Re: C++ conversion status update

[Simon Marchi <simon.marchi@polymtl.ca>]

On 15 December 2015 at 06:46, Jose E. Marchesi <jose.marchesi@oracle.com> wrote:
> g++ -g -O2   -I. -I../../gdb -I../../gdb/common -I../../gdb/config -DLOCALEDIR="\"/usr/local/share/locale\"" -DHAVE_CONFIG_H -I../../gdb/../include/opcode -I../../gdb/../opcodes/.. -I../../gdb/../readline/.. -I../../gdb/../zlib -I../bfd -I../../gdb/../bfd -I../../gdb/../include -I../libdecnumber -I../../gdb/../libdecnumber  -I../../gdb/gnulib/import -Ibuild-gnulib/import   -DTUI=1  -I/usr/include/python2.6 -I/usr/include/python2.6 -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wno-sign-compare -Wno-write-strings -Wno-narrowing -Wformat-nonliteral -Werror -c -o sparc64-tdep.o -MT sparc64-tdep.o -MMD -MP -MF .deps/sparc64-tdep.Tpo ../../gdb/sparc64-tdep.c
> cc1plus: warnings being treated as errors
> In file included from ../../gdb/target.h:74,
>                  from ../../gdb/exec.h:23,
>                  from ../../gdb/gdbcore.h:29,
>                  from ../../gdb/sparc64-tdep.c:27:
> ../../gdb/btrace.h: In function ‘size_t VEC_btrace_insn_s_embedded_size(int)’:
> ../../gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
> ../../gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
> ../../gdb/btrace.h: In function ‘VEC_btrace_insn_s* VEC_btrace_insn_s_alloc(int)’:
> ../../gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
> ../../gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
> ../../gdb/btrace.h: In function ‘VEC_btrace_insn_s* VEC_btrace_insn_s_copy(VEC_btrace_insn_s*)’:
> ../../gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
> ../../gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
> ../../gdb/btrace.h: In function ‘VEC_btrace_insn_s* VEC_btrace_insn_s_merge(VEC_btrace_insn_s*, VEC_btrace_insn_s*)’:
> ../../gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
> ../../gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
> ../../gdb/btrace.h: In function ‘int VEC_btrace_insn_s_reserve(VEC_btrace_insn_s**, int, const char*, unsigned int)’:
> ../../gdb/btrace.h:84: error: invalid access to non-static data member ‘VEC_btrace_insn_s::vec’  of NULL object
> ../../gdb/btrace.h:84: error: (perhaps the ‘offsetof’ macro was used incorrectly)
> At global scope:
> cc1plus: error: unrecognized command line option "-Wno-narrowing"
> make[2]: *** [sparc64-tdep.o] Error 1
> make[2]: Leaving directory `/home/jemarch/couts2/binutils-gdb/build/gdb'
> make[1]: *** [all-gdb] Error 2
> make[1]: Leaving directory `/home/jemarch/couts2/binutils-gdb/build'
> make: *** [all] Error 2

This can be reproduced with g++ 4.4 on any architecture, i think.

This seems to be caused by the interaction of enum_flags and the
vector implementation.  After preprocessing we have this:

  __builtin_offsetof (VEC_btrace_insn_s, vec)

However, VEC_btrace_insn_s is a non-POD data type, because it
ultimately contains an enum_flags object, which is non-POD.

  VEC_btrace_insn_s
    btrace_insn_s vec[1];
      btrace_insn_flags flags; (enum_flags <enum btrace_insn_flag>)

From what I read, using offsetof on a non-POD structure is an
undefined behavior, at least in C++98.  I don't know if it has changed
in later standard versions, but later g++ do not complain.

For g++ 4.4, it is possible to silence the warning/error with
-Wno-invalid-offsetof, if we know that it still produces good results.