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

[Torvald Riegel <triegel@redhat.com>]

On Mon, 2016-01-18 at 14:54 +0100, Torvald Riegel wrote:
> On Sun, 2016-01-17 at 18:30 -0500, David Edelsohn wrote:
> > 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.
> 
> OK.  I'm currently testing the attached patch on x86_64-linux.

No regressions in the libstdc++ and libitm tests on x86_64-linux.