) (m_enum_value & e.m_enum_value);
return *this;
}
enum_flags &operator|= (enum_flags e) &
{
m_enum_value = (enum_type) (m_enum_value | e.m_enum_value);
return *this;
}
enum_flags &operator^= (enum_flags e) &
{
m_enum_value = (enum_type) (m_enum_value ^ e.m_enum_value);
return *this;
}
/* Delete rval versions. */
void operator&= (enum_flags e) && = delete;
void operator|= (enum_flags e) && = delete;
void operator^= (enum_flags e) && = delete;
/* Like raw enums, allow conversion to the underlying type. */
constexpr operator underlying_type () const
{
return m_enum_value;
}
/* Get the underlying value as a raw enum. */
constexpr enum_type raw () const
{
return m_enum_value;
}
/* Binary operations involving some unrelated type (which would be a
bug) are implemented as non-members, and deleted. */
/* Convert this object to a std::string, using MAPPING as
enumerator-to-string mapping array. This is not meant to be
called directly. Instead, enum_flags specializations should have
their own to_string function wrapping this one, thus hiding the
mapping array from callers.
Note: this is defined outside the template class so it can use
the global operators for enum_type, which are only defined after
the template class. */
template<size_t N>
std::string to_string (const string_mapping (&mapping)[N]) const;
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;
};
template <typename E>
using is_enum_flags_enum_type_t
= decltype (is_enum_flags_enum_type (std::declval<E *> ()));
/* Global operator overloads. */
/* Generate binary operators. */
#define ENUM_FLAGS_GEN_BINOP(OPERATOR_OP, OP) \
\
/* Raw enum on both LHS/RHS. Returns raw enum type. */ \
template <typename enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_type \
OPERATOR_OP (enum_type e1, enum_type e2) \
{ \
using underlying = typename enum_flags<enum_type>::underlying_type; \
return (enum_type) (underlying (e1) OP underlying (e2)); \
} \
\
/* enum_flags on the LHS. */ \
template <typename enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_flags<enum_type> \
OPERATOR_OP (enum_flags<enum_type> e1, enum_type e2) \
{ return e1.raw () OP e2; } \
\
/* enum_flags on the RHS. */ \
template <typename enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_flags<enum_type> \
OPERATOR_OP (enum_type e1, enum_flags<enum_type> e2) \
{ return e1 OP e2.raw (); } \
\
/* enum_flags on both LHS/RHS. */ \
template <typename enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_flags<enum_type> \
OPERATOR_OP (enum_flags<enum_type> e1, enum_flags<enum_type> e2) \
{ return e1.raw () OP e2.raw (); } \
\
/* Delete cases involving unrelated types. */ \
\
template <typename enum_type, typename unrelated_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_flags<enum_type> \
OPERATOR_OP (enum_type e1, unrelated_type e2) = delete; \
\
template <typename enum_type, typename unrelated_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_flags<enum_type> \
OPERATOR_OP (unrelated_type e1, enum_type e2) = delete; \
\
template <typename enum_type, typename unrelated_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_flags<enum_type> \
OPERATOR_OP (enum_flags<enum_type> e1, unrelated_type e2) = delete; \
\
template <typename enum_type, typename unrelated_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_flags<enum_type> \
OPERATOR_OP (unrelated_type e1, enum_flags<enum_type> e2) = delete;
/* Generate non-member compound assignment operators. Only the raw
enum versions are defined here. The enum_flags versions are
defined as member functions, simply because it's less code that
way.
Note we delete operators that would allow e.g.,
"enum_type | 1" or "enum_type1 | enum_type2"
because that would allow a mistake like :
enum flags1 { F1_FLAGS1 = 1 };
enum flags2 { F2_FLAGS2 = 2 };
enum flags1 val;
switch (val) {
case F1_FLAGS1 | F2_FLAGS2:
...
If you really need to 'or' enumerators of different flag types,
cast to integer first.
*/
#define ENUM_FLAGS_GEN_COMPOUND_ASSIGN(OPERATOR_OP, OP) \
/* lval reference version. */ \
template <typename enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_type & \
OPERATOR_OP (enum_type &e1, enum_type e2) \
{ return e1 = e1 OP e2; } \
\
/* rval reference version. */ \
template <typename enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
void \
OPERATOR_OP (enum_type &&e1, enum_type e2) = delete; \
\
/* Delete compound assignment from unrelated types. */ \
\
template <typename enum_type, typename other_enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
constexpr enum_type & \
OPERATOR_OP (enum_type &e1, other_enum_type e2) = delete; \
\
template <typename enum_type, typename other_enum_type, \
typename = is_enum_flags_enum_type_t<enum_type>> \
void \
OPERATOR_OP (enum_type &&e1, other_enum_type e2) = delete;
ENUM_FLAGS_GEN_BINOP (operator|, |)
ENUM_FLAGS_GEN_BINOP (operator&, &)
ENUM_FLAGS_GEN_BINOP (operator^, ^)
ENUM_FLAGS_GEN_COMPOUND_ASSIGN (operator|=, |)
ENUM_FLAGS_GEN_COMPOUND_ASSIGN (operator&=, &)
ENUM_FLAGS_GEN_COMPOUND_ASSIGN (operator^=, ^)
/* Allow comparison with enum_flags, raw enum, and integers, only.
