[Bug c++/36960] New: Reference variable in virtually inherited base corrupted under optimization

[Bug c++/36960] New: Reference variable in virtually inherited base corrupted under optimization

[raymond at corvil dot com]

Apologies if this is a duplicate:  I searched existing reports and found tons
of hits on "virtual inheritance" and "reference", but couldn't find anything
like this.  The following program illustrates the problem:

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

struct Lower {
        const int&      ref;

        Lower(const int& ref) : ref(ref) { }
};


struct Middle : public virtual Lower {

        Middle(const int& ref) : Lower(ref) { }
};


struct Upper : public Middle {

        Upper(const int& ref) : Lower(ref), Middle(ref) { }

        int     get()
        {
                return ref;
        }
};


int     main()
{
        int i = 0;
        Upper upper(i);

        return upper.get();
}


>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Compiling this using 4.2.2 or 4.2.3 with -O2 or higher causes the resulting
binary to return a non-zero value;  3.4.2 produces a bug-free binary.  I've
seen the same effect in more complex code, where gdb gives an obviously wrong
address for the "ref" member.  Dropping to -O fixes the problem, as do several
other changes:

* Making "ref" an int (as opposed to an int&)
* Making Middle inherit non-virtually from Lower
* Accessing "ref" from Middle instead of Upper

The constness of the reference has no effect.


Issued command-line:
    g++ -Wall -W -Wundef -Wpointer-arith -g -O2 VirtuallyInheritedReference.cpp

Full output from running with "-v --save-temps" is as follows:
    g++ -v -save-temps -Wall -W -Wundef -Wpointer-arith -g -O2 -c -o /dev/null
VirtuallyInheritedReference.cpp

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: /mnt/taw/usr/Taw/tmp/gcc-4.2.2/configure --prefix=/usr
--disable-nls --libexecdir=/usr/lib --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --enable-languages=c,c++
--disable-bootstrap
Thread model: posix
gcc version 4.2.2
 /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/cc1plus -E -quiet -v -D_GNU_SOURCE
VirtuallyInheritedReference.cpp -mtune=generic -Wall -W -Wundef -Wpointer-arith
-fworking-directory -O2 -fpch-preprocess -o VirtuallyInheritedReference.ii
ignoring nonexistent directory
"/usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../include/c++/4.2.2

/usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../include/c++/4.2.2/i686-pc-linux-gnu
 /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../include/c++/4.2.2/backward
 /usr/local/include
 /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/include
 /usr/include
End of search list.
 /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/cc1plus -fpreprocessed
VirtuallyInheritedReference.ii -quiet -dumpbase VirtuallyInheritedReference.cpp
-mtune=generic -auxbase-strip /dev/null -g -O2 -Wall -W -Wundef -Wpointer-arith
-version -o VirtuallyInheritedReference.s
GNU C++ version 4.2.2 (i686-pc-linux-gnu)
        compiled by GNU C version 4.2.2.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 6ba594205d388e98f3b46dee442d61ac
 as -V -Qy -o /dev/null VirtuallyInheritedReference.s
GNU assembler version 2.18 (i686-pc-linux-gnu) using BFD version (GNU Binutils)
2.18
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>



Contents of VirtuallyInheritedReference.ii:

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
# 1 "VirtuallyInheritedReference.cpp"
# 1 "/home/raymond/src/C++/gcc-bugs//"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "VirtuallyInheritedReference.cpp"

struct Lower {
 int& ref;

 virtual ~Lower() { }

 Lower(int& ref) : ref(ref) { }
};


struct Middle : public virtual Lower {

 Middle(int& ref) : Lower(ref) { }
};


struct Upper : public Middle {

 Upper(int& ref) : Lower(ref), Middle(ref) { }

 int get()
 {
  return ref;
 }
};


int main()
{
 int i = 0;
 Upper upper(i);

 return upper.get();
}

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


-- 
           Summary: Reference variable in virtually inherited base corrupted
                    under optimization
           Product: gcc
           Version: 4.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: raymond at corvil dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36960

[Bug c++/36960] Reference variable in virtually inherited base corrupted under optimization

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-07-29 00:01 -------
  i = 0;
  upper.D.2127.ref = &i;
  upper.D.2125._vptr$Middle = &_ZTV5Upper + 12;
  return *((struct Lower *) &upper + (long unsigned int) *(int *)
&_ZTV5Upper)->ref;

This works on the trunk correctly.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36960

[Bug c++/36960] Reference variable in virtually inherited base corrupted under optimization

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2008-07-29 00:02 -------
Likewise for 4.3.0.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36960

[Bug c++/36960] Reference variable in virtually inherited base corrupted under optimization

[steven at gcc dot gnu dot org]

------- Comment #3 from steven at gcc dot gnu dot org  2010-07-13 09:41 -------
Works with GCC 4.3, 4.4, 4.5, and trunk.

GCC 4.2 is no longer maintained, so this bug will not be fixed.

Richi, perhaps you can use the test case, put it in the test suite?


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |WONTFIX


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36960

[Bug c++/36960] Reference variable in virtually inherited base corrupted under optimization

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2010-07-13 13:31 -------
Subject: Bug 36960

Author: rguenth
Date: Tue Jul 13 13:31:26 2010
New Revision: 162141

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=162141
Log:
2010-07-13  Richard Guenther  <rguenther@suse.de>

        PR tree-optimization/36960
        * g++.dg/torture/pr36960.C: New testcase.

Added:
    trunk/gcc/testsuite/g++.dg/torture/pr36960.C
Modified:
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36960