question on class layout with virtual base classes

[Yan Liu <yanliu at ca dot ibm dot com>]

I found that g++ 3.3, 3.4 and 3.5 have different class layout for the
following testcase:
struct A {char a;};

struct B: virtual public A {};

struct C:public B{
  int c;
  virtual void f() {};
};
struct D {
        char d;
};
struct E {int:3;};
struct F: public C, virtual E, public D {
        char f1;
        virtual void f(){}
};
int main()
{F f;}

g++ 3.3's information is:
Configured with: ../configure --enable-threads=posix --prefix=/usr
--with-local-prefix=/usr/local --infodir=/usr/share/info
--mandir=/usr/share/man --enable-languages=c,c++,f77,objc,java,ada
--disable-checking --libdir=/usr/lib --enable-libgcj
--with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib
--with-system-zlib --enable-shared --enable-__cxa_atexit
--host=powerpc-suse-linux --build=powerpc-suse-linux
--target=powerpc-suse-linux --enable-targets=powerpc64-suse-linux
--enable-biarch
Thread model: posix
gcc version 3.3.3 (SuSE Linux)
g++3.4's information is:
 Configured with: ../gcc/configure
--prefix=/.../torolab.ibm.com/fs/projects/vabld/run/gcc/linux_ppc/3.4-dev
--enable-languages=c,c++,f77,objc --enable-altivec --enable-shared
powerpc-suse-linux
Thread model: posix
gcc version 3.4.0 20040212 (prerelease)

g++ 3.5's information is:
Configured with: /private/var/tmp/gcc/gcc-1765.obj~1/src/configure
--disable-checking --prefix=/usr --disable-libmudflap --mandir=/share/man
--enable-languages=c,objc,c++ --disable-libada
--program-transform-name=/^[cg][^+.-]*$/s/$/-3.5/
--with-gxx-include-dir=/include/gcc/darwin/3.5/c++
--build=powerpc-apple-darwin --host=powerpc-apple-darwin
--target=powerpc-apple-darwin
Thread model: posix
gcc version 3.5.0-tree-ssa 20040321 (merged 20040529) (Apple Computer, Inc.
build 1765)

Attached files are the class and vft  layout results with three g++
compilers using -fdump-class-hierarchy option.


(See attached file: temp4.g++35.class)(See attached file:
temp4.g++33.class)(See attached file: temp4.C.g++34_32.class)

For the virtual base class A in the class F's layout, I thought tail
padding rule should only apply to non-virtual base classes. Before mapping
the virtual base class A, the current class size should be increased to the
mutiplication of alignment ( which is 4 at this moment), and A should be
put at offset 12 rather at offset 10.

I understant that C++ ABI does not explicitly specify the tail padding
rule. I would like to take this opportunity to understand  the tail padding
rules and empty base class mapping strategies you are using.

Your kind help is highly appreciated. Thanks.

Yan Liu
 email: yanliu@ca.ibm.com

Attachment: temp4.g++35.class
Description: Binary data

Attachment: temp4.g++33.class
Description: Binary data

Attachment: temp4.C.g++34_32.class
Description: Binary data