[Bug c++/48035] New: Mismatch on size of class when initializing hierarchy involving virtual inheritance and empty base classes

[scot4spam at yahoo dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48035

           Summary: Mismatch on size of class when initializing hierarchy
                    involving virtual inheritance and empty base classes
           Product: gcc
           Version: 4.4.5
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: scot4spam@yahoo.com


Created attachment 23585
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23585
preprocessed source file

The attached preprocessed source file is a much-simplified version of a more
complex class hierarchy involving several levels of virtual inheritance and
empty classes in the hierarchy.

"g++ -v" details:
Using built-in specs.
Target: i686-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-ppl --with-cloog --with-tune=generic --with-arch=i686
--build=i686-redhat-linux
Thread model: posix
gcc version 4.4.5 20101112 (Red Hat 4.4.5-2) (GCC)

Also tested with a new build:
Using built-in specs.
COLLECT_GCC=./g++
Target: i686-pc-linux-gnu
Configured with: ./configure
Thread model: posix
gcc version 4.5.2 (GCC)

Among others.

Compile with "g++ -c vtable-bug.i".

What we observe (tested on x86 and ARM) is that "operator new" is called to
allocate 12 bytes for the class (and sizeof matches that 12 byte size), but the
compiled code then proceeds to load four-byte 0's into offsets 0, 4, 8, AND 12
from the allocated buffer, stomping data at byte offsets 12-15.  (More complex
hierarchies can result in more than one 4-byte offset being stomped, and/or the
same offset being initialized repeatedly.)

Changing which classes are virtual, or adding data to empty base classes,
changes or fixes the behavior.