[binutils-gdb] gdb: fix -Wmaybe-uninitialized warning in value.c

[binutils-gdb] gdb: fix -Wmaybe-uninitialized warning in value.c

[Simon Marchi]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=344642355ce401c27ea2d57a69d83ec554a9c2bb

commit 344642355ce401c27ea2d57a69d83ec554a9c2bb
Author: Simon Marchi <simon.marchi@efficios.com>
Date:   Thu Mar 2 15:26:55 2023 -0500

    gdb: fix -Wmaybe-uninitialized warning in value.c
    
    Since commit 11470e70ea0d ("gdb: store internalvars in an std::map"), bulding
    with -O2, with g++ 11.3.0 on Ubuntu 22.04, I see:
    
          CXX    value.o
        In constructor ‘internalvar::internalvar(internalvar&&)’,
            inlined from ‘constexpr std::pair<_T1, _T2>::pair(_U1&&, _U2&&) [with _U1 = const char*&; _U2 = internalvar; typename std::enable_if<(std::_PCC<true, _T1, _T2>::_MoveConstructiblePair<_U1, _U2>() && std::_PCC<true, _T1, _T2>::_ImplicitlyMoveConvertiblePair<_U1, _U2>()), bool>::type <anonymous> = true; _T1 = const char*; _T2 = internalvar]’ at /usr/include/c++/11/bits/stl_pair.h:353:35,
            inlined from ‘constexpr std::pair<typename std::__strip_reference_wrapper<typename std::decay<_Tp>::type>::__type, typename std::__strip_reference_wrapper<typename std::decay<_Tp2>::type>::__type> std::make_pair(_T1&&, _T2&&) [with _T1 = const char*&; _T2 = internalvar]’ at /usr/include/c++/11/bits/stl_pair.h:572:72,
            inlined from ‘internalvar* create_internalvar(const char*)’ at /home/smarchi/src/binutils-gdb/gdb/value.c:1933:52:
        /home/smarchi/src/binutils-gdb/gdb/value.c:1831:8: warning: ‘<unnamed>.internalvar::u’ may be used uninitialized [-Wmaybe-uninitialized]
         1831 | struct internalvar
              |        ^~~~~~~~~~~
        /home/smarchi/src/binutils-gdb/gdb/value.c: In function ‘internalvar* create_internalvar(const char*)’:
        /home/smarchi/src/binutils-gdb/gdb/value.c:1933:76: note: ‘<anonymous>’ declared here
         1933 |   auto pair = internalvars.emplace (std::make_pair (name, internalvar (name)));
              |                                                                            ^
    
    This is because the union field internalvar::u is not initialized when
    constructing the temporary internalvar object above.  That object is then used
    for move-construction, and the (implicit) move constructor copies the
    uninitialized bytes of field u over from the temporary object to the new
    internalvar object.  The compiler therefore complains that we use uninitialized
    bytes.  I don't think it's really a problem, because the internalvar object is
    in the `kind == INTERNALVAR_VOID` state, in which the contents of the union is
    irrelevant.  Still, mute the warning by default-initializing the union.
    
    Change-Id: I70c392842f35255f50d8e63f4099cb6685366fb7
    Reviewed-By: Tom Tromey <tom@tromey.com>

Diff:
---
 gdb/value.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdb/value.c b/gdb/value.c
index 10a7ce033fd..7b4df338304 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -1853,7 +1853,7 @@ struct internalvar
 
   enum internalvar_kind kind = INTERNALVAR_VOID;
 
-  union internalvar_data u;
+  union internalvar_data u {};
 };
 
 /* Use std::map, a sorted container, to make the order of iteration (and