[binutils-gdb] Initialize gdb::optional empty payload to quiet false -Wmaybe-uninitialized warnings

[Pedro Alves <palves at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c053b65441eb70ac78a514fabc3431b857a30d2e

commit c053b65441eb70ac78a514fabc3431b857a30d2e
Author: Pedro Alves <palves@redhat.com>
Date:   Tue Apr 4 23:49:27 2017 +0100

    Initialize gdb::optional empty payload to quiet false -Wmaybe-uninitialized warnings
    
    Commit ecfb656c37b982 ("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.

Diff:
---
 gdb/ChangeLog             | 5 +++++
 gdb/common/gdb_optional.h | 5 +++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 7fcfebb..93d20b5 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2017-04-05  Pedro Alves  <palves@redhat.com>
+
+	* common/gdb_optional.h (optional::optional): Make constexpr and
+	initialize m_dummy.
+
 2017-04-04  John Baldwin  <jhb@FreeBSD.org>
 
 	* amd64-fbsd-tdep.c: Remove "bsd-uthread.h" include.
diff --git a/gdb/common/gdb_optional.h b/gdb/common/gdb_optional.h
index fef7a73..ad1119f 100644
--- a/gdb/common/gdb_optional.h
+++ b/gdb/common/gdb_optional.h
@@ -34,8 +34,9 @@ class optional
 {
 public:
 
-  optional ()
-    : m_instantiated (false)
+  constexpr optional ()
+    : m_dummy (),
+      m_instantiated (false)
   {
   }