[PATCH v2] libstdc++: Make certain exceptions transaction_safe.

[David Edelsohn]

On Sun, Jan 17, 2016 at 3:21 PM, Torvald Riegel <triegel@redhat.com> wrote:
> On Sat, 2016-01-16 at 15:38 -0500, David Edelsohn wrote:
>> On Sat, Jan 16, 2016 at 8:35 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>> > On Sat, Jan 16, 2016 at 07:47:33AM -0500, David Edelsohn wrote:
>> >> stage1 libstdc++ builds just fine.  the problem is stage2 configure
>> >> fails due to missing ITM_xxx symbols when configure tries to compile
>> >> and run conftest programs.
>> >
>> > On x86_64-linux, the _ITM_xxx symbols are undef weak ones and thus it is
>> > fine to load libstdc++ without libitm and libstdc++ doesn't depend on
>> > libitm.
>> >
>> > So, is AIX defining __GXX_WEAK__ or not?  Perhaps some other macro or
>> > configure check needs to be used to determine if undefined weak symbols
>> > work the way libstdc++ needs them to.
>>
>> __GXX_WEAK__ appears to be defined by gcc/c-family/c-cppbuiltin.c
>> based on  SUPPORTS_ONE_ONLY.  gcc/defaults.h defines SUPPORTS_ONE_ONLY
>> if the target supports MAKE_DECL_ONE_ONLY and link-once semantics.
>> AIX weak correctly supports link-once semantics.  AIX also supports
>> the definition of __GXX_WEAK__ in gcc/doc/cpp.texi, namely collapsing
>> symbols with vague linkage in multiple translation units.
>>
>> libstdc++/src/c++11/cow-stdexcept.cc appears to be using __GXX_WEAK__
>> and __attribute__ ((weak)) for references to symbols that may not be
>> defined at link time or run time.  AIX does not allow undefined symbol
>> errors by default.  And the libstdc++ inference about the semantics of
>> __GXX_WEAK__ are different than the documentation.
>>
>> AIX supports MAKE_DECL_ONE_ONLY and the documented meaning of
>> __GXX_WEAK__.  AIX does not support extension of the meaning to
>> additional SVR4 semantics not specified in the documentation.
>
> I see, so we might be assuming that __GXX_WEAK__ means more than it
> actually does (I'm saying "might" because personally, I don't know; your
> information supports this is the case, but the initial info I got was
> that __GXX_WEAK__ would mean we could have weak decls without
> definitions).

I believe that libstdc++ must continue with the weak undefined
references to the symbols as designed, but protect them with a
different macro.  For example, __GXX_WEAK_REF__ or __GXX_WEAK_UNDEF__
defined in defaults.h based on configure test or simply overridden in
config/rs6000/aix.h.  Or the macro could be local to libstdc++ and
overridden in config/os/aix/os_defines.h.

Thanks, David