[Bug c++/52796] New: [4.6/4.7 C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[Bug c++/52796] New: [4.6/4.7 C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jyasskin at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

             Bug #: 52796
           Summary: [4.6/4.7 C++11] Initialization of primitive object
                    with 0-length parameter pack fails to value-initialize
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jyasskin@gcc.gnu.org


The following program attempts to initialize a 'char' member using a variadic
parameter pack. With length 0, the initialization doesn't happen, leaving the
original value of the memory in the field. I originally noticed this in gcc-4.6
when std::list<char>(3) failed to initialize the list elements.


$ cat test.cc
#include <memory.h>
#include <iostream>

template<typename T>
class Wrapper {
 public:
#if WORK
  Wrapper()
      : t() {
  }
#endif
  template<typename ...Args>
  Wrapper(Args&&... args)
      : t(args...) {
  }

  T t;
};

int main() {
  __attribute__(aligned(alignof(Wrapper<char>))) char
space[sizeof(Wrapper<char>)];
  memset(space, '\xab', sizeof(space));
  Wrapper<char>*w = new(space) Wrapper<char>;
  std::cout << (int)w->t << '\n';
  w->~Wrapper<char>();
  memset(space, '\xab', sizeof(space));
  w = new(space) Wrapper<char>();
  std::cout << (int)w->t << '\n';
  w->~Wrapper<char>();
  memset(space, '\xab', sizeof(space));
  w = new(space) Wrapper<char>((int)'y');
  std::cout << (int)w->t << '\n';
  w->~Wrapper<char>();
}

$ g++-4.7pre --version
g++-4.7pre (GCC) 4.7.1 20120330 (prerelease)
...
$ g++-4.7pre -Wall -std=c++11 test.cc -g3 -o test && ./test
-85
0
121


Disassembling with gdb shows:

Dump of assembler code for function Wrapper<char>::Wrapper<>():
   0x0000000000400a70 <+0>:     push   %rbp
   0x0000000000400a71 <+1>:     mov    %rsp,%rbp
   0x0000000000400a74 <+4>:     mov    %rdi,-0x8(%rbp)
=> 0x0000000000400a78 <+8>:     pop    %rbp
   0x0000000000400a79 <+9>:     retq

[Bug c++/52796] [4.6/4.7 C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jyasskin at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

--- Comment #1 from Jeffrey Yasskin <jyasskin at gcc dot gnu.org> 2012-03-30 21:18:09 UTC ---
Created attachment 27050
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27050
Test file demonstrating improvement from 4.6->4.7

The attached 46_vs_47.cc shows that gcc-4.7 calls the default constructor for
pack uses like new Type(args...), while gcc-4.6 didn't. So 4.7 doesn't have the
std::list bug.  I tested this with valgrind-3.6.0:

$ g++-4.6.x --version
g++-4.6.x (GCC) 4.6.4 20120330 (prerelease)

$ g++-4.6.x -Wall -std=c++0x 46_vs_47.cc -g3 -o test && /usr/bin/valgrind
./test
...
==15840== Conditional jump or move depends on uninitialised value(s)
==15840==    at 0x4EB8074: std::ostreambuf_iterator<char,
std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char,
std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char,
std::char_traits<char> >, std::ios_base&, char, long) const
(locale_facets.tcc:872)
==15840==    by 0x4EB8395: std::num_put<char, std::ostreambuf_iterator<char,
std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char,
std::char_traits<char> >, std::ios_base&, char, long) const
(locale_facets.h:2475)
==15840==    by 0x4EC33CC: std::ostream& std::ostream::_M_insert<long>(long)
(locale_facets.h:2336)
==15840==    by 0x400BF7: void check_init<>() (46_vs_47.cc:19)
==15840==    by 0x400AC1: main (46_vs_47.cc:24)
...
0
3
==15840== Conditional jump or move depends on uninitialised value(s)
==15840==    at 0x4EB8074: std::ostreambuf_iterator<char,
std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char,
std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char,
std::char_traits<char> >, std::ios_base&, char, long) const
(locale_facets.tcc:872)
==15840==    by 0x4EB8395: std::num_put<char, std::ostreambuf_iterator<char,
std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char,
std::char_traits<char> >, std::ios_base&, char, long) const
(locale_facets.h:2475)
==15840==    by 0x4EC33CC: std::ostream& std::ostream::_M_insert<long>(long)
(locale_facets.h:2336)
==15840==    by 0x400B03: main (46_vs_47.cc:27)
...
0

