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

[David Edelsohn]

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.

Thanks, David