[cxx-abi-dev] question on the virtual base offset

[Nathan Sidwell]

Yan Liu wrote:
> 
> 
> 
> 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;}
> 
> 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)
> According to the C++ ABI online document chapter 2.4
> (http://www.codesourcery.com/cxx-abi/abi.html).  After mapping the virtual
> base class A while mapping of the most derived class F,  the dsize(F)=11,
> size(F)=11, align(F)=4, and nvsize(E)=4, nvalign(E)=4.  So, when mapping
> virtual base class E, the next available bits are at offset dsize(F)=11.  E
> should be put at offset(F) which is the dsize(F) =11, and the final class
> size should be 12. To me, g++3.4 gives the most reasonable result, I don't
> understand why g++3.5 put E at offset 12, and the final class size is 16.
> Is it means that ABI has changed or just a gcc bug?
> 
> Your kind help is highly appreciated. Thanks.

the layout 3.5 gives looks ok to me.  E requires 4 byte alignment, so
cannot be at 11.

3.4 incorrectly packed the bitfield into a char.

nathan

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