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: unassig...@gcc.gnu.org
ReportedBy: scot4s...@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.
