[Bug c++/37762] New: Member variable of empty base optimized (EBO) class appears on wrong offset

["david dot rosenborg at pantor dot com" <gcc-bugzilla at gcc dot gnu dot org>]

A member variable of a class that utilizes EBO and has a type that also
utilizes EBO appears to be laid out at the wrong offset.

In the example below, the only difference between the classes Good and Bad is
that the member of Bad inherits from a class with no members. The sizeof
printouts show that EBO is in play, but the Bad class gets an extra four byte
padding before the member. There's no reason Bad should be any larger than
Good.

Kind Regards
/David

#include <cstdio>

struct Empty { };

struct Member { int extent; };
struct MemberWithEBO : public Empty { int extent; };

struct Good : public Empty { Member m; }; // OK
struct Bad : public Empty { MemberWithEBO m; }; // Broken, see below

#define PRINT(x) printf ("%-25s %d\n", # x ":", (x))

template <typename C, typename T> long int offsetOf (T C::*m)
{
   C proto;
   return (long int)(void*)&(proto.*m) - (long int)(void *)&proto;
}

int
main (int argc, char ** argv)
{
   PRINT (sizeof (Member));
   PRINT (sizeof (Good));
   PRINT (offsetOf (&Good::m));

   PRINT (sizeof (MemberWithEBO));
   PRINT (sizeof (Bad));
   PRINT (offsetOf (&Bad::m));
}

// Printout:
//
// sizeof (Member):          4
// sizeof (Good):            4
// offsetOf (&Good::m):      0
// sizeof (MemberWithEBO):   4
// sizeof (Bad):             8  <-- Should be 4
// offsetOf (&Bad::m):       4  <-- Should be 0




> g++ -v -save-temps ebo_bug.cc

Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --prefix=/usr/local/gcc431
--with-pkgversion=gcc431 --enable-languages=c,c++
Thread model: posix
gcc version 4.3.1 (gcc431) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic'
 /usr/local/gcc431/libexec/gcc/x86_64-unknown-linux-gnu/4.3.1/cc1plus -E -quiet
-v -D_GNU_SOURCE ebo_bug.cc -mtune=generic -fpch-preprocess -o ebo_bug.ii
ignoring nonexistent directory
"/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../../../x86_64-unknown-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:

/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../../../include/c++/4.3.1

/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../../../include/c++/4.3.1/x86_64-unknown-linux-gnu

/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../../../include/c++/4.3.1/backward
 /usr/local/include
 /usr/local/gcc431/include
 /usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/include
 /usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic'
 /usr/local/gcc431/libexec/gcc/x86_64-unknown-linux-gnu/4.3.1/cc1plus
-fpreprocessed ebo_bug.ii -quiet -dumpbase ebo_bug.cc -mtune=generic -auxbase
ebo_bug -version -o ebo_bug.s
GNU C++ (gcc431) version 4.3.1 (x86_64-unknown-linux-gnu)
        compiled by GNU C version 4.3.1, GMP version 4.1.4, MPFR version 2.2.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 4926fc16e15403c044148a60354159ba
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic'
 as -V -Qy -o ebo_bug.o ebo_bug.s
GNU assembler version 2.17.50.0.6-6.el5 (x86_64-redhat-linux) using BFD version
2.17.50.0.6-6.el5 20061020
COMPILER_PATH=/usr/local/gcc431/libexec/gcc/x86_64-unknown-linux-gnu/4.3.1/:/usr/local/gcc431/libexec/gcc/x86_64-unknown-linux-gnu/4.3.1/:/usr/local/gcc431/libexec/gcc/x86_64-unknown-linux-gnu/:/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/:/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/
LIBRARY_PATH=/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/:/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic'
 /usr/local/gcc431/libexec/gcc/x86_64-unknown-linux-gnu/4.3.1/collect2
--eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2
/usr/lib/../lib64/crt1.o /usr/lib/../lib64/crti.o
/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/crtbegin.o
-L/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1
-L/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../../../lib64
-L/lib/../lib64 -L/usr/lib/../lib64
-L/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/../../.. ebo_bug.o
-lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/usr/local/gcc431/lib/gcc/x86_64-unknown-linux-gnu/4.3.1/crtend.o
/usr/lib/../lib64/crtn.o


-- 
           Summary: Member variable of empty base optimized (EBO) class
                    appears on wrong offset
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: david dot rosenborg at pantor dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37762