On Sun, Jan 17, 2016 at 3:21 PM, Torvald Riegel <trie...@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 <ja...@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