The latter case allows "== 0". As side effect, it allows comparing
with integer variables too, but that's not a common mistake to
make. It's important to disable comparison with unrelated types to
prevent accidentally comparing with unrelated enum values, which
are convertible to integer, and thus coupled with enum_flags
conversion to underlying type too, would trigger the built-in 'bool
operator==(unsigned, int)' operator. */
#define ENUM_FLAGS_GEN_COMP(OPERATOR_OP, OP) \
\
/* enum_flags OP enum_flags */ \
\
template <typename enum_type> \
constexpr bool \
OPERATOR_OP (enum_flags<enum_type> lhs, enum_flags<enum_type> rhs) \
{ return lhs.raw () OP rhs.raw (); } \
\
/* enum_flags OP other */ \
\
template <typename enum_type> \
constexpr bool \
OPERATOR_OP (enum_flags<enum_type> lhs, enum_type rhs) \
{ return lhs.raw () OP rhs; } \
\
template <typename enum_type> \
constexpr bool \
OPERATOR_OP (enum_flags<enum_type> lhs, int rhs) \
{ return lhs.raw () OP rhs; } \
\
template <typename enum_type, typename U> \
constexpr bool \
OPERATOR_OP (enum_flags<enum_type> lhs, U rhs) = delete; \
\
/* other OP enum_flags */ \
\
template <typename enum_type> \
constexpr bool \
OPERATOR_OP (enum_type lhs, enum_flags<enum_type> rhs) \
{ return lhs OP rhs.raw (); } \
\
template <typename enum_type> \
constexpr bool \
OPERATOR_OP (int lhs, enum_flags<enum_type> rhs) \
{ return lhs OP rhs.raw (); } \
\
template <typename enum_type, typename U> \
constexpr bool \
OPERATOR_OP (U lhs, enum_flags<enum_type> rhs) = delete;
ENUM_FLAGS_GEN_COMP (operator==, ==)
ENUM_FLAGS_GEN_COMP (operator!=, !=)
/* Unary operators for the raw flags enum. */
/* We require underlying type to be unsigned when using operator~ --
if it were not unsigned, undefined behavior could result. However,
asserting this in the class itself would require too many
unnecessary changes to usages of otherwise OK enum types. */
template <typename enum_type,
typename = is_enum_flags_enum_type_t<enum_type>,
typename
= gdb::Requires<enum_flags_detail::EnumIsSafeForBitwiseComplement<enum_type>>>
constexpr enum_type
operator~ (enum_type e)
{
using underlying = typename enum_flags<enum_type>::underlying_type;
/* Cast to ULONGEST first, to prevent integer promotions from enums
with fixed underlying type std::uint8_t or std::uint16_t to
signed int. This ensures we apply the bitwise complement on an
unsigned type. */
return (enum_type)(underlying) ~ULONGEST (e);
}
template <typename enum_type,
typename = is_enum_flags_enum_type_t<enum_type>,
typename = gdb::Requires<enum_flags_detail::EnumIsUnsafeForBitwiseComplement<enum_type>>>
constexpr void operator~ (enum_type e) = delete;
template <typename enum_type,
typename = is_enum_flags_enum_type_t<enum_type>,
typename
= gdb::Requires<enum_flags_detail::EnumIsSafeForBitwiseComplement<enum_type>>>
constexpr enum_flags<enum_type>
operator~ (enum_flags<enum_type> e)
{
using underlying = typename enum_flags<enum_type>::underlying_type;
/* Cast to ULONGEST first, to prevent integer promotions from enums
with fixed underlying type std::uint8_t or std::uint16_t to
signed int. This ensures we apply the bitwise complement on an
unsigned type. */
return (enum_type)(underlying) ~ULONGEST (e);
}
template <typename enum_type,
typename = is_enum_flags_enum_type_t<enum_type>,
typename = gdb::Requires<enum_flags_detail::EnumIsUnsafeForBitwiseComplement<enum_type>>>
constexpr void operator~ (enum_flags<enum_type> e) = delete;
/* Delete operator<< and operator>>. */
template <typename enum_type, typename any_type,
typename = is_enum_flags_enum_type_t<enum_type>>
void operator<< (const enum_type &, const any_type &) = delete;
template <typename enum_type, typename any_type,
typename = is_enum_flags_enum_type_t<enum_type>>
void operator<< (const enum_flags<enum_type> &, const any_type &) = delete;
template <typename enum_type, typename any_type,
typename = is_enum_flags_enum_type_t<enum_type>>
void operator>> (const enum_type &, const any_type &) = delete;
template <typename enum_type, typename any_type,
typename = is_enum_flags_enum_type_t<enum_type>>
void operator>> (const enum_flags<enum_type> &, const any_type &) = delete;
template<typename E>
template<size_t N>
std::string
enum_flags<E>::to_string (const string_mapping (&mapping)[N]) const
{
enum_type flags = raw ();
std::string res = hex_string (flags);
res += " [";
bool need_space = false;
for (const auto &entry : mapping)
{
if ((flags & entry.flag) != 0)
{
/* Work with an unsigned version of the underlying type,
because if enum_type's underlying type is signed, op~
won't be defined for it, and, bitwise operations on
signed types are implementation defined. */
using uns = typename std::make_unsigned<underlying_type>::type;
flags &= (enum_type) ~(uns) entry.flag;
if (need_space)
res += " ";
res += entry.str;
need_space = true;
}
}
/* If there were flags not included in the mapping, print them as
a hex number. */
if (flags != 0)
{
if (need_space)
res += " ";
res += hex_string (flags);
}
res += "]";
return res;
}
#endif /* GDBSUPPORT_ENUM_FLAGS_H */
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