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c053b65441
Commit ecfb656c37 ("dwarf2read.c: Make dir_index and
file_name_index strong typedefs") added a use of gdb::optional that
triggers bogus -Wmaybe-uninitialized warnings:
GCC trunk is complaining like this:
../../binutils-gdb/gdb/dwarf2read.c: In function void read_formatted_entries(bfd*, const gdb_byte**, line_header*, const comp_unit_head*, void (*)(line_header*, const char*, dir_index, unsigned int, unsigned int)):
../../binutils-gdb/gdb/dwarf2read.c:17779:65: error: fe.file_entry::length may be used uninitialized in this function [-Werror=maybe-uninitialized]
callback (lh, fe.name, fe.d_index, fe.mod_time, fe.length);
^
../../binutils-gdb/gdb/dwarf2read.c:17779:65: error: *((void*)& fe +8) may be used uninitialized in this function [-Werror=maybe-uninitialized]
../../binutils-gdb/gdb/dwarf2read.c:17779:65: error: fe.file_entry::mod_time may be used uninitialized in this function [-Werror=maybe-uninitialized]
../../binutils-gdb/gdb/dwarf2read.c:17779:65: error: fe.file_entry::name may be used uninitialized in this function [-Werror=maybe-uninitialized]
While some older GCCs are complaining like this:
../../binutils-gdb/gdb/dwarf2read.c: In function void read_formatted_entries(bfd*, const gdb_byte**, line_header*, const comp_unit_head*, void (*)(line_header*, const char*, dir_index, unsigned int, unsigned int)):
../../binutils-gdb/gdb/dwarf2read.c:17779:65: error: uint may be used uninitialized in this function [-Werror=maybe-uninitialized]
callback (lh, fe.name, fe.d_index, fe.mod_time, fe.length);
Looking around the web, I see that boost's optional implementation
triggers this kind of issue often too. See:
http://www.boost.org/doc/libs/1_63_0/libs/optional/doc/html/boost_optional/tutorial/gotchas/false_positive_with__wmaybe_uninitialized.html
I noticed that replacing the gdb::optional uses with real C++17
std::optional uses against GCC 7/trunk makes the warnings go away.
Looking at the implementation, AFAICS, libstdc++ always initializes
its "empty" union payload member (_M_empty, which is defined as an
empty class, like ours). I.e., all payload types have this ctor:
struct _Optional_payload.....
{
constexpr _Optional_payload()
: _M_empty() {}
The constexpr makes a diference too. Without it, GCC7 still warns.
So I'm applying the same treatment to our gdb::optional.
gdb/ChangeLog:
2017-04-05 Pedro Alves <palves@redhat.com>
* common/gdb_optional.h (optional::optional): Make constexpr and
initialize m_dummy.
118 lines
2.9 KiB
C++
118 lines
2.9 KiB
C++
/* An optional object.
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Copyright (C) 2017 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef GDB_OPTIONAL_H
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#define GDB_OPTIONAL_H
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namespace gdb
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{
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/* This class attempts to be a compatible subset of std::optional,
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which is slated to be available in C++17. This class optionally
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holds an object of some type -- by default it is constructed not
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holding an object, but later the object can be "emplaced". This is
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similar to using std::unique_ptr, but in-object allocation is
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guaranteed. */
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template<typename T>
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class optional
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{
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public:
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constexpr optional ()
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: m_dummy (),
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m_instantiated (false)
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{
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}
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~optional ()
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{
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if (m_instantiated)
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destroy ();
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}
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/* These aren't deleted in std::optional, but it was simpler to
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delete them here, because currently the users of this class don't
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need them, and making them depend on the definition of T is
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somewhat complicated. */
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optional (const optional &other) = delete;
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optional<T> &operator= (const optional &other) = delete;
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template<typename... Args>
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void emplace (Args &&... args)
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{
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if (m_instantiated)
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destroy ();
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new (&m_item) T (std::forward<Args>(args)...);
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m_instantiated = true;
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}
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/* Observers. */
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constexpr const T *operator-> () const
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{ return std::addressof (this->get ()); }
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T *operator-> ()
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{ return std::addressof (this->get ()); }
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constexpr const T &operator* () const &
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{ return this->get (); }
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T &operator* () &
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{ return this->get (); }
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T &&operator* () &&
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{ return std::move (this->get ()); }
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constexpr const T &&operator* () const &&
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{ return std::move (this->get ()); }
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constexpr explicit operator bool () const noexcept
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{ return m_instantiated; }
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constexpr bool has_value () const noexcept
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{ return m_instantiated; }
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private:
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/* Destroy the object. */
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void destroy ()
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{
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gdb_assert (m_instantiated);
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m_instantiated = false;
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m_item.~T ();
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}
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/* The get operations have m_instantiated as a precondition. */
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T &get () noexcept { return m_item; }
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constexpr const T &get () const noexcept { return m_item; }
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/* The object. */
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union
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{
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struct { } m_dummy;
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T m_item;
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};
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/* True if the object was ever emplaced. */
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bool m_instantiated;
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};
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}
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#endif /* GDB_OPTIONAL_H */
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