[Bug c++/54740] New: Empty base optimization gets confused when a member variable derives from the same empty base.

[avenikov at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54740



             Bug #: 54740

           Summary: Empty base optimization gets confused when a member

                    variable derives from the same empty base.

    Classification: Unclassified

           Product: gcc

           Version: unknown

            Status: UNCONFIRMED

          Severity: major

          Priority: P3

         Component: c++

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: aveni...@google.com





struct empty_base {};

struct another_empty_base {};



struct inner : empty_base {};

struct outer : empty_base {

    inner i_;

};





int main(int, char **)

{

    return sizeof(outer);

}



This returns 2;

If you change outer to derive from another_empty_base, it returns 1 (which is

correct). Whatever the correct result should be, they at least should be the

same.



I tried it on 4.7.2 and on 4.4.3 - same problem is present.

The optimization flags don't seem to make any difference.



Setting it to major, as this seems like a very common idiom and people expect

empty base optimization to kick in in such cases.





Andy.



P.S. Just in case: partial output from my gcc -v:

Using built-in specs.

COLLECT_GCC=/<...>/stage-4.7.2/bin/gcc

COLLECT_LTO_WRAPPER=/<...>/stage-4.7.2/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.7.2/lto-wrapper

Target: x86_64-unknown-linux-gnu

Configured with: ../gcc-4.7.2/configure --prefix=/<...>/stage-4.7.2

--with-gmp=/<...>/gmp-4.3.2/ --with-mpfr=/<...>/mpfr-3.0.0/

--with-mpc=/<...>/mpc-0.9/ --enable-languages=c,c++

Thread model: posix

gcc version 4.7.2 (GCC) 



It's a ubuntu-based system.