[Bug c++/12053] New: ABI difference between default g++ 3.3 and g++ 3.2

["grigory at stl dot sarov dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12053

           Summary: ABI difference between default g++ 3.3 and g++ 3.2
           Product: gcc
           Version: 3.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: grigory at stl dot sarov dot ru
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i386-redhat-linux
  GCC host triplet: i386-redhat-linux
GCC target triplet: i386-redhat-linux

Overview
--------
IF the last byte (a byte on the alignment boundary) of either virtual or non-
virtual part of the class contains tail of a non-POD base class's bitfield 
THEN, besides the bit alignment (to the end of that byte), g++ 3.3 compiler 
increases class size by the value of alignment.

That makes object layout generated by g++ 3.3 incompatible with g++ 3.2, while 
it's stated that ABI version left unchanged between these two version of g++.

Details
-------
The following class may introduce a bit-field tail in the byte on alignment 
boundary. Class must be non-POD!
	struct C4
	{
		int b:30;
		C4(){};
	};

Next class gives the desired situation. C4 is the last class laid out in C1's 
virtual part, b's tail goes into the last byte.

	struct C1:  virtual C4
	{
	    int i;
	};

Expected size of C1 on ia-32 is 12 bytes. G++ 3.2 gives the required dozen, 
while g++ 3.3 makes it 16. 

Next class illustrates same problem but in non-virtual part. Expected size - 4; 
g++ 3.3 gives 8.

	struct C2:  C4
	{
	};

Notes
-----
Given problem exists in g++ 3.3 and g++ 3.3.1 in default configuration only. It 
is gone when we use -fabi-version=0 key.