$ g++-4.7pre --version
g++-4.7pre (GCC) 4.7.1 20120330 (prerelease)
$ g++-4.7pre -Wall -std=c++11 test.cc -g3 -o test && /usr/bin/valgrind ./test
==8931== Memcheck, a memory error detector
==8931== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==8931== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for
copyright info
==8931== Command: ./test
==8931== 
0
3
0

[Bug c++/52796] [4.6/4.7/4.8 C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jyasskin at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

Jeffrey Yasskin <jyasskin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[4.6/4.7 C++11]             |[4.6/4.7/4.8 C++11]
                   |Initialization of primitive |Initialization of primitive
                   |object with 0-length        |object with 0-length
                   |parameter pack fails to     |parameter pack fails to
                   |value-initialize            |value-initialize

--- Comment #2 from Jeffrey Yasskin <jyasskin at gcc dot gnu.org> 2012-03-30 21:22:30 UTC ---
And 4.8.0 20120330 matches 4.7's behavior for both test cases.

[Bug c++/52796] [C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2012-04-03
                 CC|                            |jason at gcc dot gnu.org
         AssignedTo|unassigned at gcc dot       |jason at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

[Bug c++/52796] [C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> 2012-04-03 22:51:14 UTC ---
Author: jason
Date: Tue Apr  3 22:51:08 2012
New Revision: 186120

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=186120
Log:
    PR c++/52796
    * pt.c (tsubst_initializer_list): A pack expansion with no elements
    means value-initialization.

Added:
    branches/gcc-4_6-branch/gcc/testsuite/g++.dg/cpp0x/variadic-value1.C
Modified:
    branches/gcc-4_6-branch/gcc/cp/ChangeLog
    branches/gcc-4_6-branch/gcc/cp/pt.c
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog

[Bug c++/52796] [C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> 2012-04-03 23:38:25 UTC ---
Author: jason
Date: Tue Apr  3 23:38:21 2012
New Revision: 186122

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=186122
Log:
    PR c++/52796
    * pt.c (tsubst_initializer_list): A pack expansion with no elements
    means value-initialization.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/variadic-value1.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/52796] [C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> 2012-04-03 23:37:16 UTC ---
Author: jason
Date: Tue Apr  3 23:37:11 2012
New Revision: 186121

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=186121
Log:
    PR c++/52796
    * pt.c (tsubst_initializer_list): A pack expansion with no elements
    means value-initialization.

Added:
    branches/gcc-4_7-branch/gcc/testsuite/g++.dg/cpp0x/variadic-value1.C
Modified:
    branches/gcc-4_7-branch/gcc/cp/ChangeLog
    branches/gcc-4_7-branch/gcc/cp/pt.c
    branches/gcc-4_7-branch/gcc/testsuite/ChangeLog

[Bug c++/52796] [C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.6.4

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> 2012-04-04 01:38:20 UTC ---
Fixed for 4.6.4, 4.7.1 and 4.8.

[Bug c++/52796] [C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[bkoz at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

Benjamin Kosnik <bkoz at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bkoz at gcc dot gnu.org

--- Comment #7 from Benjamin Kosnik <bkoz at gcc dot gnu.org> 2012-04-05 15:06:21 UTC ---
Nice work on this Jeffrey.

[Bug c++/52796] [C++11] Initialization of primitive object with 0-length parameter pack fails to value-initialize

[michaelh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52796

--- Comment #8 from michaelh at gcc dot gnu.org 2012-05-23 21:34:54 UTC ---
Author: michaelh
Date: Wed May 23 21:34:50 2012
New Revision: 187816

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=187816
Log:
Fix the PR entry in the ChangeLog.  Should be:

2012-05-23  Michael Hope  <michael.hope@linaro.org>

    PR c++/52796
    * g++.dg/cpp0x/variadic-value1.C: Change selector for explicit
    options.


Modified:
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog