[Bug c++/13114] [ABI] wrong class hierarchy

["yanliu at ca dot ibm dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From yanliu at ca dot ibm dot com  2003-11-25 03:11 -------
(In reply to comment #3)
> not a bug. X is an empty base, and so can be overlaid. Please refer to the ABI
> documentation linked to from the gcc site.

Thanks for the quick reply. But I am not convinced :-). In this particular 
testcase, X is a virtual empty base. According to C++ ABI, before mapping X, 
the current dsize(ZZ) =8 ( which includes 4 bytes for its vft pointer, 4 bytes 
for its data member -- int i), so, when it's time to mapping X, X should be 
mapped at offset 8( not at offset 4). With option -fabi-version=2, the layout 
of X is indeed at offset 8. But I don't understand why X's W is put at offset 
12. 

According to C++ ABI, " 3. If D is an empty proper base class: 
Its allocation is similar to case (2) above, except that additional candidate 
offsets are considered before starting at dsize(C). First, attempt to place D 
at offset zero. If unsuccessful (due to a component type conflict), proceed 
with attempts at dsize(C) as for non-empty bases. As for that case, if there is 
a type conflict at dsize(C) (with alignment updated as necessary), increment 
the candidate offset by nvalign(D), and try again, repeating until success 
occurs.

Once offset(D) has been chosen, update sizeof(C) to max (sizeof(C), offset(D)
+sizeof(D)). Note that nvalign(D) is 1, so no update of align(C) is needed. 
Similarly, since D is an empty base class, no update of dsize(C) is needed".  

Accordingly, after mapping X, the dsize(ZZ) should not be updated, thus W 
should be put at the same offset as X -- offset 12. 


The following class hierarchy is generated from IBM's VAC++ compiler, what we 
did is first try to put X at offset 0, but since X's W has type confliction 
with Y's W at offset 0, therefor X is put at dsize(ZZ), which is 8. This time, 
there is not type confliction, so both X and X are put at offset 8.

Class ZZ
   size=8 align=4
ZZ (0x00000000) 0
    vptridx=0 vptr=((&ZZ::_ZTV2ZZ) + 12)
  Z (0x00000000) 0 nearly-empty
      primary-for ZZ (0x00000000)
      subvttidx=4
    X (0x00000000) 8 empty virtual canonical
        vbaseoffset=-12
      W (0x00000000) 8 empty
    Y (0x00000000) 0 empty
      W (0x00000000) 0 empty






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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13114