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

[Mark Mitchell]

Nathan Sidwell wrote:

> 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.

If E should have 4-byte alignment (as indicated above by "nvalign(E)"), 
then I agree.

Of course, independently of which choice is correct, we don't want G++ 
changing the ABI without explicit use of -fabi-version.  However, I see 
the size of F as 12 with both G++ 3.4.1 and the current mainline, on 
i686-pc-linux-gnu.

Yan, are you using PowerPC instead of IA32?

Thanks,

-- 
Mark Mitchell
CodeSourcery, LLC
(916) 791-8304
mark at codesourcery.com